Honda CB750 Restoration Project

Honda CB750 Restoration

Importing a Bike From The US

I decided that I fancied taking on a Honda CB750 restoration project. It’s one of the superbikes from my youth that I had never owned, a Honda CB750 K.  When they first came out in 1969 I was a staunch British bike fan, and like a lot of people at the time I could come up with a lot of reasons why British was better than Japanese.

Over the years though I have had plenty of other Japanese bikes, 3 or 4 Fireblades, a couple of VFRs not to mention the other brands that I made up my long list. 

Now that I wanted to find a CB, I started to do my research.  Most of the Honda CB750 K series are very similar, and over half a million of them were produced over around 10 years.  So I thought it shouldn’t be too hard to find one. Having discussed it with Young Spanner we decided that I should look for a K0 preferably a Sandcast, but not essentially. 

The reason we decided upon the K0 model was so that when the day came to let it go, we would get the best price for it. As the number following the K gets bigger the value of the bikes gets lower, with the K7’s and 8’s despite being technically the best models, also being the cheapest.

Unfortunately, this price differential is also reflected in the project bike prices, with a project K0 costing more than a pretty good condition K7. And if it is a Sandcast (basically the same bike but with the engine cases made using a simpler casting method) then add an additional £8k to the price. 

Having looked for a while I spotted one listed by a company in the USA on eBay.  I had noticed their listings of a variety of interesting bikes on eBay offering free delivery to the UK for a couple of years.  But I hadn’t been brave enough to buy one.

This one was a K0 diecast that had been chopperised at some time in the past. A none runner with high bars and a king and queen seat.  Having looked at the pictures very closely I could see that a lot of the bike looked in quite good condition. There was also a box of some of the harder to find parts, air filter, badges etc.

The bike was on bidding, but the starting price looked attractive, and as it stood the bike was unattractive enough to hopefully repel other bidders. I contacted the company to check up on some details they had missed out of the listing, and got positive information back from them, and the contact number of the person in the UK who looked after distribution.

After a conversation with the guy in the UK, I felt quite confident that there was a good chance of the deal working out well.

Honda CB750 For Sale

In the end somebody else did bid, but I still managed to win the bidding at a price I felt was good.  I won the item just before Christmas, and although the company shipped it within 3 weeks it was still near the middle of March before I actually got it in my workshop.

It was interesting to track the ship on its way across the Atlantic, then from its first drop off in Antwerp then on to Liverpool where the container containing the bike was unloaded.  If you are going to buy a bike in the same way, you will need a lot of patience while you are waiting.

Delivery of my Honda CB750 Restoration Project

The bike arrived in the evening so it was the next day before we could give the bike a thorough looking at.  All of the parts that came with the bike were packaged well and had arrived safely.

As I looked over the bike, I could see that it was as good as it looked in the pictures,  I was very pleased with what I had bought.

Honda CB750 Restoration Project

The engine turned over and the gears selected well.  The clutch was stuck, but that was expected after 30 plus years of inactivity.

The front brake wasn’t working, but I didn’t expect it to be, hydraulic brakes don’t take well to decades of storage.  The back brake being a drum was working fine.

The tyres didn’t look bad, but they are decades old and as they have equally old inner tubes will need to be changed for peace of mind.  

Having looked at all of the obvious things, the next step was to get the bike into the workshop and up on a bike lift so you can see things from a better perspective.  The main things I spotted was some suspicious welding on the oil filter bolt, and a lot of wiring issues, but more of that later.

Draining the CB750’s Oil and Removing the Oil Filter

I checked the oil in the tank, and there was nothing showing on the dipstick.  I wondered if it had been drained before being shipped. Just to make sure though I undid the drain plug at the bottom.

There was still about 150ml of oil still in it, and typically, although I had a bowl to catch it, it missed and dribbled out all over the rear brake light switch 🙂

Next I undid the sump plug, this time I had the catch bowl carefully placed.  I discovered that this was where the oil was, obviously the bike had wet sumped over the years.  The oil that came out was not too bad, although not clean, it wasn’t contaminated with water.

I felt quite reassured that the oil was still quite fluid, and would have been protecting all of the bottom end of the engine over the years.  I will have to see if it still wet sumps after I get it going.

The final thing to look at was the oil filter.  I had already noticed the bolt head at the head of the bolt had a lot of weld around it!!

At first I thought that the previous owner had welded the bolt to the filter housing to stop a leak 🙂 But this wasn’t the case.  The head of these filter bolts sits exactly where the front wheel will pepper it with road debris and water so frequently do cause a problem.

The previous owner had welded a larger bolt head on, doing quite a neat job. I have subsequently found out that Honda reduced the size of the bolt head from 17mm to 12mm early in the model life.

Apparently this was to prevent the filter bolt from being over tightened and damaging the crankcase. I’ll have to bear this in mind when I put it back.

The oil filter didn’t have any nasty surprises, there weren’t any bits of metal or other engine parts in there.  That and the fact that the oil seems in a reasonable uncontaminated state gives me more hope that I won’t have to strip the bottom end.

How to Free the Clutch on an Old Motorcycle

  1. Disconnect clutch cable
  2. Remove clutch cover
  3. Remove clutch lifter bolts and springs
  4. Remove clutch centre nut (requires a special tool) and washers
  5. Pull out the clutch middle and plates
  6. Split out the parts, friction plates, driven plates and clutch centre
  7. Check parts for damage and wear
  8. Friction plates (3.1mm minimum thickness)
  9. Slots may have scalloping, this may be dressed with a file if not too bad
  10. Check the clutch lifter is free, there is an O ring on the adjuster screw
  11. Clean parts, re-oil friction plates
  12. Rebuild clutch centre with friction and driven plates
  13. Return parts to the clutch basket, lining up tabs into the slots
  14. Replace thrust washer, concave in towards the engine
  15. Position the lock washer correctly
  16. Tighten the centre nut (with special tool) to 32-36 ft/lb then fold lock washer tab to hold the bolt
  17. Replace springs and clutch lifter, bolts tightened to 9 ft/lb
  18. Adjust lifter, screw in until bottomed, then back off ¼ turn, then tighten the lock nut
  19. Replace cover, screws tightened to 9 ft/lb
  20. Replace clutch cable

The clutch was stuck on this Honda CB750 restoration project, which is usually the case with bikes that haven’t been used for a long time.  I had a Triumph twin that would stick after a couple of weeks 🙂 so this wasn’t a surprise. I decided to take the clutch plates out to check them.

I could see that the clutch cover had allen headed screws, which it didn’t have on the other covers, so it was obvious that somebody had been in there before, not just replacing the screws.  After taking the clutch adjuster cover off and removing the clutch cable and its actuation arm, the clutch casing came off quite easily.

The clutch lifter and springs came out easily after removing the 4 retaining bolts.  The compression of the engine was sufficient to hold the clutch in place while undoing them.  The bearing in the clutch lifter ran freely so I knew it would be OK when put back.

This left the next thing to undo being the clutch centre lock nut. This is a special nut which is round with 4 cutouts which the special tool goes into.  This type of nut is used so that a lock washer with tabs can be used to stop it coming undone in use. One of the tabs of the lock washer fits very securely into one of the slots in the nut when fitted correctly.

Having bent the tab washer out of the way I used a drift to start undoing the nut.  Ideally I would have used the correct special tool, but at this time I didn’t have one.

I could see that the nut had been removed and possibly tightened using a drift in the past.  Not ideal, as there is no way of knowing what toque you have done it up to 🙂 Having undone the nut I removed it and the washer and a thrust washer that was behind it.

Now I could pull out the clutch centre and plates as one piece.  I little pushing and pulling and the odd curse here and there soon had it out without too much difficulty.  When I separated the pieces on the bench I found that there was very little wear on any part of the clutch.

The friction plates actually looked virtually new, so maybe they had been replaced just before the bike was laid up.  The slots on the clutch centre and basket were all pristine and like new. I have seen well-used clutches in the past where these have ended up with a row of scallop grooves where they have worn, so I was very pleased these were good.

I cleaned all of the plates and re-oiled them before putting them and the clutch center back together.  Putting it back in the engine is a bit fiddly, as you have to get the keyways on the center to line up while at the same time getting the tabs on the outside of the friction plate to go into the basket.

I then put the thrust washer back, this goes with the concave part in towards the engine. This was followed by the lock washer that needed to be positioned so that it engaged with the locking tab on the clutch centre.   Finally, the special nut, for which I now needed the special tool which I didn’t have.

Luckily I had watched Dino’s Hackaweek video about this on YouTube.  He made a tool out of a 7/8th inch socket. I didn’t have that size but I did have a couple of 22mm sockets which are pretty much an equivalent size.  Having 2 I was quite happy to sacrifice one. Using an angle grinder I cut the socket so that it had the 4 prongs to engage in the clutch lock nut.

Using the nut to mark up the socket so that I didn’t cut off too much worked out quite well.  After angle grinding, I dressed the edges with a file. The important thing was for the 4 prongs to engage well, and I managed this without too much trouble. 

Honda CB750 Clutch Tool DIY Socket.

I need to torque the clutch nut up to 32 – 36 ft/lb and the new tool I made worked well.  I had more of a problem locking the clutch than using the tool. I haven’t got the clutch locking tool, but a large pipe wrench can be used to keep the clutch still.

All you need to do is put the jaws into the webbing cast into the clutch centre and use your torque wrench on the nut. The only real problem was not having 3 hands, but a little help from somebody else this would have been a lot easier.

When I put the clutch lifter back in I had a problem with one of the bolts.  It turned out that it wasn’t one of the originals and must have been one of a lesser tensile strength.  Luckily I noticed that it wasn’t going in correctly and stopped before it snapped.

When I removed it I could see that it was waisting (going thinner) and would have snapped if I hadn’t stopped.  I had an equivalent bolt which was marked as having a strength of 8.8 which I could use. Once this was in I torqued all of the bolts up to the 9 ft/lb they required.

I then replace the clutch cover putting all of the Allen screws back in torquing them up to 9 ft/lbs as per the spec.  Finally I replaced the actuation arm and fitted the clutch cable. The nut for the actuation arm is also the lock nut for the clutch adjustment.

To set this you tighten the centre screw until it bottoms out, then you back it off one-quarter of a turn. Only then do you tighten the lock nut being careful not to change the adjustment.

With all of the clutch back together I could then replace the outer cover.  This also stops the little spring on the actuation arm coming off its peg. With this all done I could route the cable and connect it to the lever.

I couldn’t fully complete this yet as I still had to sort out the handlebars and switchgear that the clutch lever is part of, so that would be the next job.

Removing Motorcycle Headlight and Handlebar Swap

The handlebars were the z shaped high rise ones that are popular with some custom bike builders.  I think they were very much in vogue in the ‘70s, although I don’t remember wanting them myself at the time.

I discovered that the handlebar wiring had been cut where it came out of the bottom behind the headlight. I had already taken the master cylinder off so I just needed to remove the switches and disconnect the clutch cable.  The clamp bolts came undone quite easily, so the handlebars were quite quickly removed.

The original headlight ears had been removed and the previous owner had replaced the headlight with a small Bates style chromed one.  This seemed to be held on by the indicator arms, so I undid these first, but once I had removed the rim and found all of the wiring was soldered and couldn’t be just pulled through.

I then decided that it would be easier to just undo the brackets, which were held on by a bolt each to the bottom fork clamp. Once these were undone the headlight was quickly removed.

Honda CB750 Headlight Rebuild

The headlight that came with my 1970 Honda CB750 restoration project was definitely the wrong type.  When it had been chopperised in the ‘70s it had a small chrome headlight instead.

Probably done to emulate the Bates style headlight, but this one was held on by the indicators at the side rather than a bracket at the bottom.  

Honda CB750 Restoration Headlight and Handle Bars.

When I removed the front rim I could see that all of the wires were soldered rather than using connectors.  Whoever had done the work on the electrics on this bike in the past had done a really good job of soldering, obviously they never intended it to come apart again! 🙂

Although not starting a full restoration at this time, I still wanted the bike to be as standard as possible, so I decided to get the original type parts.  They are all available new, either “genuine” from Honda, or as reproduction parts.

I decided that where possible I would get original parts, but I wasn’t going to get too precious about it.  Also as I don’t think I will be going for a “better” than new restoration at any time so parts with a little patina would be acceptable.

I managed to get a set of headlight ears including all of the reflectors and other small parts.  They were from a candy gold bike and the paint was in pretty poor condition. If the rest of the bike’s paint was gold with a similar patina I would have left them as they are.

But as the bike has been painted black that has deteriorated badly I will need to repaint anyway. The original colour is candy blue green, and although I did originally think of making it candy ruby red, I will go with it’s original colour. 

I stripped the ears down to get them ready for painting.  The gaiters that were still attached had become very brittle and like hard plastic, and the other rubber parts had also gone very hard.

I would replace the gaiters, but I will try one of the rubber rejuvenation potions for the other rubber parts. I will paint the ears, and may powdercoat the chrome ring when I come to refinish it.

Hond CB750 Headlight Ears in Candy Gold.

I then found a sound headlight bowl, which was in red. The plastic of the bowl is quite thick so had stood up to the passage of time quite well. It had a few dents and dings but no cracks so should be fine to repaint.  

I now need a rim and a bulb.  There are repro rims available and the price isn’t too high.  But before I spotted them, I had found an original set which I had bought.  When the rim parts arrived there were none of the associated fasteners with them.  I had to use the parts list to try to work out what I needed.

Motorcycle Headlight Rebuild.

The parts are available by part number from many suppliers, but typically when there are lots of small parts the price suddenly gets unreasonable.  So I decided to work out what threads and lengths were required and buy them from a fixings supplier.

I know this means that they won’t be Japanese Industrial Standard heads on the screws, but I don’t think that is too much of an issue with a working restoration.

In the end I got virtually everything I required from Spalding Fasteners.  The only thing they didn’t seem to have was a spring for the lateral adjustment screw.

They also didn’t have the little top hat type spacers used for the screws that hold the bowl to the rim.  However, they did have Rivet Nuts which I could cut down to use instead.

I decided to get stainless fasteners when ordering.  The cost of them over zinc plated isn’t very much, and although I don’t think it will make a difference to me, long term subsequent owners may benefit 🙂 

I already had a 7 inch sealed beam bulb made by General Electric.  It came with a ‘72 Triumph Bonneville that I had previously resurrected.  I was pleasantly surprised to find that both the filaments worked when I tried them by connecting them to a battery.

Another bonus was that I still had the right connector block connected to it, which I hadn’t needed when I replaced the headlight on the Bonnie.

The rim is a little like a chinese puzzle, it does take a little time to work out which way they all go together.  However there are tabs and slots that line up when you get it all right.

When I first put it together I found that I had underestimated the adjustment screw length, at 30mm, but I soon found a 40mm screw to replace it.

It all went together quite well in the end, I still needed the adjustment spring, and to paint the bowl, but that will be done later.  

Repairing Switchgear and CB750 Handlebar Wiring

Having removed the Z style high rise bars from my Honda CB750 restoration project I now had the task of removing the switch gear and wiring and fitting them to the original bars.  The wiring ran through the bars and removal was more of an issue than I expected.

I already knew that the start button was missing from the right-hand switches. It had been replaced by a chrome aftermarket button but the wire from that was also missing. So I would have to address this before I could use them.

I have absolutely no idea how the wiring was originally put through the Z bars.  I actually started to think that I wouldn’t be able to get them out. Luckily at each angle in the bars there was a little drain hole, through which I could put a piece of stiff wire to catch the internal wiring and pull it.

I literally could only move the wire about a millimetre at a time, pushing the end, pulling through the drain holes, pulling at the switchgear end. Sometimes it just didn’t move, then I’d gain another millimetre. It took a lot of time and patience, but in the end, I had them both out.  

I assume that part of the problem was that the sheaving covering around the wires had stiffened up over the years.  One side, the left, indicator one, seemed to have stayed in reasonable condition. However the covering on the right hand set was a lot stiffer and had holes and tears, which may have been from my removal efforts, but may have been from the original fitting.

I decided to replace the right-hand side sheaving with new heat shrink tubing, as the bundle of wires was around 1cm across, I ordered 2cm unshrunk heat shrink tubing to match the original black colour and it worked out well.

I decided to put the wiring through the set of original handlebars, which I had received with the bike, before addressing where the wiring had been cut. By running a piece of flexible wire through the bars then taping it to the end of the wiring I pulled it through reasonably easily.  The original bars do not have the same sharp bends as the Z ones, and the length through the bars was considerably shorter. 

I then put crimped bullet connectors on the end of the wires.  The wiring had only just been long enough to come through the Z bars, but through the original style handlebars it gave a lot more free length.  This is in keeping with how the wiring was originally, with the connections being on the left side of the frame under the front of the fuel tank.

Motorcycle Wiring Loom Repair.

When I opened the right hand switchgear, I could see that the starter button, brass fitting and spring were missing.  When I started to research this, I found that replacement buttons are available, so I immediately ordered one.

The starter switch button is the same as the horn button and is mostly listed as a horn button if you are looking for one.  When I came to fit it I found that it is a very fiddly job. The trick seems to be to put the button and spring in first held in by the connector on the end of the wire, then compress the spring to slip the earth connector in between the spring and wire connector before screwing it in place.

I made sure that I cleaned all of the contacts in the switchgear.  The brass was tarnished from years of inactivity. I used a piece of wet and dry paper to clean the contacts back to a bright finish.

The final thing I did before I finished with this part of the wiring was to use a multimeter to test each of the switches to make sure they worked as expected.  I noticed that some of the wire colours for the indicator switchgear were very faded, with the blue, green and light green all looking very similar, so while I had the multimeter out I labelled them to make it easier for the future.

Motorcycle Wiring Harness Repair – Honda CB750 Restoration

When the Z bars had been fitted to the bike the wiring from near the front of the frame to the handlebar switches had been extended.  Whoever had done this had simply cut off the original connectors and had solder black wire between the ends of the two wires. The changed headlight had been done in the same way.

It looked like at some point there had been a short that had melted some of the wires including the main positive strap to the engine.  Maybe it was when this was happening that somebody had cut the wires where they came out of the handlebars, maybe this was an emergency measure because wires were melting?

It also seemed that there might have been panniers or some kind of extended lighting system at the back of the bike, because some of the wiring there had also had some of the black wire extension treatment.  That and the fact that the rear indicators were missing along with most of the bolts that held the rear mudguard together.

So there was a lot of work to be done with the wiring.  Before starting I went online to find wiring diagrams for the bike. Initially I wanted to repair the existing wiring, making a decision about getting a new look some time in the future if repairs were not appropriate.  Matching things to a wiring diagram would give me the best chance of success.  

I started by cutting out all of the black extended wiring and cleaning the ends of what was left.  Having already worked on the handlebar switch gear, I also cleaned up the ends of the wiring from the speedo and rev counter using new connectors where necessary.   

Honda CB750 Wiring Restoration

This now left me with deciding what to do to join the main loom to them.  I didn’t think that because of the trimming the wires had had in the past that simply fitting a connector would work, as some of the wires wouldn’t reach.  I also spotted that some of the wires joined together in 3s or 4s especially earth wires which appear to be mainly green.

I had bought a number of crimp type connectors both singles and doubles and I do have a supply of wire that I can recycle so that I decided to methodically go through the wiring diagram making bridging pieces to join the wires.

The bullet connectors for Japanese bikes are 3.9mm in diameter compared to the 4.7mm on British bikes. There was another benefit of doing this. I could check out all of the components that I came across while doing this with a multimeter or by putting power across them.  Surprisingly all of the bulbs were still fine!  

I ticked the wire diagram for each wire I had joined and each component I had tested.  It took some time to work through all of the wiring, and by the end there did seem to be quite a tangled nest of wires by the left of the headstock.

Motorcycle Wiring Harness Repair.

Probably worse than standard, but I had noticed in the past that not only Honda but Triumph and other makes seemed to have an aversion to using block connectors for associated sets of wiring in this area.  It always seems that there is a tangle of wires hidden behind black tape in this area. Maybe it is so that they can squeeze them under the tank.

CB750 Carburetor Rebuild

The next step was to overhaul the carbs and get them cleaned. Carbs are a common issue when it comes to getting a classic bike started up again, and I decided to take this out of the equation on this build by going to town and acquiring an ultrasonic carb cleaner.

I have done a separate post covering the full motorcycle carb cleaning, rebuilding and adjustment process for the CB750 which you can check out.

There are also some helpful videos on there too, showing the results and more detail step-by-step on how I rebuilt the carbs for this Honda.

Setting Valve Clearances on a Honda CB750 Restoration

The correct valve clearance is important to ensure good running, and prevent damage to the engine. If they are too tight, there is a danger that the valves may not seat properly, reducing compression and valve cooling.  This could even lead to the valves burning due to them becoming too hot. Mostly wear in engine components tends to cause the clearances to increase so this is not usually a problem. If they are too slack then the valves are remaining open for a slightly shorter time, reducing the time that the petrol air mixture can get into the engine, or the exhaust to get out. Although this doesn’t tend to be overly noticeable, unlike the extra noise that this causes 🙂

Setting the valve clearances on Honda CB750 K and F series engines is fairly easy.  The workshop manual explains a two step process, setting the number 1 cylinder to TDC on the firing stroke first.  Then setting the clearances on number 1 and the inlet on 2 and the exhaust on 3 at the same time, then rotating the engine to cylinder number 4 being at TDC to setting the other clearances.  To check that the engine is in the correct position the points cover should be removed so that the timing mark can be checked to be lined up with T mark.

The more traditional way to position the engine is to use the fact that the firing order is 1, 2, 4, 3. So the engine is set to cylinder 1 being at TDC on the firing stroke by turning the engine until you see the inlet rocker for that cylinder closing then going another 180 degrees.  This can be checked using the timing marks. The clearances for this cylinder are then set, .05mm for inlet and .08mm for exhaust. Then the engine is rotated 180 degrees, so that cylinder 2 is set TDC on the firing stroke, so that the clearances for that cylinder can be set. Then another 180 degrees putting cylinder 4 in the right position, followed by another 180 degrees to position cylinder 3.  Although this had more steps than the workshop manual method, I prefer to work on one cylinder at a time, so I used this method.

Although some people will use feeler gauges to check the clearances and judge whether they need changing or not, I tend to reset each one anyway. Having gone to the effort to get to them it just seems a reasonable thing to do 🙂

To set each clearance, first slacken the lock nut.  Position the feeler gauge between the tip of the rocker and the end of the valve. Tighten the adjuster screw until the feeler will not move, then slacken it off slightly so that the feeler can be moved, but you can still feel that it is being very slightly gripped.  Holding the screw in this position with the screwdriver the lock nut then needs to be tightened back up. Tightening the lock nut may affect the clearance so you need to check that the feeler still feels the same slight drag when you have tightened it, if not do it again. 

At the end I would go through all of the clearances again starting with cylinder 1 just to check them.  If I find any that don’t feel “right” I would reset them. Maybe a little over the top, but it hasn’t done me any harm 🙂  Finally I would replace the tappet covers after checking the ignition timing, so that you can look at the rockers to check for the firing stroke.

Setting Initial Ignition Timing on a Motorcycle

Having set the valve clearances I now gave the ignition timing my attention. This bike still has its original points system, if I decide along the way to keep the bike I may consider putting an electronic ignition system on it.  In reality the points system doesn’t give many more issues than an electronic system, especially on a hobby bike that you actually do want to fiddle with now and again 🙂 However if I thought I was going to do a reasonable amount of miles on one of these the reduced maintenance of an electronic system would be worthwhile.

The CB750 has a wasted spark system.  This means that cylinders get a spark at the same time.  One spark fires the petrol air mixture and on the other cylinder it is wasted by firing just before the exhaust stroke. It reduces the number of components and settings required in the ignition system, meaning that you can set 2 cylinders at a time, also saving time.

The steps to set the ignition are:

  1. Set the 1 and 4 points gap
  2. Set the 2 and 3 points gap
  3. Set the static timing for 1 and 4
  4. Finally set the static timing for 2 and 3.

The points should be opened and given a wipe, and checked for any burning.  Some slight marking can be removed using a suitable file or abrasive, but if there is anything that is worse than superficial I would replace the points set. 

The points are opened by a central cam that the heel of the points bears against. To set the gap you need to turn the engine so that the points are open at their widest.  In reality this isn’t a single position, the points stay wide open for a reasonable amount of rotation of the engine so it isn’t hard to find. 

Then using feeler gauges the gap can be checked.  It should be between .3 and .4mm. When the points open the engine sparks, and this gap alters the duration that the coil has to recover before the next spark, when the points close again. If the gap is incorrect, you undo the lock screw and then using a flat screwdriver in the slot on the points plate, you move the contact until it is correct.  

I tend to go for a middle value so in this case .35mm.  But if when I check the gap is wider than the smallest, .3mm, and tighter than the widest, .4mm, then I would not change them.  I set the 1 and 4 points first, followed by the 2 and 3. But for this setting the order you do this isn’t overly important, but you must do 1 and 4 first for the timing setting.

The next setting is the ignition timing for cylinders 1 and 4.  1 and 4 must be done first because the plate for 2 and 3 is screwed to the backing plate that is adjusted for 1 and 4 so they are changed as well if adjustment is required.

To get the engine in the correct position it must be rotated in the correct direction until the timing mark lines up.  The timing mark is cast into the crankcase, with the adjustment marks visible through a hole in the points backing plate. The marks to be lined up are on the advance mechanism backing plate.  There is a T for TDC and an F for firing along with the appropriate 1.4 or 2.3 marked besides them for each pair of cylinders. You have to line the 1.4 F mark up with the mark on the crankcase.

I turn the engine by putting it in gear and moving the back wheel. But if the spark plugs are out you can use the nut on the end of the ignition cam.  If using the cam nut be careful, it is easy to put a slight bend into the timing cam causing run out which will affect the timing. Although this can be fixed, it is better not to cause this problem in the first place.

Once the 1.4 F mark is lined up with the crankcase mark. I slacken the screws to the backing plate and clip a test light between the moving contact arm and earth.  With the ignition on, when the points open the light will come on, so by moving the back plate to the points you can move it in each direction until you find the point at which the light just comes on.  The light coming on is when the engine would fire, so once you have found this point, tighten the backing plate screws and that is 1 and 4 set. I would rotate the engine through a full rotation to make sure the timing marks lined up next time the light came on just to check.

Then I swapped the clip on the light to the moving contact on the 2.3 points. Then turn the engine so that the 2.3 F mark is lined up with the crankcase mark.  The 2.3 contact breaker is on a little plate that can be moved separately to the back plate, when the screws are undone. In a similar way to setting the 1.4 timing the little plate is moved until the light is just coming on, then tightened back up.  Again I turn the engine over a couple of times checking when the light comes on, ensuring that it when the marks line up.

That’s it, now the ignition timing is set.  Old school mechanics would usually be happy to stop there.  Personally I will use a strobe to check it once I have the bike running, but from experience you don’t usually find that there is much of an issue.  If you time using a strobe, then check again a week or so later, you tend to find the same kind of variation anyway!

Preparing to start a motorcycle after a long lay up for the first time – Honda CB750 Restoration

When a bike is not run for a long time, in the case of this bike probably 30 years, then a lot of issues need to be addressed before trying to start it.  There are lots of YouTube “will it run videos”, some are good, but some are potential acts of vandalism. It might not matter too much if it is a model that isn’t ever going to be a collectors piece, or is maybe going to be stripped for parts, but I always think some care should be taken not to cause unnecessary damage.

To reduce the chance of damage and to give the best chance of starting I would always go through lubrication, fuel and ignition first.  So I would go through the following list.

First Start Checklist:

  1. Replace oil and filter, if possible clean any oil passages that can be accessed.
  2. Make sure oil is circulating.
  3. Turn the engine over to make sure there aren’t any obvious mechanical issues
  4. Clean and check carburettors or fuel system.
  5. Check the electrical system to ensure that the ignition system will work, and that the rest won’t burst into flames.
  6. Check the ignition system to ensure there will be a spark at the right time.
  7. Use fresh fuel, with a clean tank and fuel lines.

Honda CB750 Project – First Start Preparation

I had already drained the oil and gone through the electrical system so I was a little ahead. The oil I had taken out wasn’t contaminated or too far gone.  So I was reasonably confident that after I put in a new oil filter and oil it should be OK. I then used the kick starter to get it moving through the engine, checking that it was arriving at the top end by removing a tappet cover to look.  In doing this I could also feel sure that there were no obviously terminal mechanical issues.

I had already thoroughly cleaned the carbs and replaced them on the bike, giving the carb synchronisation an initial setting.  I have a cheap remote fuel reservoir which I use instead of the fuel tank, this also gives more access to the engine while preparing and attempting to start it.

I had already set the valve clearances so I was happy enough with them.  This also meant that I had checked that there were no obvious mechanical issues with the rocker gear.  I had also done quite a bit of work on the wiring so I was confident that that was in good order.

The ignition timing was set so that the sparks would arrive at the right time.  I have a remote fuel tank, so I connected this to the fuel lines. Because the CB750 K0 has a seperate fuel line to each pair of carbs I needed a Y shaped hose connector to connect the tank.

Honda CB750 K0 Project Bike First Start

Before making the first attempt, you do need to think of safety as a priority.  Although not a common occurrence there is the chance that something may catch fire.  All of your checks should have helped you avoid this, but S4!T happens! You are using a highly combustible fuel, and providing a spark to make it burn 🙂 If possible I would look to do this outside, see the T140 first start, <link to video>.  But as in this case it may not be possible ( or easy enough for you to be bothered 🙂 ) to move the bike outside. So I did make sure I had a fire extinguisher and a clear exit route. Also it is prudent to think through what steps you would do if it did happen, and to remove any other flammable liquids.

Another thing you need to consider is if there is some kind of mechanical failure that could cause harm.  Personally I never put myself in the way of anything that could fly off, or blow out. It is worth thinking about.

If you have gone through everything to ensure there is fuel, spark and sufficient OOMPH to turn the engine over then there is no reason it will not start.  I have found in the past that some engines fire into life straight away, but others seem to need to build up to it. This one was one that needed a build up!

It took about 4 attempts before it started.  In fact the first time I pressed the starter it did sound as though it was nearly there, with some cylinders firing occasionally but not enough for it to burst into life.  I went through the usual steps, adding some extra fuel, attaching a booster battery, and the odd prayer!!

When it did start I hadn’t actually done anything very different.  Although I had given the wiring a good looking at, made sure it still had a spark, checked that the fuel was getting through.  It was more like it decided you’ve done enough now I’ll start up. It is always a very good moment when an engine that hasn’t run in decades burst into life, this is immediately tempered by listening for unwanted noises!!  

The other thing that I have noticed is that one cylinder will always join the party a little later than the others! Usually with a bit of a backfire and some smoke.  Just to keep you in your place 🙂 

Once it had started the first time, it now starts quite easily,  I just needed to set the tick over to a level that kept it running. 

4 thoughts on “Honda CB750 Restoration”

  1. Thoroughly enjoyed watching all the YouTube videos on this bike. Did you have two 750s one green and one candy yellow?

    Did you keep them?

    1. Hi Ben, really glad you enjoyed the videos, it was a labour of love on this project. I’m working on a Candy Gold CB750 at the moment, which is a bit of a “bitsa” really, so I’m a little torn as to which way I should take the project.
      The only other was this Candy Blue one, which sadly I decided to sell on. It ended up being a really lovely bike, but I’d gone as far as I was going to with it and it was time for a new project. The new owner added it to a great collection of CBs in various colours, so I know it’s being well looked after at least. 🙂

      1. Hi. I wrote a short message here the other day. It seems to have disappeared.
        I was wondering what your plans with the exhausts were, since I am looking for such a set.

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