Throttle Jerk

[rickmcdougald]

Q. Just what do the mixture screws DO anyway?

Hey guys,
Just what do the mixture screws DO anyway?

I'm asking about the Mikunis on the XJ specifically.
Do they change the FUEL or AIR in the pilot circuit?

I don't have a Mikuni manual for these carbs. Is there one "out there" someplace?

Thanks!

[apolloant]

they are for fuel. In for leaner, out for richer. They alter the mixture of fuel and air from idle to about 1/8 throttle. If you very gently seat them, and then turn 'em back out 1 and 1/2 turns, this should give you a mixture of about 14.5 to 1. Ideal mix for combustion, or close. The only way I have found to truly get this mix right-on is to use a color-tune. Just type "colortune" into the search box and read around. I also found out that if you adjust and there is no change, you need to take the darn things out and make sure you didn't break the tip off. It gets stuck in the metering hole and you only get a lean adjustment, pop and backfire from that cylinder.

[rickmcdougald]

How does it change the idle mixture?

I don't quite understand.
How does it change the idle mixture? Does the little hole (that the screw tip gets broken off in) let more air in to mix with idle jet fuel, or what?

Thanks again!

[rickmcdougald]

No, I'm NOT using Jetfuel! lol

I meant, "... fuel through the idle jets!"

[apolloant]

I believe the fuel comes out of the float bowl, up thru the pilot jet, and is fed into the engine via that tiny hole that the pilot screw, or idle screw, seats in. Therefore, as you back it out, more fuel is allowed to pass into the engine. One in a half turns is usually enough clearance for a stock engine. But I have found that when you get headers and/or a k&n air filter that airflow is increased, therefor you need to increase fuel to the motor, or you run lean, ie, not enough fuel in the mix. go to www.motorcyclecarbs.com and hit the jet recommendation link, that will explain some. Any carb gurus out there want to help me out?

[rickmcdougald]

Thanks to ApolloAnt !

I've found the answer to my question:
"How does it change the idle mixture? Does the little hole (that the screw tip gets broken off in) let more air in to mix with idle jet fuel, or what?"

Thanks to ApolloAnt I followed the link to:
http://www.motorcyclecarbs.com/support_W124.cfm

Where I found a very descriptive PDF:
- Carbs101 Tutorial on Carburetor and Jetting theory.
http://www.motorcyclecarbs.com/carbs101.pdf

The answer is on:
Page 1, Fig. 2

"The pilot circuit has two adjustable parts, fig 2.
The pilot air screw and pilot jet. The air screw can be located either near the back side of the carburetor or near the front of the carburetor. If the screw is located near the back, it regulates how much air enters the circuit. If the screw is turned in, it reduces the amount of air and richens the mixture. If it is turned out, it opens the passage more and allows more air into the circuit which results in a lean mixture. If the screw is located near the front, it regulated fuel. The mixture will be leaner if it is screwed in and richer if screwed out. If the air screw has to be turned more than 2 turns out for best idling and performance, the next smaller or bigger size pilot jet will be needed."