slow letdown to idle? sticking but not cables??? HELP!!!


29 replies to this topic
  • cocomunga11

Posted February 27, 2006 - 12:48 PM

#21

Well i bought the bike from my dad... he didnt THINK that it did this, BUT he didnt go ride indoor arena cross when its 30 degrees outside either. IT has not been very long since i did the valves and from my experience I think this might be a jetting issue in the COLD. ( the more i thikn about this the more i think that it wasent this bad on the warmer days)since the carb is already off and the jet is easy to get to Im gonna try that first.... if that fails then all signs point to the valves (which will suck because I will be adjusting the valves every 3 rides from here on out ) :thumbsup: as soon as the jet gets here i will let everyone know !!!

  • grayracer513

Posted February 27, 2006 - 01:14 PM

#22

.... if that fails then all signs point to the valves (which will suck because I will be adjusting the valves every 3 rides from here on out )

If it is a YZ400, then it will have stainless steel valves, which do not wear in the same way that Ti valves do. They are hardened, but the hardening is deeper, and the underlying steel is naturally hard than Ti is, so they don't ordinarily start wearing rapidly once they wear a few thousandths.

  • cocomunga11

Posted February 27, 2006 - 01:19 PM

#23

thats good to hear!!

  • DaveJ

Posted February 27, 2006 - 05:59 PM

#24

Quite the contrary, in fact. Leaking intake valves reduce manifold vacuum by allowing positive pressure from the preceding compression, power, and exhaust strokes into the intake port, which leads to a weaker intake pulse, and less gas delivered per intake stroke, and so, a leaner mixture.


Hmm....interesting.

:thumbsup:

So I assume that the mixture becomes lean given that the air from the combustion chamber is recycled back into the intake maniford, which is then reintroduced at the next intake cycle. Yes?

But we still don't have an answer as to why the engine will idle faster. I mean, I can see it running lean due to valve matters, which should stall it out or just cause it to be lean, but something has to be going on with the carb in order for the engine to get the additional RPMs. In words, how is a lean condition producing more power? Or how is more power produced without additional fuel?

  • cocomunga11

Posted February 27, 2006 - 07:56 PM

#25

Hmm....interesting.

:thumbsup:

So I assume that the mixture becomes lean given that the air from the combustion chamber is recycled back into the intake maniford, which is then reintroduced at the next intake cycle. Yes?

But we still don't have an answer as to why the engine will idle faster. I mean, I can see it running lean due to valve matters, which should stall it out or just cause it to be lean, but something has to be going on with the carb in order for the engine to get the additional RPMs. In words, how is a lean condition producing more power? Or how is more power produced without additional fuel?


Must be that the extra air is what is causing the throttel to " stay on " there IS fuel going in even if the throttle is closed ... so the extra air makes it lean out from there ( a little leaner is higher RPM no matter where in the range)

Visit the ThumperTalk Store for the lowest prices on motorcycle / ATV parts and accessories - Guaranteed
  • One Louder

Posted April 01, 2006 - 11:16 PM

#26

I have 98 yz400 and I just got new cables, a 180 main, and a 48 pilot and my bike started doing the same thing. Harder to start, dying and variable idle. on the plus side it it was a lot stronger. Is a bigger pilot # jet Richer or leaner? My old jet was a 45. Any thaughts? :thumbsup: :thumbsup:

  • Fastest1

Posted April 02, 2006 - 09:58 AM

#27

Hmm....interesting.

:thumbsup:

So I assume that the mixture becomes lean given that the air from the combustion chamber is recycled back into the intake maniford, which is then reintroduced at the next intake cycle. Yes?

But we still don't have an answer as to why the engine will idle faster. I mean, I can see it running lean due to valve matters, which should stall it out or just cause it to be lean, but something has to be going on with the carb in order for the engine to get the additional RPMs. In words, how is a lean condition producing more power? Or how is more power produced without additional fuel?


What I interpreted from GR's statement was that the air was being forced back up the intake tract and therefore reducing vacuum if only momentarily. Lean does cause a higher idle and it develops more power. I am pretty sure that is why so many people seize motors otherwise they would just foul plugs if richer was the way to power. More air and fuel increase power not just fuel.

  • cocomunga11

Posted April 02, 2006 - 11:36 AM

#28

a 48 is richer. i am trying to get one for my bike but no one can give me the right one !!! :thumbsup: But you should no longer be having an idle problem? is your bike stock? the only reason i am needing a bigger piolt jet is because mines ported and polished with the 450 cams and 450 exhaust...

  • One Louder

Posted April 03, 2006 - 08:20 AM

#29

Oh by the way.... the part #'s for the sarter jet and the pilot jet are switched in all of the scematics , perts lists, and in the manual. It is a major error. I just got one from my local yamaha dealer. we ordered a "starter jet" but I got my size 48 air jet. hope this helps. I'm actually going to go back to my 45 to see if that helps my hanging idle problem :thumbsup:

  • wpsnowman

Posted April 03, 2006 - 08:34 AM

#30

When you run out of things to blame it on, check your valve clearances. This is a result of a too lean idle mix, and tight intakes are one of the causes.


I totally agree, Also an air leak would cause it to lean out. But if it was fine before and now its not I would not lean towards replacing jets. If the jet is clean and worked good before why change or resize. More than likely your valves need to be adj.





Related Content

Forums
Photo

yz250x vs 450 4t comparisons (preferably desert riding) needed by JakeNorthrupYZ450F


Dirt Bike   Make / Model Specific   Yamaha   Yamaha 2-Stroke
  • 9 replies
Forums
Photo

Michigan Motocross Tires by 288yz450


Dirt Bike   Dirt Bike Regional Discussion   North
  • 1 reply
Forums
Photo

First Hare scramble tips by dhend8


Dirt Bike   General Dirt Bike Forums   General Dirt Bike Discussion
  • Hot  33 replies
Forums
Photo

Megabomb Fitment by 288yz450


Dirt Bike   Make / Model Specific   Yamaha   YZ 400/426/450
  • 1 reply
Forums
Photo

Is it worth it? (Cam and High compression piston) by macgi77


Dirt Bike   Make / Model Specific   Honda   CRF 150/230 F/L
  • Hot  36 replies
 
x

Join Our Community!

Even if you don't want to post, registered members get access to tools that make finding & following the good stuff easier.

If you enjoyed reading about "" here in the ThumperTalk archive, you'll LOVE our community. Come join ThumperTalk today!

The views and opinions expressed on this page are strictly those of the author, and have not been reviewed or approved by ThumperTalk.