The only way to know what jet kit was installed would be to remove each jet and compare them to the manual specs.

Meant to say mine's 98, not an 86.  Old age + ADD.

Firefighter wrote on 12/31/69 at 16:00:10:


That screw is the idle speed screw.

The idle fuel mixture screw is on the right side...upper front with a brass plug (and a dab of white paint and a small hole).  You drill out the plug to get to the screw.  Easy to do.

My email is:
r0464@indygov.org

Thanks!

The idle mixture screw changes your idle and when you barely crack the throttle off idle.  It may also help with your backfire condition.  To figure out where you want the mixture screw, it's always good to know where you started from.  That way you can always put it back like it was before.

To figure out where the screw is now, take a little screwdriver and count the clockwise (IN) turns it takes to seat the misture screw -- Gently, don't force it, it's a delicate little needle on the end.  Then turn your mixture screw right back out to where it was before you started, same # of turns counter clockwise, so it's exactly like it was before you started.  Now you know no matter how messed up you think you may get, you can set it back like it was before you messed with it.

Next, fire up the bike and warm it up by riding it around for a while.  You must get the engine up to operating temp for the mixture adjustments to mean anything.  Bring it home and WITH IT RUNNING, turn the idle mixture screw IN (clockwise) till you notice the idle slow down.  It may not slow down for you without the white spacer, but if it does, you have found the leanest setting you want -- least fuel at idle.  

Then turn the idle screw OUT (counterclockwise) and you will probably notice the idle speed up and then if you keep going, the idle will get bad again.  That's the richest setting you want -- most fuel at idle.  You may also see black smoke from the tailpipe from the rich condition.   Without the white spacer, and with presumably bigger jets, you'll probably see rich pretty quick.  But, who knows till you try.

If you can find the lean and rich endpoints, put the screw about in the middle of the two and see how that works for you -- starting, idling, killing the bike, backfiring, etc.

When you find the mixture screw setting you like, kill the bike and count the number of turns to seat the screw, then back it back out again.  Now you know where you want it today.  When it gets really cold you might want to change it a bit.

Someday you'll probably play with the idle speed screw and notice when you change the idle speed screw you should go back and change the mixture screw as well, especially with no spacer.  Play with it and find the idle that works best for you.

My bike's mixture screw was 1/2 turn out from the factory when I started playing with it, before removing the white spacer.  My bike never leaned out at idle after I yanked out the white spacer, so my idle screw is now all the way in and the bike is happy.  If I turn it out past a half turn now, the idle slows and black smoke comes from the pipe (too rich).

Keep us posted, email if you want.  Good luck with it!

Probably, and that's a good thing -- means you didn't have to drill it out and dress up the top of the screw from the drill boogering it up.

Plugs over the mixture screw are relatively new -- old carbs had the mixture screw out in the open like the good Lord intended.  EPA regulations mandated less emissions, and tamper-proof carburetors were a result.  OK when new, but need to be adjusted after a few years.

What year is your bike?

Guys, I really don't want to start something here as I am new to this forum and my 'Savage' S40 .  But I am NOT new to these carbs and engines and have two points of concern for you to consider:

1. If the idle mixture screw is all the way in then there is a problem with another circuit in your carb (or your pilot jet is too big but most folks and jet kits don't touch this).  Your mixture screw should be out around 1 - 2 turns.

2. Taking out that white spacer without AT LEAST replacing it with a thinner washer is a HACK JOB.  If you are willing to adjust your needle, get a Dynojet kit (is there any other out there?) and use the adjustments on that to do it right.  Also, if adjusting a bike with a kit already installed, be aware that Dynojets main jet sizes do NOT correspond to Mikuni's sizes.  I would guess that a 150 Mikuni is close to a 140 Dynojet. I figured this out when jetting my carb on my Z400 when I put in an intake cam and high compression piston.

Anyways, it might be that the problem in (1) is related to the 'fix' in (2).

I temporarily fixed the backfire problem by properly adjusting the idle mixture screw and touching nothing else.  Ended up around 1.5 turns out (was 1/16th out from the factory, how bout THAT EPA crap...).  I will rejet in the spring with a K&N/Supertrapp/Dynojet kit.