yz 400 popping on decel


13 replies to this topic
  • ozonefirefighter

Posted February 24, 2009 - 05:50 PM

#1

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

  • grayracer513

Posted February 24, 2009 - 08:19 PM

#2

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.thumperta...599#post3710599

  • ozonefirefighter

Posted February 25, 2009 - 08:18 AM

#3

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

  • DRZ04

Posted February 25, 2009 - 08:44 PM

#4

Fairly common if you are jetted slightly lean.

  • grayracer513

Posted February 25, 2009 - 10:14 PM

#5

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.

  • travisn

Posted February 27, 2009 - 06:37 PM

#6

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.thumperta...599#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...

  • ozonefirefighter

Posted February 28, 2009 - 10:28 AM

#7

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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  • travisn

Posted February 28, 2009 - 10:44 AM

#8

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

  • ozonefirefighter

Posted February 28, 2009 - 11:06 AM

#9

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.

  • grayracer513

Posted February 28, 2009 - 02:03 PM

#10

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.

  • Humai

Posted February 28, 2009 - 02:45 PM

#11

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.

  • grayracer513

Posted February 28, 2009 - 03:01 PM

#12

2) Airleak at the exhaust port/header junction

The leak can and often does occur at the header to mid pipe joint, also.

  • ozonefirefighter

Posted February 28, 2009 - 06:36 PM

#13

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

  • 400f mech

Posted March 01, 2009 - 06:24 PM

#14

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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