FCR - 185 Main and Still Too Lean- WTF?

[Quadzilla]

Hey guys,

I just got my 416 (10.8:1 piston with custon Cometic gaskets to pump the compression up to 11.2:1 or so) back with all the mods below. I started with a 170 main and it was starving for fuel on top. I pulled the plug and it was ash white. I just tried the only larger size that I had, a 185 main. It ran a good bit better (not starving quite as much) and the plug was a extremely light brown/ashy mix (also a little better than with the 170 main). I've never heard of anyone running this large or larger in a FCR 39mm. I did also get a 3 angle vavle job and some significant porting, Hotcomas stage 2, etc. Could this be causing my quad to be so thirsty? I had a can of STP carb cleaner and sprayed it all around the carb and head while it was running thinking that if there was a leak, it would cause the motor to rev a little. Nothing happened which hopefully means that there are no leaks. Oh, I also have a EHS airbox lid and a sparks airfilter setup (larger, Uni-style filter and billet mounting ring). So back to my question, anybody running a main jet this large ir larger in a 39mm FCR? HELP!!!

[the yella yak]

A buddy of mine that I race with has a full ct440 with a 41mm fcr, and was running a 210. He then decided he wanted some more power, so he put a new piston in. It was running bad, and me all the 400exs we race with all were running 160s and 165s in our 41mm fcrs, so we jetted his down to a 165, and it seemed to run good. We did it a few minutes before the races started, and didn't have time to check the plug. About half way through the first race his bike died. Upon further inspection we found that he melted his piston and jug due to running lean. We fixed everything on it and put a 210 in and in ran like a champ.



So to answer your question-a 200-range main jet is not unheard of in a 400. I think a lot of it depends on your headwork. Cuz his headwork is different that all of ours.

Sorry if I rambled on-hope it helped ya.

[cals400ex]

i don't know. that seems large to me. i would think you would notice a loss of top speed with that size of a mainjet in.

[cals400ex]

by the way, if you run a leaner needle (fatter), it is quite possible that you may need a 180 or so. this is what i read anyways.

[Quadzilla]

Yes the needle is fat (OEM needle for YZ400F). I'm also going to further investigate airleaks. Has anyone noticed that even when the carb is wide open, your needle is still totally visiable in the carb body (I still can't see the bottom of the needle. Is this normal? On CV carbs, I think that I usually see the needle retract up enough when wide open that I can see the bottom of it. I checked to make sure that my cable was adjusted all the way and that the slide is going all the way up. Just seems strange that the bottom of the needle is still below the carb throat (it's still in the float bowl area). Thanks again guys!

[Quadzilla]

Bump

[EXDUNER127]

Have you tried adjusting the needle at all, because mine was runnin way lean and i raised the jet size a couple and adj. my needle?

[jimboatv]

I have a 465ex with a 41 mm FCR and the head proted with 1mm over size intake valves. I'm running a 170 main jet. I ran my bike on a Dyno last week it was still a little rich, wide open with the 170.

It made 40 HP and 50 on the nitrous.:cool:

[Quadzilla]

I have the needle set to the 3rd position from the bottom (fairly rich). It is a pretty fat and not extremely tappered needle. Again, is the bottom of the needle is still below the carb throat (it's still in the float bowl area) when the throttle is wide open? Seems strange...

[cals400ex]

i am thinking mine was the same way. i would check really quick but i have it boxed up ready to ship out because i sold it. however, i am almost positive that you still saw the whole needle when the throttle was pinned.

[shamisc]

I had the same prob with my 39fcr. I went all the way to a 210 before it ran right and plug looked good. My tech told me it should never go that high in jet size. We also tried WD-40 around the intake and everything, but no changes in idle. When we pulled the carb off, the boot broke in two instead of the carb pulling out. The boot appeared to be dry rotted and cracked. You could tell in had been cracked for a while. So, I ordered a new boot and it was way too rich. Now I'm back to a 170 and it runs great and great plug color! So, if I were you, I would take a close look at the intake boot.;)

[cals400ex]

yes, they revised the boots in 2001 to help prevent that problem.

[Quadzilla]

I'll look for leaks tonight. Sounds like that is probably my problem... Thanks guys!