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ozonefirefighter

yz 400 popping on decel

14 posts in this topic

I just re shimed my valves on my yz400. Now when i ride it it pops on decel. Its not bad it just pops twice on decel sometimes maybe 3times. what is this and should i worry about it. The bike runs great otherwise. also When I re installed my timing chain tensioner I dont have a way of torquing it. I adjusted it to where i thought it was before but im just worried about it jumping or something. Or can it be too tight? thanks

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Ok thanks. I was just wondering y it started after re shimming. it wasnt before but it sounds like its popping should be normal the way it is

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Depends on the individual engine. When the pilot is set up correctly on most YZ250F's, for instance, it will just about never pop off on decel. A YZ450, set up by the same parameters and with a perfect, by the book fuel mixture will crackle a little some of the time. It actually makes a good audible indicator of being set up right.

There again, this assumes there are no air leaks in the exhaust.

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The cam chain tensioner the bike came with is entirely self adjusting. If you have a manually adjusted one, it's an aftermarket part, and I personally recommend you replace it with one from Yamaha.

As far as the popping goes:

http://www.thumpertalk.com/forum/showthread.php?p=3710599#post3710599

I think he meant torquing it after installing it, so he is thinking if he didnt tighten it properly it may move, which would effect the tension on the timing...

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Exactly what I was asking about the torquing it. And the popping is really bugging me. The bike runs like a scalded cat but the popping on the decel is whats really bugging me. I havnt messed with the exaust so there shouldnt be any leaks but how do I check for them? I cant hear anything. And the bike ran great no popping ever before the valve work so it wouldnt be the jetting would it? I am just really bugged about it :lol:

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My uneducated guess is you shimmed the exhaust valve too much so it doesnt close all the way so you are hearing combustion on decel?

I really dont know but that might happen. lol

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Well i am just a beginner with mechanics to but the valves were tight to I had to shim down. The had the stock shims in there and they were like a 169 and a 172. since they were tight I had to get smaller ones and since they only sell them in increments of 5 which made both of the stock closer to 170 i shimmed down to 165. So I do not think that is the problem if i am thinking correctly. thanks for the input though.

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Exactly what I was asking about the torquing it. And the popping is really bugging me. The bike runs like a scalded cat but the popping on the decel is whats really bugging me. I havnt messed with the exaust so there shouldnt be any leaks but how do I check for them? I cant hear anything. And the bike ran great no popping ever before the valve work so it wouldnt be the jetting would it? I am just really bugged about it :lol:
The cause of the popping is as I stated, regardless of the work done.

On the question of the tensioner, does your tensioner have a bolt and a lock nut in the center that you must adjust and lock down? If so, replace it with a new stock one. They are spring loaded and self adjusting, and more dependable.

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And the popping is really bugging me. The bike runs like a scalded cat but the popping on the decel is whats really bugging me.

Like the others have said, it's one or both of:

1) Lean idle mixture

2) Airleak at the exhaust port/header junction

I still have my 98 YZ 400F I bought brand new. When new, it popped/backfired like all buggery on decel so I went a size or two richer on the pilot and readjusted the idle mixture and voila - problem solved.

The early YZ 400s were notoriously lean at low throttle - pilot/idle and needle should all be richened a step or two.

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thanks a bunch. I had found those threads when I did my serch I just didnt know how I would have got an exaust leak when it was running fine before and i havnt touched the exaust or carb

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hey, i own a 98400f and i had the same problem, asked my mech and he said it was running to lean. check your hotstart to make sure it is closing properly and try jetting it a little richer

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