top end breaking up after mods

[gimphand]

I have a 400ex, put in a K/n with no lid, bought a full exhaust and the dude gave me a jet too. He said it was 4 sizes bigger and told me how to set it up.. The needle is on the third groove from the top.. Now my mid is awsome the top end breaks up when I have it wide open... Whats the problem? What do I need to do.. I am told that the jet is a 152. Top end is sputtering when wide open..

[Cody_300ex]

Bigger main jet.

[Sjorge450R]

Originally posted by Cody_300ex
Bigger main jet.

yep your draining your bowl out.

[GPracer2500]

Originally posted by Sjorge300EX
yep your draining your bowl out.

:ermm: ....nah. The main doesn't feed the bowl, the bowl feeds the main.

[Vista Cruiser]

Is there a number on the main jet. It sounds to me like you are running too rich, not too lean. You will need a smaller jet, like a 158 or 160 Honda main jet. If it is a dynojet, the numbers are different.

[bwamos]

I agree the main jet is off.

Is it sputtering or bogging down?

If it's sputtering it is Rich
If it is bogging it is Lean.

[phildogg]

prob running lean.. with a full exhaust and no lid might try a bigger jet depending on your location. I'm running a 152 with a hmf s/o and k&n filter.
phil

[Wheelie]

If the main jet is 4 sizes bigger, you have a 158. Sputtering on the top end is too rich. Try a 152 or 155.

[gimphand]

So the jet is a 152. I started messing with the fuel/air screw a bit more and the top end is fine. First gear is breaking up a little now.

[GPracer2500]

Originally posted by gimphand
So the jet is a 152. I started messing with the fuel/air screw a bit more and the top end is fine. First gear is breaking up a little now.

Focus on throttle postions not gears or rpms. The different carb circuits relate to throttle postions only. I see in your first post you made a point to mention WOT, so maybe you know this but I'll continue for the benefit of others.

Sometimes a miss-jetted circuit will show symptoms of being off in one gear or rpm range more easily than in another gear or rpm range but it's still the throttle position that matters. For example, it's very hard to evaluate the main jet in the lower gears because it's hard to hold the throttle wide-open without running out of rpm. Also, high loads will unmask incorrect jetting more easily and it's hard to maintain high loads in the lower gears unless you're climbing a mean hill. That's why when people say "its missing/sputtering/breaking-up on the top end" they are usually dealing with miss-jetted main jet. It's not the fact that they are at high rpm or in a high gear, it's that they are holding it at or near full throttle.

0-1/4 throttle: pilot circuit (pilot jet and fuel screw)
1/8-3/4 throttle: needle clip position
3/4-full throttle: main jet

That is a simplified but useful guide to figuring out which circuit in the carb is not jetted right. There are actually several other carb variables that come into play (slide cuttout, needle taper, and more) but the above is mostly what you need to worry about.

It helps to put marks on the throttle grip that shows what postition you're at. That way you can look an see that, for example, at 0-1/2 throttle and above 3/4 throttle it doesn't miss but from mid throttle to 3/4 it is sputtering. That tells you it's the needle that needs attention.

Where is the throttle at when the "breaking up" occurs?

ETA: I should note that there is some bleed-over between the different circuits. For that reason it is best (for most carb types) to start with the pilot circuit and then do the needle and lastly the main. If you do the pilot last, then richening or leaning that circuit will have some affect on the other circuits that you worked so hard to get right. But the bleed-over between the ciruits doesn't go nearly as far in the other direction--changing the main isn't going to affect your pilot.

[ohsobad_chevy]

Originally posted by Vista Cruiser
Is there a number on the main jet. It sounds to me like you are running too rich, not too lean. You will need a smaller jet, like a 158 or 160 Honda main jet. If it is a dynojet, the numbers are different.

I second that, spitting and sputtering on the top end is a sign of a rich mixture. Pull your plug, Im willing to bet it will be dark black and sooty. Let us know what your plugs look like.

[Vista Cruiser]

If first gear is breaking up, maybe you're just hitting the rev limiter.