why does my yz450f give pops on deceleration

25 replies to this topic
  • yamaboy23

Posted October 14, 2004 - 10:17 AM


a pop thru the carb is a "backfire"
a pop thru the exhaust is a "afterfire"...which usually is a rich condition
but chopping the throttle from a high rpm setting youll notice this even on one that is perfectly jetted
just watch nascars when they enter turns and you see that flame coming out the exhaust ...thats afterfire
its more noticed with after-market exhausts rather than a stocker

  • grayracer513

Posted October 14, 2004 - 07:49 PM


We called it backfiring long before anyone invented the term "afterfire". That's pretty silly semantics. I mean unless you know of someone whose carb backfires on deceleration, that is.

When you think about the aftermarket exhausts, remember that a number of them, like my FMF Ti-4, don't use a mid pipe clamp. And mine pops some most of the time. It's because it leaks air.

  • YZ250F_Rider

Posted October 15, 2004 - 03:22 AM


bet you a nickel that if you turn the fuel screw or go up a min it wont pop.

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  • mxmhero

Posted October 15, 2004 - 01:35 PM


Hey guys found this on Dubach racing web site, thought u would like to see it .

Why your 4-stroke pops on deceleration

The sound your hear comes from left over gas in the head pipe that is igniting.

#1 Why does this happen?
When you close the throttle there is an excess of fuel in the head pipe. As the cold air is drawing in through the end of the pipe it dries the gas to a lean enough mixture that it becomes combustible. This small combustion is was you hear.

#2 Does this effect performance?
No, it is no more than an annoying noise.

#3 What can I do to fix it?
Short of putting a WR style carb on your bike, there are some things you can do to minimize it.

Note: The Yamaha WR style carb has a diaphragm in it that at low intake pressure (when throttle is closed) it opens allowing some extra fuel to bleed into the motor to keep the mixture from getting to lean.

You can richen up the pilot circuit of your carb, however this can have a small effect on the popping and can have an effect on the engine performance.

Check to see if there is an exhaust leak. The closure to the engine the cold air is drawn the bigger the effect of popping.

  • grayracer513

Posted October 15, 2004 - 02:45 PM


The last time I tried that, it blubbered a bit on the transition off the idle circuit into the slide, indicating the pilot circuit was too rich. Didn't entirely get rid of the pop anyway, and I frankly don't care that much.

As I mentioned once before, I can cause my son's bike to do this anytime (it has no decel backfire at all now) by simply loosening his mid pipe clamp. What I did not mention is that mine does not do it with the stock mid pipe (clamped and sealed) and the same carb settings. And please don't anyone try to suggest that the stock exhaust enrichens the F/A ratio at an idle, OK?

  • YZ250F_Rider

Posted October 15, 2004 - 03:29 PM


thats why I suggested a step up on the main. when the pilot is at the edge the main can make the difference.


the popping has to be happening on heavy decel since that high vacumn state is enough to pull from the main as well. If it happens on mild decels it wont help.

The stock pipe doesnt enrich the fuel mix, but due to it's high back pressure it prohibits complete evacuation of the cylinder. It leaves much too much burnt exhaust in there that could be exchanged with fresh fuel air mix with a good pipe. It has the same affect as the egr valve on cars. for cars it's a good thing, but for balls out machines it robs power.

I forgot to add. I have seen a couple guys just wipe the inside of the midpipe with hi-temp silicone. It seals it fine, and is easy to separate when you go to do any work.

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