Jump to content
Sign in to follow this  

YZ250F Pilot jettng seems to small tobe too rich?

Recommended Posts

Specs:

08 YZ250F

fresh head from fastheads with stainless valves and matching springs.

Properly shimed

New timing chain

New wiseco piston and rings

New cylinder works cylinder

New wiseco gaskets

New UNI filter

Hotrods crank

Supposedly has hot cams, but no way to verify other than PO's word and a sticker on the tank

DRD head pipe

DRD can w/spark arrestor

Pilot jet #42

aftermarket fuel screw

91 octane with booster (it's the best I can do around here)

 

I ride high altitude 4300+ feet above sea level (will be doing some riding at 6000-9000 feet asl but none so far)

Temp has been 80-100 degrees and varying humidity. It's monsoon season here in the high deserts of New Mexico, so humidity jumps around a lot.

 

Problem(?):

42 pilot jet, engine doesn't stumble or die till the fuel screw is nearly all the way in

I noticed that the rpms pick up when I pull the hot start while the engine is still cool (haven't had the chance to try it warmed up), and I had to use the hot start to get it started even with the engine cooled down last time I tried.

 

I never rode the bike when I first bought it (aside from a test ride in a parking lot). I rebuilt the engine first thing upon purchase. It came with a 45 pilot jet and and aftermarket fuel screw. Upon completion of the rebuild it was hard to start and ran poorly. I noticed that I could turn the fuel screw almost all the way in before it would die. It seemed to run best at about 1/8-3/8 turn out. I read the pilot jetting guides on this forum and others as well and determined I needed a smaller pilot jet. I swapped out the 45 for a 42 pilot, at this point I also adjusted the exhaust shims(they were just a tad on the tight side) and it seems to run better when cool, but the fuel screw does nearly the same thing, runs best at about 1/2 turn, only dies when it's all the way in.

 

I tried to ride it Sunday, and it did well while it was cool, started second or third kick, but I found that after shutting it off in the middle of the desert, it took me probably 10-15 min to get it restarted. Fortunately I eventually did get i started, and was able to make it back to my truck with no further issues. Now mind you I was only riding it easy (1/2ish throttle, fairly level ground)as I am still breaking in the new rings and cylinder, so I don't know how it will do when I thrash on it.

 

I know that a 42 pilot is generally considered small for a YZ250F with an exhaust, but do you think it may need to go even smaller with my temp and alltitude?

 

Share this post


Link to post
Share on other sites

What size slow air jet?

Confirmed float height?

Allow or brass fuel screw? Some aslloy fuel screws are poorly made and cause a much richer mix.

42 pilot is normal size on a yz. Exhaust makes next to no difference.

Share this post


Link to post
Share on other sites

almost all the way in before it lean misfires isn't bad, you should be about 1 1/4 -1 1/2 out from where it misfires,  would have you in a normal range.

Share this post


Link to post
Share on other sites

As William said, check the slow air jet and float height. Also, pull the fuel screw out and check to make sure the rubber o-ring and washer are in good condition. I recommend going with the R&D Flex fuel screw. It is much better quality than those cheap aluminum ones and makes adjusting on the fly super easy. 

 

If all of the above looks good, you might want to go down one more size to a #40 pilot. Being a 4,300 feet above sea level you will need to lean out the mixture a little bit. 

Share this post


Link to post
Share on other sites

Well, So far have found that it is likely due to operator error. When I removed the pilot jet I realized that it was in fact a 48 not a 42. So I put the 45 back in and will see if it does any better now that the exhaust shims are better. I will also be looking into the R&D fuel screw since I still don't think this one is acting right, and I really like the idea of the easy to reach adjustment knob since my riding can go from 4300' ASL to 9000+' ASL with drastic temp changes in a single ride. Last year on my 400ex I was fine at the base of the mountain and ended up running into a rain/hail storm at the summit, really wish I would have had an adjustable fuel screw (and a hoodie!). It ran like crap once the storm hit and I was at high altitude. I think I'm going to buy one for all my bikes and quads.

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Reply with:

Sign in to follow this  

×
×
  • Create New...