A mind of it`s own ? By rob63 (Read 565 times)

[rob63]

Thought I`d post this one up to see if anyone has any ideas. One of the fleet hire scoots (Sym Euro 50cc 2 stroke) at work has developed a slight problem or two about a week ago.
Firstly it won`t idle when cold, hold the throttle for a minute and it then settles down to a reasonably steady idle. However after it`s warmed up after about 5-10 minutes the thing slowly starts increasing it`s revs all by itself to the point where the engine is going full bore.
We`ve set the carb up and then reset it trying slightly different adjustments each time. I`ve checked resistance on the choke unit, checked current feed to it. Removed it to visually check it`s doing what it`s supposed to. Sprayed light oil liberally all round the inlets, carb, filter areas to no avail. Checked jets, needle, throttle cable and slide etc.
Had the same model in which was accident damaged but running fine, so yesterday we swapped the whole carb/choke unit over to the problem scoot, but the same symptoms occur.
I`m now erring on the electrical side as a potential problem but not sure whether there is anything there which would cause it to rev it`s nuts off by itself.
Any ideas ladies and gents ?

[sprocket]

Have you check that the throttle is in the idle position? I have seen lawn mowers do that and it is usually the butterfly valve opening by vibration...

Is there a vacuum petcock on it? it could be leaking and dumping raw fuel into the intake elbow...from the vacuum line...

It has to be fuel... even if the electricals went crazy the engine would just stall from lack of fuel...

[kz1000st]

It sounds like a seal is leaking on the crankshaft. Air is getting into the bottom end which is the same as an intake leak on the manifold.

[sprocket]

Two strokes are the devil's spawn..give me something that needs valves to be adjusted ;-)

[rob63]

May 24, 2012 15:01:57 GMT -6 sprocket said:

Have you check that the throttle is in the idle position? I have seen lawn mowers do that and it is usually the butterfly valve opening by vibration...

Is there a vacuum petcock on it? it could be leaking and dumping raw fuel into the intake elbow...from the vacuum line...

It has to be fuel... even if the electricals went crazy the engine would just stall from lack of fuel...

It`s a little slide carb so no butterfly.
The tank is in the footwell with a vacuum pulse pump to get the fuel up to the carb. I`ll have to check the vacuum pipe, although it`s a pain to get to as it dissappears under the barrel cowl into the crankcase, not into the inlet manifold.

It sounds like a seal is leaking on the crankshaft. Air is getting into the bottom end which is the same as an intake leak on the manifold.

Mmm food for thought....

Thanks chaps, it`ll give me a few more avenues of exploration.

[sprocket]

You may be lean..try adjusting the mixture screw to make it richer...more fuel to air...if that doesn't do it ...air leak as KZ said..