Zama C1U Carb testing

[arbCub Cadet 126]

I have a Zama carburetor, Model C1U-S77 ,off a Stihl Ms-210 chainsaw. The inlet needle and seat hold pressure but if you tickle the metering diaphragm to pressurize the metering chamber with the high and low speed needles seated the metering chamber looses pressure . I am not sure if this means there is a problem with the high or low needle not seating , or if there is a problem with the nozzle or idle check valve. Are the check valves located after the high and low speed needles in the fuel flow circut? The carburetor doesn't have a primer. The problem I am having with the saw is that it will start and idle but won't accelerate . I checked the air filter, the ignition , the compression ,the spark arestor screen, the fuel filter, the fuel line, and pressure checked the crankcase, I also checked the metering lever height and the inlet screen. Some times you can get it to accelerate if you play with the throttle trigger. I also tried changing the high and low speed adjustments but it didn't make a difference.

 

[Brent Nowell]

I don’t know what you mean by ‘tickle’ but it sounds like you are trying to see if the metering side should hold pressure. The answer is yes.

Walbro has a great YouTube video on this, it’s something along the lines of small carb servicing etc

With the carb assembled, apply 8-10 psi to the fuel inlet nipple, it should not drop at all. If it drops you have a problem with the needle and/or seat, and most likely the needle is dirty or worn. This will cause richness at idle, and usually will allow fuel to pool up in the crankcase.


Now block off the fuel inlet
If you have a primer bulb you can apply 5 inches of mercury vacuum to the primer bulb nipple connection. It should hold, if it does not then most likely your main nozzle check is leaking, this can cause other issues with running. Usually similar symptoms to the diaphragm.

The main nozzle has to be punched out to replace it and if your not sure how to do this it may be easier to just order a new carb.

One or both of those tests usually fail with a poor running engine that is mechanically sound.

 

[DND 9000]

That can be a problem with the accelerator pump. The pump and it`s parts can be replaced, but in most cases it`s not worth it. Normally not only the pump piston and o-ring is worn, also the seat of it is worn. In this case it is better to use a new carburetor, because a new pump piston in a worn seat will not last long. This can also give you an air leak which can cause engine seizure.

Did you clean the carburetor properly? Ultrasonic cleaning would be the best. There must not be any dirt in the small passages of the idle circuit and elsewhere.