Revs rise and fall

Just bought a Huatian QT50 scooter which had been off the road at least a year as non runner. Fuel system was choked up so flushed and dried tank, fresh petrol, replaced fuel vacuum tap and carburettor with brand new units. After a bit of cranking, engine fired up and ran quietly, quite fast, as electronic choke operated. Adjusted idle screws and engine started to settle down. I was chuffed, until the engine revs rose high, then fell low, and continued to do this until it dropped and stopped. No leaks on inlet,, and fuel fine. Hard to start again, then when it did, the same thing happened. It being a 4-stroke, there's no reed valve, so am at a loss what to look for,as at my advanced age I need the scoot to be relilable. Help much appreciated.

 

Is the auto choke unit getting warm when running ? Also check valve clearance and adjust mixture screw at tickover.

 

Sounds like the mixture to me, but check your valve clearances

 

Auto choke seems fine (its a brand new unit) and checked mixture from 1.3/4 turns to 2. 1/4 turn out. Haven't checked clearances yet, but not noisy.

 

as replied to Marsh "Auto choke seems fine (its a brand new unit) and checked mixture from 1.3/4 turns to 2. 1/4 turn out. Haven't checked clearances yet, but not noisy."

 

With the mixture you need to do it dynamic, as in, turn it a quarter, give it a few revs and let it rest again and see how it is.

 

OK thanks will do. Question on same topic. Even though I have a new carb, could the auto choke give this problem. If so, How does it work. I assumed it would work off temperature somehow, but as the scoot is air cooled then ?????

 

Move the circlip on the slide needle to it's lowest position.
It's possible you may need to upjet the new carb a little

 

Hi. Thanks, but this carb is butterfly flap rather than piston and needle. It does look identical to the old one.

 

Post a photo of the new and old carbs please.
Your scooter is a typical GY6 50cc made in china, many many identical parts, across many many scooters.
Any new carb you buy WILL need setting up properly.

 

Hope you can see the new carb propeerly. I didn't want to take it off today.

 

Ok.
You do have the standard carb.
Take the big black lid off, underneath is a rubber diaphragm and slide.
In the middle of the slide is the needle.
It has a circlip on it.
Move it down one or two places, this raises the needle point out of the main jet hole slightly allowing more fuel through.
If the bike will not idle properly you will have to remove the float bowl and change the main jet either up or down in size for it to run properly..

 

Thanks Steve. I had no idea that was the needle system, from my motorbike and car experience I assumed it didn't work that way. Learn something new every day. I'll get on to that tomorrow when I'm not hung over haha.

 

HI again. I've checked both carbs, niether has provision for adjustment on the needle, just a palin needle held in either bu a plastic retainer on the old, and a clamp with screw on new. The diaphram moves freelt and is in correct position, spring is fine, autochoke warms up and allows its needle to drop. When first started it settled down and I adjusted mixure screw for even idling at 2.1/2 turns. Fine for about a minute then revs rose again, stayed medium high, then dropped to fairly moderate speed. then up, down, up down. As to main jet, wouldn't it either be too high or low without up and down? Fianlly, as a last thought before I shoot myself, could the cdi be at fault here?

 

I personally wouldn't be looking at the cdi.
I know it means taking the new carb off again but......
take it off and compare the numbers on the main jet with that of the old carb main jet
It's just a case of removing the float bowl screws thats all. I'm scratching but I think they may be different sizes.
Here's another little thing to try:
Take the air filter off, start the bike, don't try and rev it, see if the revs rise and fall or stay either high or low.

 

I had a problem like this when I first got the piaggio, not saying yours is the same, but the rubber manifold to the engine had a split so the mixture would vary with suction. you could also try putting a jubilee clip on yours where the carb meets the rubber. I also changed the autochoke first lol.

 

Hi Steve. Well checked jets both identical. Tried with or withour air filter, harder to start without, but not much. Put old carb back, did start but felt "choked-up" and cut out even at revs.New carb back on, disconnected autochoke when cold, no difference to starting, still difficult, still same trick of revving up, gradually settling down a bit,, but even a tiny touch on throttle sent revs up again. WTF

 

Hi following your chain of thought I removed the manifold and spacer. The manifold seems sound, but its O ring had been slightly damaged, and the O ring on the spacer was flattened. Would these O rings be the cause do you think? Also the manifold has two nipples. One goes to the vaccum fuel tap, but the other disappears under the belly pan. Where does that one go, and would it help to block that one off?? Thanks guys for your help.

 

If youve got a manifold that's creating a vacuum for the petrol tap on one nipple and a pipe going to fresh air on the other it's not going to be much of a vacuum and it will be sucking in air upsetting the mixture into the engine too..ide block it off and seal up all the carb and cylinder mating faces to ensure no leaks..can't hurt to give it a try

 

Well, I'm now at wits end. New manifold and spacer with new O rings and clamp.. Blocked off secondary vacuum pipe. Thought I'd fixed it as it started quite well, responded properly to throttle. Idled fine for a couple of miutes.Then. revs rose up to half throttle again, stayed there, then eventually stuttered and any touch on throttle sent revs high again. So no idea now.