Fuel Screw turn out

Hey, I just did some carb modifications wiht new jets and needle - EMM needle, on the 4th clip (4th down from the flat end). Put in a 38 pilot jet (PJ), a 70 pilot air jet (PAJ), and a 158 or 160 main jet (MJ). I have removed snorkel under hte tank and removed both baffels onteh muffler.

I'm wondering how I should set the fuel screw and how the fuel screw effects the carb. (ie) how many turns out should I go and what does additional turns out do to the carb?

I ride in the warmer cliamtes from 0-4000 feet.


The fuel screw adjusts the fuel/air misture from say

0-1/4 throttle. Turning it in (clockwise) reduces the

ammount of fuel in the air/fuel mixture. Turning it

out (counter clockwise) increases the ammount of fuel

in the air/fuel mixture.

Lots of backfiring is a sign you need to adjust your

fuel screw. Also the bike won't run as crisp if the

fuel screw isn't adjusted right.

If you go significantly up in altitude their would be

less air, so you would need to turn it in (clockwise)

to reduce the fuel to compensate for the less air. If

you go down in altitude you would need to do the


In the March edition of Motocross Action on pg. 162

there is a great article on the fuel screw for 4


Little summarized snippet that I found interesting..

"Once the bike is warmed up bring idle up to around

1800 rpms using the idle adjustment screw. With the

bike running at a steady idle turn the fuel screw in

using a adjustment tool slowly in until the rpm drops

and nearly dies. Now, slowly turn the screw back out.

You will hear the engine rpm begin to speed up and the

exhaust note will become crisper. Stop turning the

fuel screw the exact moment when the engine hits peak

rpm (You are now dialed in). When you reach that point

the idle won't become faster the more you turn the

fuel mixture screw out. If you decide to keep turning

the screw, the rpm's will stay up but the exhaust note

will become dull, flat and lumpy.(Bad) Train your ear

to hear what peak rpm sounds like and crispness of the

exhaust note."

If I had to guess I'd say one turn out from closed. The setting will be unique to you though.


Thanks Matt. That is good info. I'm going to print out your reply and give it a try.

Anyone know where I can get one of those fancy fuel screws that don't need a screwdriver?

Good info.

I've been doing it slightly different.

By reading this I think my setting might be slightly leaner than this way shows. (I split the difference between the 2 settings) I can't help but wonder if it can be just a bit better, so I'll be checking this technique out over the weekend.

It's not popping so I know I'm close but....


Here ya go Billywizz-




Good point about setting it between the two points. I do it that way too. I think either way is good.


I think either way is good.

Me too. With it being so easy to get to it's just something fun and easy to compare. Once the settings are marked it should be fun camparing them back to back.

I actually think some of my buds will want a piece of this test. If they do I'll do a blind test with them and have them do it for me. That'll add a little game to the weekend fun.

I'll let you know.

I did the same carb mods just this weekend and was wondering the same. Will try that test mentioned above. Please keep us posted as to what you find.



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