Popping, misfiring, stalling

I posted a couple weeks ago about this but am still having problems.

I've got a '00 426 which suddenly started misfiring badly about 5 weeks ago, especially noticable when chopping the throttle. Flames blow out the exhaust. It tends to stall very easily when the throttle is closed. The plug has heavy, black, dry carbon and will foul out after about 20 minutes. Everything was fine before this including the jetting. Here's a list of what I've tried:

Carb completely disassembled and cleaned

Stock jetting, float height OK

-Tried going out on the pilot screw

-Tried larger pilot jet

Clean air filter

All resistances checked OK per owners manual

New secondary coil

New stator

Tried '01 CDI

Disconnected TPS

Disconnected kill switch

Disconnected neutral switch

Removed wiring harness and inspected

Cleaned grounds

What the heck is left???? Am I missing something obvious? The local Yamaha dealer is as stumped as I am.

[ November 13, 2001: Message edited by: motobob911 ]

It sounds like you've got the correct air/fuel mixture and you've verified spark, other things that come to mind are the timing, verifying that your exhaust isn't plugged somehow, and the quality of your fuel. Probably not the solution, but some more things to rule out at least.


Here are some possibilities.

1. The accleration pump diaphram is not functioning properly. Sometimes when the diapham gets old or is cleaned with carb clean, it causes fuel to get pumped into the engine after the throttle is closed. Long story.

Remove the carb or airbox, and watch and monitor the result of the fuel stream.

2. The boots around the choke and/or hot start valve are torn, or the valves are not seated against the carb body. Check.

3. The carb has a leak in the idle circuit. If you took the the two carb halves apart, (and I don't mean the float bowl), then for sure this is your problem. Write me back and I'll tell you how to fix it.

If none of the above, run these tests.

Warm the bike up until it's hot, then pull open the choke. Let me know what happens.

Warm the bike up, then rev the engine. Note how quickly the bike returns to idle or dies out.


If you have done all of the above, correctly, I would have to guess that you have a leak in the carb.

Pull open the choke when the engine is warm. If the engine races, your leak is most likely between the two carb bodies.

Just to be safe, check to make sure the choke and hot start boots are not cracked, and that both of these valves are tight.

Let me know what you find.


Thanks for the quick responses!


I have not checked the timing with the new stator.

I checked the exhaust and it's not plugged. The gas was drained and replaced.


1. Before I put the carb back on, I did the BK mod. Fuel stream from nozzle just misses the slide, stream lasts approx. .4 sec.

2. I pulled both the hot start and choke knobs checking for any problems. The boots are OK. The valves are seated against the body.

3. I didn't split the two carb halves. (I guess I didn't take the carb COMPLETELY apart.) I did notice two of the screws holding the two halves together protruded into the float bowl area. I was thinking of covering them with epoxy. What do you think?

Could there be an internal problem with the carb? The local dealer is going to lend me a carb from an '01 to see if it clears things up. The other thing he mentioned was the possibility of a burnt exhaust valve, but I'd think there would be a loss of compression which doesn't appear to be the case.

If you are going to get a loaner carb, that's the best way to eliminate this possibility. I don't recommend taking it apart any further until you know otherwise.

And no need to cover those screws. They have a seal around them between the two halves.

And I left out the possibility of a burnt valve or a cam timing issue. Just plain forgot. A compession check would be your best source for that.

By the way, when and how did this problem develop?



I first noticed occasional popping on decel one day during the week. It was colder out, about 55 degrees. Went out 1/8 turn on the screw. Practice Sunday it seemed to run fine, temp was about 35 degrees. Five minutes into the first moto (about 45 degrees), terrible backfiring and popping began, along with a loss in power.

By the way, I replaced the secondary coil because the dealer said it was definitely bad. The stator was replaced because they said the output was low and was probably the problem if replacing the coil didn't fix it. It is running slightly better after replacing those two components, but not much.


Sorry, I jumped to conclusions.

It's just that I seldom comes across electrical failures. It's the mechanicals of these bikes that are less reliable, and these carbs can be problematic as well.


So with this information, you're going to have to eliminate a few factors. Mainly, a good spark at the right time. If you can't measure coil output, at least confirm a blue spark. If the engine runs worse when warmed up, try this test with the engine...well warmed up.

You're next issue would be that of spark timing and proper advance. Since this is not a variable, let's assume it's correct unless someone installed something incorrectly or the CDI is freaking out. Do make sure you're getting a spark every cycle.

Next on the list is engine compression, or at least an assembled leak down. If you don't have a compression guage, you should have enough of a feel of the kick this bike has to know if you have a hole in the valve, (or whatever). It also should not lose compression over time. So find the compression stroke, let it sit, and see if you then easily move past it.

If all this checks out, it's a simple check to verify that your cams are in time. See the manual for details.

If all this checks out, wait until you can borrow the carb.

Let me know what you find. And I still may be missing something here, so be open to what other may advise.


Motobob, putting epoxy in one hole from the inside

of the float bowl area, cured my 01.

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