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Status update:  I rebuilt the carburetor with the best parts from the old and new ones (did not find any problems with diaphragms); enriched the idle fuel mixture; did more work to tighten up all vacuum lines (including installing better hose clamps); installed a deluxe fuel filter; and gave the variator and clutch a good cleaning.  The idle speed is much more steady now, the bubbles in the fuel line are gone, and I believe the clutch is engaging at a slightly higher RPM.  So there's less difference between the high and low RPM ranges, and less tendency for the clutch to want to engage while I'm waiting at a stoplight  (then try to die from low RPMs at the next stoplight). 

I put everything back together, and plan to ride for a week or two, to see if the problem is really cured, and dial in the best idle speed.  Thanks, everyone, for your helpful suggestions!

P.S. -- I would like to replace my clutch springs (the small ones) with slightly stiffer ones that would cause the clutch to engage at higher RPMs.  I bought some a while back that were supposed to fit 250cc scooters, but they were too long and didn't work.  Can anyone recommend a good source of springs for our 250cc scoots?

Forgot to mention -- I already have an Iridium plug -- I always install them -- thanks!

Hi all, I'm stumped by an idle issue with my 2007 Benjhou scooter -- equivalent to a Roketa MC-54B with the 257cc Linhai-style engine. I have been riding this style scooter for about 6 years now, and am familiar with valve adjustment, carb rebuilds, and other normal maintenance on these and other scooters.

Scooter has about 8K miles on the odometer, 5K of which were put on by me. It generally runs well, but for most of the time I've had the scooter, it has wanted to idle either too high or too low. Using the idle speed screw, I could adjust it to 2000-3000 RPM, and the clutch would engage. Then, when I back out the screw, at some point the idle speed would drop to 800-1000, or even less. I have almost never achieved a consistent idle speed in the 1500 rpm range. Recently, the extremes have been even greater, so that some time after the auto-enricher closes and the cooling fan has come on once or twice, the idle speed rather suddenly goes so low that the engine dies at a stoplight. Here's what I have tried, other than simply adjusting the idle speed screw:

* adjusted valves (multiple times) .004 intake, .005 exhaust.

* adjusted the idle mixture screw (after engine warmup). Just when I think I have the idle speed optimized, it may suddenly drop in rpm at unpredictable times.

* replaced vacuum lines and checked for vacuum leaks

* replaced intake manifold o-rings (and added red RTV for additional sealing, and checked the manifold for cracks or leaks)

* replaced the fuel tank vent canister, then got rid of it altogether, plugging the hoses and installing a fuel filter on the tank vent hose.

* eventually replaced the entire carburetor, with no apparent improvement.(my working assumption is that this eliminates the auto-enricher as the culprit, since behavior didn't change with the new carb and enricher)

* swapped out the performance CDI that I had installed with the original, with no apparent change.

The air filter box is stock (with additional sealant around the joints), and the other emissions components have not been touched. I also had previously installed a 'performance' coil, which I haven't swapped out lately. Have not checked compression.

So at this point, if I adjust the idle speed screw for a 2000 rpm idle when cold, the engine starts to die at stoplights after about 10 minutes. If I adjust the idle speed so it doesn't die when warm, the idle speed is out of control at the next cold start. My plan is to keep looking for vacuum leaks, and continue tweaking the idle mixture screw in case the last time I adjusted it, the enricher may not have been completely closed. But I seem to be running out of possibilities to try.

One thing that puzzles me:  having replaced my fuel lines with transparent plastic, I can see that when the engine is idling,  there are bubbles coming up from the fuel filter, through the fuel pump, and up to the carburetor. There are no visible fuel leaks when the engine is off. Where could these bubbles be coming from, and is it possible that the idle is dropping due to fuel starvation?  I did replace the gas cap, and the tank seems to be adequately ventilated.

So, what have I overlooked? What would you check next? Any and all input will be gratefully accepted. Thanks in advance, and sorry for the long post!

Happy New Year!  I'm sharing a problem/solution that I just experienced in hopes that it might help someone else solve a similar pesky problem.

I have a Benjhou Road Queen (gotta love that name) -- it's almost identical to a Roketa MC-54B with the horizontal Linhai 257 engine.  For some time now, I've been having problems consistent with a vacuum leak or fuel delivery problem -- really hard cold-starting, wandering idle speed, dying of fuel starvation during high-speed runs... So my holiday project was to install a Mikuni DF44 fuel pump and replace all my vacuum lines; also checked the valve adjustment, carb jets, and float level, while I had the plastics off.

Much to my dismay, the engine didn't run any better after all that work.  I pulled the carb again, and I'm not sure why, but I hooked up a hose to the ACV vacuum port and sucked on it.  I found that I could suck or blow air almost unrestricted.  I pulled the top cover off the ACV and saw that the rubber diaphragm was apparently in good condition, but the tiny O-ring that is supposed to seal the gap between the ACV base and top cover didn't look so good, and was apparently the source of the vacuum leak.  On deceleration, high vacuum is supposed to go from the hose/port on the base of the ACV, up through a small hole in the middle of the tiny O-ring, to a passage in the ACV cover, where it pulls up on the diaphragm causing the tiny air valve  below it to close. But the O-ring was letting large amounts of air in instead.  Check the photos of 250cc carbs posted by alleyoop elsewhere on the board to help visualize this part of the carb.

Not having a spare O-ring (and they are not usually included in carb rebuild kits), I carefully applied some Permatex gasket material with a small screwdriver to the top and bottom of the O-ring (kind of like icing a tiny doughnut while trying not to get any gasket stuff in the center hole/air passage).  After reassembling and letting the sealant dry, I was able to pull a good vacuum with minimal air leakage.  And when I reassembled everything, the scooter ran MUCH better.  It starts immediately first thing in the (cold) morning, gets to a stable idle speed quickly, holds its idle setting better, and so far, hasn't died of fuel starvation.

I hope this tale helps someone else with a similar problem in the future -- I think it's probably a pretty rare problem, but that's what made it so hard to identify.  My only question for the experts is -- do you have any idea what size O-ring or seal could be used to replace that little part?

Thanks guys -- I will try both those suggestions. phssthpok, I had a low-voltage issue on my 250cc scooter just like you describe -- thought the enricher was bad, but it turned out to be a bad regulator that was delivering low voltage to the enricher, which caused it to warm up more slowly. Here in Texas, that usually means too rich for too long...

I need to remove the flywheel and stator on a GY6-150 engine, and the nut seems very tight.  I haven't done one of these in a few years, and can't remember whether the threads on the crankshaft end are standard right-hand thread, or left-handed.  Can anyone give me a quick confirmation of the thread direction before I maybe put a lot of pressure on it in the wrong direction?  Also, in your experience, do the manufacturers typically use a thread locking compound on these nuts?

Thanks!

I got a direct replacement for the large trunk from an eBay seller for about $125. They claim that they use higher grade plastic than the original. So far it has held up well.

Regarding the hook for hanging a helmet: There's a slot under the left side of the seat that was designed for a special hook to be placed in it. You put the hook through the D-ring of your helmet, then drop the hook into the slot and lock the seat. I found that an old Toyota ignition key is a pretty good substitute for the helmet hook, which, as far as I can tell, is never actually supplied with these Chinese Reflex clones...