BS34 Mikuni air fuel mix screw

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rocketoctopus

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Hey all wise in the knowledge of everything XS
Me? I have an 81' xs650 engine w stock bs34 Mikuni,pods, and open pipes.
I'm been relentlessly working at getting the fuel mix correct
Bought a kit from mikes with new jets (145) needles,and drill bit for drilling out the slide.
Runs wo choke on now but still runs lean.
Engine and pipes real hot and light back fire,white smoke in idle down and lengthy idle down at that
Little smoke and light back fire right in the middle range.
One step on the directions for installing the kit lists an air fuel screw adjustment.
The area where that screw is is plugged up by factory due to epa standards.
Can I drill this plug out and locate the screw?
Or should I go up in sizes on my main pilot?
Drilled out my stock ones w a 1/16 drill bit to experiment and no great results
Was gonna move up from there just to experiment.
Any suggestions greatly appreciated
Thanks
 
The plug is only an 1/8 inch or so thick so be careful you don't plunge through and damage the mix screw. A wood screw (or other coarse thread) in the drilled hole and pull the plug out.
It may be better to buy a selection of jets as opposed to drilling your own?
 
You can drill it out though?

Yea,I'm just experimenting with drilling the old jet to kinda give me an idea of the size I need because there's 5 sizes greater than 145 at around 8$ a set
 
Hi Rocket and welcome,
You bet your ass those needle jet plugs gotta go!
Not only do they lock the carbs into the EPA Nazis mandated hyper-lean idle settings, they mean that the carbs ain't been cleaned properly from new.
BTW, open pipes may be your thing but the XS650 engine don't like 'em, they are a bitch to tune unless they see some back pressure.
 
Yes, you can carefully drill it and then pull it using a sheet metal screw. Note that the plug has a small "breather" hole through the center .....

6w594s8.jpg


Drill a little bit and check the hole, drill a little more and check. Watch that little center hole. As soon as it starts getting bigger, stop drilling as you are almost all the way through. File the point off the end of the screw you're going to use .....

WwXCtog.jpg


Screw it in until it bottoms in the hole and starts to spin the plug. Now you can grab the screw head with some vice grips and yank the plug out. A little twisting motion as you pull will help .....

1He45qM.jpg


imPf1SZ.jpg


The mix screw is usually set too lean from the factory. You will probably find it somewhere in the 2 to 2.5 turns out range. This carb set prefers settings in the 3 to 3.5 turns out range.

Your other problem could be that kit from Mike's. I'm assuming it's that Heiden Tuning kit. Most report it doesn't work very well on your typical "pods and pipes" modded 650. Apparently Heiden designed it for use with a 750 big bore kit and their own 2-1 exhaust. You may get better results by using the stock needle and just changing mains and pilots. Most do. A 145 main is about as big as should be needed. Most don't even go that big.
 
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Very helpful fellas. Thanks to you.

5twins,I was thinking the exact same thing after trouble shooting every slot on the new needles. Gonna throw the stocks in there and take notes. Drilling out the slide lift holes won't effect the stock needles performance in any way would it? Cause that's already been done.
 
No, the slide mod is simply to enhance the lifting, make it quicker. It's something some do even on stock set-ups, although I've never tried it.

If you compare the length of that Heiden needle to the stock one, I think you'll find it quite different. In order for a replacement needle to work, it needs to be near the same length as the original. Mike's also offers what they call the "Canadian" needle. This is pretty much just an adjustable version of the stocker, but it is a little shorter (richer). They also offer the "Canadian" needle jet and say it should be run with that needle as a matched pair.

If simple main and pilot jet swapping won't get you tuned good enough and you think an adjustable needle may help, this eBay seller offers a nice kit .....

http://www.ebay.com/itm/161720238856?_trksid=p2060778.m1438.l2649&ssPageName=STRK:MEBIDX:IT

The kit does two carbs so isn't much more, if any more, than the basic rebuild kits Mike's offers. But you get more, a needle jet and not one but two adjustable needles.
 
Well I'm getting no where here.
Exposed the fuel mixture screws.
Messed about with them and changed back to the stock needles.
Very little improvements on engine temps and that's about it
Even if I get it to run rich thru adjusting the mix screw the temps are still seemingly higher than normal,the throttle down time is long, and the white smoke is still present on the throttle down.
Also at a steady 1/2 throttle it "misses" and white smoke proofs out.
That smoke also emenates from the pipes after shut down and the carbs,either right or left,pop.
I dunno
Not sure if I'm even on the right path here
All valves checked while cold and all compression checked and perfect as well
Gonna start ripping my hair out and see what that does
 
The usual routine for your BS34 carbs with "pipe and pod" mods is 1 or 2 up on the pilots and 2 to 4 up on the mains. You still have the 145 mains in there? That may be too big. White smoke can be an indication of too rich a mix. The excess fuel washes the oil out of the cylinders and that's where the white smoke comes from. The type of pods you have can also have a large effect. The cheapo K&N copies can block the air ports around the mouth of the carb and that results in a richer than normal mix.
 
Thats where Im getting a little confused as well.
From all the info I've gathered I'm led to believe its running lean.
For instance,the heat of the engine especially the pipes which have gotten red hot at an earlier stage,the longer than normal idling down and idling down to a high idle,only running on choke early in the process. Uprgrading all the needles and jets has helped these problems to a point.
But I also have signs of a rich mixture which are wet plugs,excessive white smoke,running erratically with increased vibration.

gonna go to a 135 main and try to focus on the air mix thru the fuel mix screw rather than trying to increase the fuel mix thru the needles and mains I guess???

gonna start doing this behind my back to mix it up a litttle cause god knows ive had enough practice to acheive it
 
When running, are you using a fan for cooling? Just a good idea, if not.

Drop the needle back to stock height...carbs synced?

Have you tried to do a dead cylinder method for tuning each carb at the lowest idle possible? See if you can get each cylinder to run independently at like 600-800rpm. Then check the plugs, and see if they're wet/dry/lean, etc?

Not sure what's going on...but that's what I would try.
 
Hi Rocket,
you didn't post that after you pulled the EPA plugs you carefully cleaned out every little port and passage that was down the needle hole and behind the enrichening circuit now that you could get at them.
If you didn't do that, you really should.
 
I prolly should do that though fredintoon
Just getting really burnt on this
Been at it for 2weeks
At least I'm learning
Thanks for the good advice
 
Still just no luck
Tried syncing
Tried mix screw
Had the effect that messing with those things would produce,but still hanging the same issues with heat,significant smoke on the idle down.
Messing w the mix screw helped w the length of the idle down though.
Decided to tear down carbs,completely this time meaning removal of mix screw and the choke parts as well.
That's when I ran into a problem of some little bitty tiny stainless springs and bearing falling out of a location that I did not see in an order that is unknown.
Any help as to where these may go and the order in which they should be installed?
 
if you removed the choke rod, that's were the spring and ball live. The choke will still work because the other carb has the same spring and ball.
 
thanks angus 67

had just actually figured it out

when I see little springs and bearings mysteriously appear my first instinct is to panic lol
 
its a bitch to get those back in. put some bearing grease on the end of the spring, stick it in there. The thick grease will keep it in there until you get the ball and rod were they belong. It eliminates your want of a third hand.
 

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