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sCvHeaVenS

Possible Jetting Issue - Popping On decelerration (Grayracer help lol)

6 posts in this topic

Ok, for starters I have searched through a ton of threads and this is what I have questions on, I am aware of hot start lever / exhaust leak checking.....

I have a 04' YZ450F and whenever I get on it hard it and let off the throttle completely it backfires / afterlites w.e. you want to call it (flames out muffler, 2-3 "pops").

Starts up one kick, idles fine, and has good throttle response. It seems to only happen when im on it hard - throttle 3/4 to all the way open, then let all the way off.

Now I've read that its normal if its not excessivley doing so and thats the big question. Is this condition considered excessive if it does it every time im on it hard, but it doesnt if I dont open it up all the way before I let off ?

It has stock jetting, the previous owner put a FMF pipe on it (didnt change jetting) and I have only rode the bike in cooler weather (had some popping, definately less... but I wasnt comfortable on the bike so I was never on it full throttle like I am sometimes now).

The only conditions that have changed are me running 87 (all I had) instead of 93, and the fact that its about 15 degrees warmer than what I have been riding in.

I would like to also get some clarification on the 4S jetting... some say that running lean can start melting things, others say its almost impossible b/c 4S will run so bad with lean conditions. The bike isnt puking any coolant, wouldnt it be doing this if it was running hot enough to do damage b/c of lean condition?

Thanks!

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Popping on deceleration is indicative of a lean condition, more than likely your pilot circuit (jet) from my experience. I would expect the same for you..

#1) You're riding in cooler than normal tepms - air is more dense & leaning your air/fuel mixture ratio.

#2) The bike has an aftermarket pipe on it, but hasn't been jetted for it.

When you "let off the throttle completely" the engine is being fed fuel via your pilot circuit, and like I said popping on decel indicates a lean fuel condition. I'd start by going up 1 size on your pilot jet, then set your fuel screw :

http://www.thumperta...ircuit-setting/

Try that out, I'd be very surprised if that doesn't solve your problem. If it doesn't, then I'd suspect a cracked header, or air making it's way into the intake tract where it shouldn't be. While you're putting a bigger pilot in, I'd also go up 1 size on the main and raise your needle by 1 position to help with the FMF pipe.

Edited by crf450319

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Popping on deceleration is indicative of a lean condition, more than likely your pilot circuit (jet) from my experience. I would expect the same for you..

#1) You're riding in cooler than normal tepms - air is more dense & leaning your air/fuel mixture ratio.

#2) The bike has an aftermarket pipe on it, but hasn't been jetted for it.

When you "let off the throttle completely" the engine is being fed fuel via your pilot circuit, and like I said popping on decel indicates a lean fuel condition. I'd start by going up 1 size on your pilot jet, then set your fuel screw :

http://www.thumperta...ircuit-setting/

Try that out, I'd be very surprised if that doesn't solve your problem. If it doesn't, then I'd suspect a cracked header, or air making it's way into the intake tract where it shouldn't be. While you're putting a bigger pilot in, I'd also go up 1 size on the main and raise your needle by 1 position to help with the FMF pipe.

Im riding in warmer weather @ sea level. So when they say yz450f's all have the pop as long as its not excessive? What is "not" excessive amount of popping?

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Normal would be when it pops a little (1-3 pops) 60-75% of the time. They are more prone to it if you are at full throttle and then chop it suddenly, so that's normal, too.

Aftermarket exhausts do not automatically mean the bike needs to be rejetted. Some do and some don't need it, and some of them actually cause the bike to run rich, rather than lean, as everyone seems to assume. That, and the fact that the '04 was set up reasonably well as delivered. Look into the link posted above to verify your pilot jet setting, then read this one:

http://www.thumperta...699#post2881699

The fact that the bike starts well most of the time is an indication that the pilot at least is not far off, if at all, but it bears looking at.

In all cases where one is trying to reduce decel popping, one first needs to be sure that there is not a significant air leak in the exhaust system. Trying to jet out a pop from a bike with a leaky exhaust will net you a bike that runs too rich and pops anyway, so check that first, then set your jetting up according to the information you've been given, then check for leaks again if it still acts up.

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Im riding in warmer weather @ sea level. So when they say yz450f's all have the pop as long as its not excessive? What is "not" excessive amount of popping?

Oh, my bad.. I thought you were riding in colder than normal temps. Watch this video, it's the only video of "excessive" popping I could find :

Warmer temps or not, I'd still try the bigger pilot if the popping is excessive.. I've dealt with the lean pilot circuit/popping/hanging idle issue with my '09's and the jetting changes solved the problem. I don't have any experience with the '04's though and don't know what to expect with them.

Play around, look for leaks as grayracer said if you can't find any.. move on.

Edited by crf450319

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Normal would be when it pops a little (1-3 pops) 60-75% of the time. They are more prone to it if you are at full throttle and then chop it suddenly, so that's normal, too.

Aftermarket exhausts do not automatically mean the bike needs to be rejetted. Some do and some don't need it, and some of them actually cause the bike to run rich, rather than lean, as everyone seems to assume. That, and the fact that the '04 was set up reasonably well as delivered. Look into the link posted above to verify your pilot jet setting, then read this one:

http://www.thumperta...699#post2881699

The fact that the bike starts well most of the time is an indication that the pilot at least is not far off, if at all, but it bears looking at.

In all cases where one is trying to reduce decel popping, one first needs to be sure that there is not a significant air leak in the exhaust system. Trying to jet out a pop from a bike with a leaky exhaust will net you a bike that runs too rich and pops anyway, so check that first, then set your jetting up according to the information you've been given, then check for leaks again if it still acts up.

So to me it sounds normal, i.e. it doesnt really pop unless im on full throttle and pop, then it does it anyways. I will check for exhaust leaks and give it a good ride this weekend and see how it does... I didnt really notice it popping until full throttle and chopping... Ill do some test with full throttle and slowly letting off / half throttle and chopping.

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