Using Carburetor Synchronizer Gauges To Improve Honda CB750 Cafe Racer Motorcycle Running | 27

In this video I am still trying to improve the running of my CB750 SOHC Cafe Racer project bike following major work on the engine. It isn’t running on all cylinders consistently but I think if the carbs are synchronized it will solve the problem.

Things that I used:
Carburetor Synchronizer Gauges: https://ebay.us/g2cm5F
Motorcycle Auxiliary Fuel Tank: https://ebay.us/7aeVud

After giving the carburettors another clean in the last episode I am sure that all cylinders run, but that number 2 doesn’t run as well as the others. I only know this by measuring the temperature of the exhaust where it comes out of the cylinder. For cylinder 2 it is the same as the others on choke, but gets cooler when the choke is off, showing it isn’t burning as much fuel as the others. As I commented in the previous video I suspect it is running well enough so that it wouldn’t be obvious when riding the bike. Carburettors on motorcycles often go out of synchronisation, with various symptoms, including; loss of power; poor idling; flat spots; stutters. Frequently these changes happen slowly and the rider adjusts around them, often they are not bad enough to stop you riding the bike.

Synchronisation of the carburettors is the way to make each cylinder give the same power all the way through the rev range. Most people think that it is getting the slides or butterflies to open at the same time, but really it is to get the amount of fuel and air going into the engine equal. You measure this with a vacuum gauge so that each cylinder is pulling the same amount of fuel and air.

Each cylinder will be slightly different, either the inlet tract might be slightly longer or a different shape the actual tolerances on each component might be a little different, but the power each produces is simply based on how much fuel and air it burns, so getting this the same for all cylinders will make it run much better.

Obviously each carburettor needs to have the same jetting, so this is the first thing that needs to be right. Also before synchronisation the valve clearances need to be right, otherwise some cylinders will be sucking for longer or shorter times than the others, even if only by a very short period of time.

I have frequently used a mercury manometer in the past, but if you watch an earlier video you will see some of the mercury has been lost. So now I have some vacuum gauges which do the same job. Basically they measure the vacuum, how hard the cylinder is sucking, measured just before the fuel and air goes into the inlet.

It is either a screw, or in this case a nut that is used to adjust the height of the slides/butterfiles. You adjust each until they are all showing an equal vacuum. In reality you can only do this at idle, but if you have skill, you can also get a view on how the vacuum is at other parts of the rev range.
As I haven’t fitted the air filters this setting will still not be completely right, but I did manage to make them all equal, and it was noticeable that the engine seemed to rev and idle a lot better.

While working on the carbs I noted that the main jets are 110s. With the cone filters I am going to run, and the very free flowing exhaust I do not think these, standard, jets will flow enough fuel at higher revs, but it shouldn't make too much difference at low revs. I am going to get some larger jets and fit them before I ride the bike, so I will need to sync the carbs again once I have made that change.

It is such fun playing with engines 🙂

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