Backfires when de-accelerating?

Got a european 426. Noticed that after hard acceleration when slowing down can backfire. Is this ok or an indication of too lean? If I need to make it richer, what do I need to do? Thanks for your help. Rich

p.s. Airbox lid has been removed.

[ April 22, 2002: Message edited by: Follett ]

It also happens when I have stalled and trying to re-start. If I pull the compressor in and kick through a few cycles then on the 3-4 it bangs!

[ April 22, 2002: Message edited by: Follett ]

It depends on how much. There is a tendency to pop some on decel. In the US, late model WRs have a 'air cut valve' or 'deceleration enrichening circuit' that is supposed to reduce this. If you have and ACV and haven't disabled it and have hardly any at all, I would consider richening it up some. Without the ACV, it could pop more before worrying about it.


hey man, i have a canadian wr 426 which i believe is exacly the same. It has backfired since the day i got it and i haven't been able to figure it out. sometimes it backfires alot and sometimes not at all? it only does it with the throtle closed, so it has to be in the idle circuit, which is realy only the mixture fuel screw on the bottom on the carb. i have screwed it in and out with no luck. if you ever figure it out, please let me know because this has stumped me for some time now. thanx matt.

p.s. it hasn't done any damage that i know of yet, but i sure feel like a wanker when i am riding down the street and my bike gives off a shotgun blast and everybody is looking at me.

if you've got standard jetting it's probably a sign that you're too rich and all that unburnt mixture gets in that orange hot exhaust and you get a second combustion. this in turn turns the pipe even orangier!! and so on.

turn your pilot screw in 1/2 turn. alternatively go stand at the whitehouse and spook a few of those grey suits. now that would be funny!


Everyone but Taffy ignore this. This is not information, it is a question. It would probably just reinforce wrong opinions if read.


I know the jetting is close to stock so this isn't the case here but doesn't the needle straight affect idle and overlap the pilot ciruit/fuel screw?

Is his backfire, which I assume is through the pipe, the same as the popping normally associated with lean? I historically associate loud backfires with ignition/plug problems. If it was too rich in the cylinder, wouldn't the partially burned exhaust gasses be even richer in the exhaust pipe where the pressures and temperatures are far lower? Where would the extra air come from to ignite it there? I may have just answered my own question, the loose fitting manifold to header. But even so, it's easier for me to picture unignited A/F (from ignition/plug problems) building up in exhaust causing backfires (not just pops) than once burned gasses.

He mentioned the fuel screw but wouldn't the pilot jet be equally associated?


Originally posted by Mark Cantrell:

[QB]Everyone but Taffy ignore this. This is not information, it is a question. It would probably just reinforce wrong opinions if read.

This is why we have personal messages. LOL

Point taken.



that's a hard one to answer because i only go on experience this time and not the mixture of knowledge and experience i can usually draw upon. like you i have to guess what's happening.

whenever a roadbike or roadracer pops back under decel you could say it was leaking air in at one of the joints. therefore the fresh oxygen was helping burn the unburnt fuel. igniting it appears to be the least of ANY bikes problems.

if you get a rich mixture it gives a loud bang nearer the rear tailpipe where as a lean mixture on decel seems to be up near the header and doesn't reach the exit. agree?

i do know that the correct mixture has a temp of 800f whilst a lean mixture shoots up to 1300f. a rich mixture is i think about 6-700f.

so it must be the sheer heat that causes the back pop on the lean engine.

the combustion chamber is screaming for A/F but all it gets is a very small amount of a relatively rich mixture. after all you've seen that slide cutaway and you'll note that there is a 1.5mm high arch only 5mm wide. not exactly going to feed the 5,000 is it.

what's the compression ratio on closed throttle? 1 to 1? 2 to 1? does it even squeeze it up enough to burn it at all. a lot of the heat is caused by the sheer compression process itself. look at deisels for instance.

i would normally back myself heavily but i'm just surmising here on this one.

like i said before your in a narrow gully with unburnt fuel on one side and massive temperatures on the other. doesn't take much to switch from one to the other.

i only say adjust the fuel screw because it is all you can do without buying and changing. your right it is the PC that needs sorting. but if it's standard it really shouldn't need much should it.

the needle straight is the first yard and after that it's pilot circuit (PC). i've always held this view, i suppose you could say it's up to 1/4 throttle but that's a little unfair. that's one thing i don't agree with on the PB write up.

as previously mentioned if you ride flat out and kick down to 4th coming up to a bend. if at first it softly pops it's the needle that's lean and if it does it right on top of the corner it's the PC.



Thank you very much.

A little curious note for a different application. I used to fly quite a bit. In small planes, we had a carb or no feedback fuel injector controlled from a knob on the panel. The proper procedure for almost all is to run full rich on takeoff and climbout (when little air flow for cooling) and at altitude lean out. There are two strategies. With Exhaust Gas Temperature guages installed, lean until the temperature peaks, then richen 75 degrees C from peak temp. The EGTs were calibrated in 25 degree C increments but didn't report an absolute value. Without EGTs, lean until it ran rough (literally began popping and loosing power, happened at exactly the same time, i.e., no loss of power until popping) and enrichen 1/2 turn.

At altitudes over a few thousand feet, the enrichening process would take a long time to register on the gauges, they would be dead until several turns out. Then they would rise very slowly for a while. This was due to the fact there was raw unburned fuel simply being wasted for its cooling value. After it got past the raw fuel stage, the guage would climb more quickly as we were in the 10:1 or so range, leaning up to the 14:1 or 15:1 target. The power did not change until the point the guage started to rise more quickly and then had some more power (guessing 5% or so) until right at the peak temperature.

Some planes, notably the Piper Malibu, relatively high performance for a modern piston single, the leaning procedure was to run 50 degrees lean of peak, very hot. Most pilots were unwilling.

Most planes had a best power and best economy. The best power was slightly rich (1/4 turn or another 25-50 degrees rich) of best economy. This always seemed strange to me because the power felt noticably higher toward lean. May have had to do with being able to handle more throttle cooler.

Anyway, I know this is way off topic, but even with fuel, engine, altitude, etc., it still may have some pertinence.


My oz 01 426 was extremely rich standard causing the same problem, ended up dropping the needle to clip #3 and standard mj165

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