Post by marinervella on Mar 2, 2017 23:05:36 GMT -5
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!
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!
