I originally thought I had it all figured out...that the mikuni has a FUEL screw, not an AIR screw to adjust idle. Therefore, you back the screw out to richen, and turn it in to lean it. Is this info correct? I was reading some old threads, and either a few people have it backwards, or I learned it backwards (I am 90% sure that I am correct though)

I have never had much luck tuning carbs (one reason I plan to hit a dyno soon) and I seem to have the biggest problem with the screw and needle settings. I have KN (in box) ported intake (dont know if that affects much, but thought id mention it) hmf slip on (run it open, no quiet core or S/A) and run on 89 octane.

I usually dont jack with my jetting much because it seems to be "ok" at where it is at. I currently have a 175 (mikuni) main in, CLIP on the lowest position (with a washer below the e-clip) on the mikuni needle, a 45 pilot, and air screw approx 2 turns out. I am thinking the 45 pilot is too rich, because I can turn that screw almost all the way in and still have it idle pretty good. I also noticed that my quad dosnt seem to respond to the method of "adjust the screw both ways till she bogs, and put it in the center" because mine dosnt bog until almost totally seated (as expected) and I can go pretty damn far out with it as well.

What idle rpm should this all be checked at? I usually have idle set @ 1300 give or take. My only thought is that this is too high, and that is why the screw dosnt have much affect?

Last question... I cant seem to remember, but does popping out the exhaust generally mean RICH, or LEAN? I thought backfires through the airbox meant lean, and out the exhaust meant rich (extra fuel being burn in the exhaust pipe?)

I noticed last trip at the badlands it was popping quite a bit, and sometimes it would die while going down hills (I would have to keep the throttle up so it wouldnt die) <- I spent a solid 30 minutes adjusting the idle screw and going down the same damn hill trying to get it to not die, and no setting be it 1/2 turn out to 4+ turns out seemed to make much of a difference.

Alright time for work, so I will cut my mini-novel short [img]i/expressions/face-icon-small-wink.gif[/img]

 

Four stroke carbs have fuel screws, 2 stroke carbs have air screws. Out on a fuel screw is richer, out on an air screw is leaner.

 

Yes you are correct...popping out the exhaust is rich and through the carb is lean

Yes also rpm is 1300 +/- 100

 

Set screw out 3 turns. Start motor and let idle or go for short run.
To set pilot screw motor must be hot. Turn idle down till almosts quits running.
Turn screw in till lugs or stops running- Turn off motor - turn screw in and count turns.
Go back to 3 turns and start motor.
Turn out till motor lugs or stops running.Turn off motor - turn screw in and count turns.
Subtract in reading from out reading and that number divide by 2.
Take that number and either add to low reading or subtract form high reading.

Example: In reading 2 turns - Out reading 8 turns
8-2=6/2=3
Add 3 to 2 = 5 turns or subtract 3 from 8 = 5 turns

The key is to have the motor running at the lowest rpm without turning off and motor hot.

Reset idle speed to 1400rpm
Important - If motor idle is set too low the pilot orifice into throttle body is covered and motor will not start easy. For you guys that complain about starting.

 

Oldman's method is sweet, very well described. I reinforce the fact that motor must be fully warmed to op temp.


On the subject of air screw vs. fuel screw, some carbs have one, some have the other, but it actually has nothing to do with 2 stroke or 4 stroke. My TM carb has an air screw that controls the amount of air thru the pilot circuit. But the stock carb has a fuel screw. If the screw is in between the bowl and the head then it is usually a fuel screw. If the screw is between the bowl and the filter, then it is usually an air screw.

 

Traditionally on a 4 stroke carb, the screw is in front of the slide and vice versa on a 2 stroke carb. There are exceptions, but on the majority of cases it holds true.

 

alright, thanks for clearing that up guys. I was pretty damn sure I was right, but after reading a lot of the old posts I thought maybe I was the one wrong!

 

I would like to review your jet needle.
You state washer below e-clip.
Below the e-clip should be first the nylon washer, then the thick metal washer.
Some of us have ground the thick washer to match the offset in bottom of throttle slide body.
Above e-clip should be the thin washer. Plunger with spring pushes and holds the assembly down.
If you have lost your nylon washer, this would put you in the 3 groove,too lean.
Maybe this is your midrange backfiring.

 

ok, the thick white (think it was white...been a LONG time since ive seen it!) went BELOW the clip?

This would easily explain why I feel lean in the midrange, yet clip is in the lowest position.

The first time I took the carb apart I had too much going on to remember the order of the washers, and thought the nylong went on top for some reason (and eventually just lost it but figured since it was on top it didnt matter)

How thick was that nylong washer? maybe I can just make up for it with some other washers.

Also, I know the hole in the slide that the needle sits in has that little lip you are supposed to grind, but how do you get down in there to grind it so its flat, and the washer seats properly?

Thanks

 

The nylon washer is .270 diameter X .099 thick. With a .120 hole
You can go to a hardware store like Ace and buy 3MM flat washers. .272 dia. x .020 thick
They are .020 thick - so buy 5 Ask the clerk to verify diameter and thickness.
There should be a washer above and below the e-clip.
If you have lost them, then buy 7
You need 6 washers below and 1 above e-clip.

I would put the e-clip into the 4th groove from top of needle.
5th groove may be too rich for trails. Use for drags with inlet hoses to airbox mounted to side and tied to frame so not to touch exhaust.

Recheck jetting when weather is above 65 degree's.
Don't worry about notch at bottom of slide. You would need a thicker 3mm washer.
If you find a 3mm that is .040 thick then:
You grind the side of washer half way to the center hole.
Install washer and position so that is fits the notch at bottom of slide.