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

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 !!!

.... 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.

thats good to hear!!

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?

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)

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:

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.

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...

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:

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.

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