Motorcycle Carb Cleaning & Carburetor Adjustment

When fuel is left in a vehicle for decades it starts to cause a lot of problems leading to you needing to undertake motorcycle carb cleaning and a rebuild/adjustment.

As fuel evaporates it can form hard varnish type coatings, gungy rusty coloured thick soup, block jets, and uncover parts that start to corrode.  The carburettors can get into a real mess.

I was lucky with this Honda CB750 motorcycle I’m restoring, whoever had laid it up seems to have emptied the float bowls, this one act saved a lot of work as this is the main area that causes problems.  Even so the carbs needed to be checked and cleaned.

I have a small, 3 litre, ultrasonic cleaner so I could use this to make sure that the internal passages were really clean.  Because it is a small cleaner, I had to do one carb at a time, but as this isn’t something I do too regularly this wasn’t too big a deal

I stripped all of the carbs making sure that all of the parts for each carb were kept separately ready for the motorcycle carb cleaning process. I had a set of carb repair kits for these carbs.  Due to the quite good condition, I didn’t need to replace all of the jets but did need to use the new seals and gaskets. If possible I always like to reuse the original parts, they have been there for a long time so it would seem a shame to just bin them if they are still serviceable.

Ultrasonic Carb Cleaning

In the past motorcycle carb cleaning carbs was a real faff.  The best method I had found was to gently boil them in a mixture of degreaser and detergent, but this frequently left the inner passages blocked.  So poking them through with wires and small files was needed, with some being virtually impossible to get at. 

Ultrasonic cleaners work using a very good feature, I believe it is called cavitation, the vibrations cause bubbles to form on the surface of the item, which then burst giving a scrubbing effect. As this happens inside the small passageways in the carbs this is really useful. Along with heat and specialist motorcycle carb cleaning fluids carb cleaning has become a pleasure.

I used a specific carb cleaner solution in the ultrasonic carb cleaner’s tank, depending upon how dirty the carbs are this can usually be used for more than one time.  I start by adding hot water to the tank, because its inbuilt heater can take quite a time to heat up.

Obviously if reusing cleaning fluid you just have to accept this.  I set the temperature to 60c, you could go for a higher temperature, but as I tend to pull things out to check with my fingers I don’t want to scald myself 🙂 

I usually set the timer to 15 minutes giving the items a check and then resetting it.  Usually, I give 2 sessions, even if the parts are looking good after the first one. But with some repeat cleaning with maybe a little scrub between sessions in the tank might be necessary.

I have made a video of using the ultrasonic carb cleaner available on YouTube which can be seen here.

How to Rebuild Carbs on a CB750

After I had cleaned all of the parts, I used compressed air to blow through all passageways to make sure they were not blocked.  To prevent seizing in the future I put the thinnest smear of lithium grease on most threads. At this point, your motorcycle carb cleaning process is complete and you’re on to the rebuild and carburetor adjustment stage.

When replacing screws and jets you need to remember that carbs are made out of what appears to be a very weak alloy of zinc or something, so it is important not to damage the threads.

I always take extra care to avoid cross-threading anything, and if it feels even a little bit stiff, I back it off and check. When I have tightened the jets and screws finger tight, I give them no more than about 1/16th of a turn as a final tweak. 

For the air screw I checked the manual to ensure they were set correctly.  For this bike I noticed that when I removed the screws they were 1 and a half turns out, but the manual indicated that this should be 1 turn. I decided to put them back with the 1 and a half turns out to start off with, but know I will need to check this when I get the bike started.

Setting Carburetor Float Heights for a CB750

Turning the carbs upside down and looking at the float heights I could see that they were all different.  This is quite an important basic setting for the bike to run well. If the floats are set too low the fueling will be lean, and too rich if it is set too high.

I discovered that they should be set to 26mm, although it wasn’t fully clear whether this was from the gasket face, gasket or rim around the outside of the float bowl.  There are specific gauges for this setting but I have never had one, or even seen one! However looking at videos and images of them it looks like the measurement should be from the gasket face, which to me made the most sense.

I used vernier caliper to set my floats.  Measuring from the gasket face to the bottom of the float.  To change the heights you bend the little tab that goes against the float valve.  Although this sound fiddly it seems to work out quite well.

Once I had set the float heights I replaced the bowls and all of the fuel and breather hoses before putting the carbs back on the bike.

Initial Carb Synchronisation for a CB750

For best running each of the slides in the carbs should rise and fall together, with each allowing the same amount of air through. If the carburettors are not synchronised properly then each cylinder can work as if it was a single, with some of them being slightly ahead (giving more power) than the others.

So in effect maybe pulling the other cylinders along.  This wastes power and can cause uneven running. The only way to make sure this is accurate is to use gauges with the engine running, however, an initial setting can be done on the bike.

With later CB750s there is a much better way of adjusting each carb with a screw adjuster for each carburettor, or the three that you match to the master one.

With a K0 the carbs don’t have this adjuster, so the carbs need to be changed by adjusting the throttle cables using a screw adjuster where they enter the top of the carbs.  This is a little more fiddly but not too difficult. The main disadvantage is that the carbs have to be back on the bike, whereas the later ones can be set on the bench.

The best way to set them is to get a set of drill bits that fit exactly into the gap between the slide and the bottom of the carb.  Drill bits are suggested because you can get them in a larger variety of sizes, so when you have worked out which one fits you can get another three.  With the drill bits in place you then adjust the cable in this way.

First, take any other slack out of the system using the adjuster at the twist grip end.  Then use the adjuster for each carb in turn until you can see the drill bit just start to move. The trick is to get them set so that each drill bit moves at the same time. When you have got them right, remove the drill bits, then make sure you adjust the twist grip again so that there is the appropriate amount of free play.

I have a set of mercury manometers for synching carbs. Once I have the bike running I will use these to make sure they are right.  With these instead of the drill bits, you adjust the cables with the engine running making sure each gauge reads the same.