Inconsistent Idle

[ray]

I had done some work on my Taotao ATM50 scooter at the end of last summer. Replaced CDI, carb, spark plug, belt. For the most part scooter ran great but the rpms at idle were inconsistent. I remember following the guide to adjust the carb, but because the rpms at idle would go up then down (like in a sinusoidal wave), it was impossible. I did check the valve spacing, and when it finally got warm this past week I checked the valves again. It seems to do a little better but the rpms still go up and down. When the rpms go down, it almost seems like it will die but never does.


When riding, under throttle it runs great. When you come to a stop and the rpms drop and you give it throttle to go, sometimes the scooter will die. It will start but it sounds like it has to build up to get running again. so you have to hold down the starter button longer than normal before the engine will run by itself.

Suggestions appreciated.

[90GTVert]

Have you checked for vacuum leaks? Do sight checks by looking for hoses out of place, damaged hoses, or barbs left open. While the engine is running you can use an unlit propane torch to find leaks. Move it around near the intake and carb and if there's a leak the idle should change or it may cut out.

[humanshield]

Ray, these small engines are very sensitive to heat and they endure a lot of it.
The parts are very small and there is usually no water cooling.

After you do what Brent suggested, I suggest a compression check. Personally, I almost always perform one when encountering an engine with performance issues or idling issues because regardless of what else you do, everything hinges around the basic health of the engine's internals.

Once you rule that out, it's just a matter of diagnostics and common sense.

[Fox]

If it has a PAIR system on it that could be a potential vacuum leak. It's non-essential smog device stuff that really only serves to add to the maze of tubes running all over. More stuff to go wrong. Brent made a video showing how to remove all that stuff.

www.youtube.com/watch?v=ubUosWJUhM4

On some models there's also an evap. canister hooked to the gas tank vent that has a vacuum line. That's another thing that can be deleted if you so choose. Brent prolly made a vid for that too but I cant find it.

[90GTVert]

Sorry, no vids on the evaporative emissions stuff. Never owned or worked on a scoot with that yet.

[ray]

Alright, I will run the lines in the next few days and see what I find. I was thinking about throwing all of that mess out and replacing them at some point anyways. I will check the compression as well. Thanks for the suggestions.

[Fox]

I remember an old thread about the same symptom and it turned out the intake manifold was cracked where the clamp wraps around it on the bottom side so it went unnoticed for a while.


If you had the intake boot off then the O ring may have fallen off and you didn't notice it. Just throwing out ideas..

[tiny]

Sounds like an air leak around the airbox or manifold. But also check the fuel lines for a blockage or debris slowing down the fuel flow.

[rocketdog]

I'd say a vacuum leak or it's running too lean at idle. Could be the valve lash. Try the old carb. cleaner trick. Spray carb. cleaner at the carb. to manifold connection, the manifold to engine connection, and any vacuum hoses leading to the intake. If it picks up speed you have a leak.

If you get nothing, try richening the slow speed jet with the mixture screw. Counter clockwise is richer. A 1/4 turn at a time and see if it picks up idle speed.

If all of that fails it might be the valves are too tight. Look into Brents how too's to see how to solve that problem.

[ray]

I checked a few things, but the weather here is kind of crazy right now. (70 today and snow tomorrow evening)

I checked with a propane torch with the engine running and no vacuum leaks were detected.

I did remove the valve cover and discovered one of the bolts was loose, which lead me to discover that two of the bolts are shorter than the others. I put it back together and assumed the two shorter bolts, went to the snorkel part of the pair system. I am assuming this was correct?

After that I started up the scooter and the sound changed from a smooth hum sound to a more pop-pop sound. The idle was consistent now and I took it for a quick ride around the block and it seemed to have a bit more power.

Even with the consistent idle, it still wanted to die on occasion. I still need to do a compression test on it.

I also noticed some air coming from the front of the engine? I am not sure if this is normal, I thought it was from fan off the drive belt, but I didn't feel much air coming from that area. I can't seem to figure out where it's coming from, it's on the drive belt side of the engine.

[Fox]

To make it stop wanting to die, you can try adjusting the idle/idle mixture according to the info here:
49ccscoot.proboards.com/thread/429/idle-mixture-adjustment

The pop pop sound isn't a good thing. There should be a gasket on the Pair snorkel flange you may have let fall to the ground.

The air you feel is most likely coming from the cooling fan on the dipstick side of the engine. Air gets sucked in on that side by the fan and is blown around the cylinder inside the black plastic shroud that covers the cylinder part of the engine. That cools off the cylinder. It escapes through an outlet vent on the lower left hand side of the shroud.

The arrows in these pics indicate the air flow for cooling:




This pic shows the outlet vent holes for the air to escape right next to the exhaust pipe:



This shows how the engine looks without the shrouds. The arrow is the spark plug location.

[ray]

Yeah, that gasket actually came off so I made to put it back in place.

Ahh that makes sense on the air flow. It was confusing me to where it was coming from.