Bike stalls and backfires

Hi, stupid question perhaps, but I couldn’t find the answer searching:

My bike started stalling when I come to a stop quickly (but at no other time) a couple weeks ago, especially in the first 5 minutes of running. I assumed the mix was too lean, and asked the mechanic, who turned something under the seat. I assume he was setting it richer.

Now it doesn’t stall as much, but still does sometimes. I’d assume it needs to be turned a click richer, except that it now also backfires now and then, which implies to me (but what do I know?) that it’s also too rich.

It can’t be too lean AND too rich, right? So I assume it’s something else. Any advice? Thanks in advance!

It might help if you told us what kind of bike it is? :wink:

He heh…

Kymco 125 cc scooter.

How’s the idle speed when it’s warm? I’m guessing your idle speed was a little low. The automatic choke cuts out pretty quickly after start up, so if the idle speed screw is on the low side then it could stall once the clutch lets the engine go to idle… Could also be that the choke jet is dirty. It’s pretty easy to get the airbox off and give all the carb orifices a good blast with carb cleaner or compressed air.

It’s common for scooters to stall when stopping during cooler weather, and withing the first 5 minutes of operation. A good way to adjust the carb properly is to go to a shop that has the emissions tester…this way they can see if your carb is set within it’s parameters…you’ll see right away if you have a carb problem once they have it running in the emissions test. Then the air can also be gauged and adjusted.


Great, thanks! Now I just have to figure out how to say that in Chinese, and I’m all set! :smiley:

I wonder if a new spark plug wouldn’t solve the problem.

How often should one change those? (insert sheepish yokel grin)

@ around $100nt, it’s not actually a bad suggestion by Toasty.

Oh, I completely agree; much easier than finding a place with an emissions tester. But that wasn’t the question… :wink:

Dunno, to tell the truth. My bike recently hit 8000kms and was starting to stall alot and run crappy when not fully warmed up. It dawned on me that I had never changed the plug and this might be why it was not firing properly. I had it taken out and it looked nasty. The new plug solved the problems.

It’s not that hard to find a shop with an exhaust sniffer. They get a decent kickback from the government for doing emissions testing, so it’s a worthwhile investment. The real problem is that noone has yet figured out that it also comes in very handy as a diagnostic tool.

A bad spark plug will most likely cause stalling only once it’s really hot so I’d rule that out. It’s cheap though and should be replaced periodically. I’d say about twice yearly.

Kymco 125cc have an anti-pollution system on them and it usually breaks down within 5 years or sooner causing backfires when decelerating. The parts are expensive and mechanics never replace them because the bike will run the same without it.

If your bike backfires when decelerating, I suggest you disable the anti-pollution. Often the kymcos have an access panel in front of the seat where you put your feet. If you open that, you will see a big hose 3/4" diameter. Unplug it and stuff your old spark plug into it, it’s the perfect size and it plugs the hose permanently. Your anti-pollution is disabled and the backfire issue should be solved.

If the bike backfires when accelerating and stalls when you stop quickly, I suspect your CDI could be faulty. Thats about 800nt installed on a kymco.

DO NOT let anyone blow pressured air into your carb in an attempt to clean the air orifices! These carbs have a thin rubber membrane and forced air will ruin your carb. If a mechanic adjusted your carb and the problem remains, I think it’s either the anti-pollution system or a weak spark(CDI).

Many shops will have a spare CDI they can install for a test drive. I would do that first and see how the bike runs.

Good luck


DO NOT let anyone blow pressured air into your carb in an attempt to clean the air orifices! [/quote]A quick squirt, even into the diaphragm chamber, won’t hurt as long as you don’t try and plug the hole and pressurize the chamber. Not unless the diaphragm is old and about to fail anyway. In DB’s case he only needs to clear out the idle system, choke circuit and emulsion tube passages anyway. Those are the orifices at the bottom and side of the main bore. There’s nothing in there that compressed air will damage. Leave the top one alone.
You are right, people can get overzealous with the air gun, and carb cleaner is a much safer option.

[quote=“redwagon”]people can get overzealous with the air gun, and carb cleaner is a much safer option.[/quote] Exactly. Carb cleaner is safe but many mechanics use air presure as well. It often ruins the carb, especially if the membrane is older and the rubber is dried up.

Your bike could have two problems I think. With increased air the bike started backfiring so I still suspect the anti-pollution to be faulty. As for stalling, it’s either electrical or a carb issue.

Here’s another thing that could be looked at which many mechanics rarely inspect. If a air hose attached to the carb has a leak on it,(damaged hose) the bike will tend to stall easily and/or the idle speed will vary. Sometimes higher, sometimes lower to the point of stalling. Something worth checking since hoses are very cheap and quick to replace.


Well, the bike’s 1.5 years old and I’ve never changed it. I’ll just do that first. The problem’s minor anyway.

Here’s our “how to say it to the repairman in Chinese” thread: … 824#325824
So it’s "qĭng bāng wo3 jian3ch

A loose exhaust connection to the engine can cause backfiring.

Someone mentioned it’s normal for scooters to stall at a stop during cold weather…if not warmed up for 3 min…maybe. But a well maintained scooter shouldn’t stall…period.

[quote=“Mordeth”]A loose exhaust connection to the engine can cause backfiring.
[/quote]I was thinking about that. If you had the muffler removed for things like rear brakes maintenance or replacing the tire, the muffler can cause backfires. Replacing the seal at a shop would be max. 100nt.

My BWS had the same symptoms as your bike DB when the CDI went bad. If I went fast and suddenly decreased the throttle to a desired coasting speed, the bike would misfire and BOOM! The bike would idle perfectly but often would just stall as if I turned they key off. No hesitation, no stuttering, it would just die. Press the start button and it would restart immediately.

Made in Taiwan CDI’s have a very variable life span. From not working right to begin with, to lasting as long as Japanese parts. You can buy a new one, bring it back and no one will argue with you…They’ll echange it without a question.

Best is to change your spark plug first and see what happens. You can tell how your bike is doing by looking at the spark plug. If it’s really bad, that just might be your problem. 18 months is enough to stuff it up probably. It’s bad for your engine too btw.


That may be dead on. The same day I took it in to have him turn it richer, I also had him replace the rear tire, and he of course pulled the muffler. That’s when the backfires started.

Exact match for my conditions.

Thanks! I’ll have him check the spark plug, and check the muffler connection at the same time. I’ll ask about the CDI too if I can figure out what that is in Chinese, he heh… :help: [/quote]

Popping in the exhaust on the overrun is caused by one of two things. Rich idle mixture + leak in the exhaust OR very lean idle condition and high EGTs.