Little help understanding the carb guide

[rshelbert]

81 xs running VM34 carbs. Have adjusted valves, chain and new oil. Synced the carbs as close as I can get them.

The bike starts easily, idles fine, although the right side seems a bit lean at idle as it has a slight pop until it warms up. Once it warms up it idles fine. It rolls on throttle with no hesitation or lingering.

The floats are set about 24 mm. And despite the boots being the stock bs34 ones I cannot get any indication they are leaking with starting fluid or pen oil.

The jets are as recommended in the guide.
180 mains
22.5 pilots
159 p6 needle jet
6F9 needle in 3rd groove
Air jets removed.

The left cylinder mix screw is about 1 1/2 turns out, the right is about 1

The problem is I am unsure what to change next to correct a lean mid range plug chop (about 45 mph roughly 2500-3000 rpm range. There is hardly any smoke ring in the right plug. The left has little more of a ring but both are pretty light.

All that background to finally get to my question, Should I raise the needle or up the pilot for these symptoms?

 

[Kathos]

At mid range you will probably need to adjust the needle. This doesnt always mean a full step ( I tune vm34 not bs34). We can do half steps by using washers under the clamp to space it. Sometimes this new richer needle position means you need to downsize the main or pilot depending on how it adjusts.
Also, its really worth it to make a vacuum gauge. Its ridiculously easy and very worth it. My engine seems to run so much more wacky on one side when its not perfectly sync'd.
Hope this helps.

 

[TwoManyXS1Bs]

VM34s? If you have the dual left carb kit (not the proper left/right combo), you may be able to switch the slides, if the idle screw ramp is on the same side. At that 45mph and 3000 rpm setting, the cutaways have a strong influence. Very slight variances in slide machining can give significant mixture changes. See if the plug color follows the slides...

 

[rshelbert]

Went out and changed the slides as suggested, It possibly made it worst if any change at all. (its also cooling down now the suns down so that may be part of it) Ran a 3 mile chop and plug was white as new, ground was very white. No sign of a ring. The guide would have me up the main if I have to go to the bottom of the needle but I had one more slot before that.

I figured I might as well try the needle, Dropped it one clip to the 2nd to last on bottom. Put slides back and tried again. It was a little better but still very lean looking with barely visible ring.

I have not messed with the main yet and I started with the 22.5 pilot. I can up the pilot to a 25 (the biggest I have and see) Or I can go as high as a 190 main (also the biggest I have) I also do not understand the function of the air jet that I removed (could putting that back in help?)

 

[grizld1]

24 mm. is the max float height (min. fuel level) specified for the VM34. Lower your float height to 22 mm. (not ABOUT 22 mm., do it right) and see what happens.

 

[rshelbert]

Ok I went back through the carbs just to make sure and I found that I had messed up the float adjustment on the right side. The float was set to about 28-29mm on one of the tangs, not sure how that happened but it must have caught on something. Not sure how much that will affect things but I will resume this journey tomorrow.

I cannot do anymore testing tonight its too late and too loud and too cold.

OK it is at 24 now, will try 22 if that does not help. I try to make it exact but Honestly I cannot see that good, when I say about its within half a mm or so.

 

[weekendrider]

Hand you just got one of the best tuners on the forums to answer you.
I would heed his words.

 

[Kathos]

Hand you just got one of the best tuners on the forums to answer you.
I would heed his words.

Lol.
On a side note. If the float height is different on each carb, that's enough to throw them out of sync.
On my bike Im running open exhaust and vm34s. Im currently using 27.5 pilot, Middle notch 6F9 and 195 main. Im still dialing everything in.

 

[grizld1]

Kathos, float height has nothing the hell to do with throttle synch. It has plenty to do with fuel mixture through the range. The high float setting on the OP's right carbie forced the richer mixture screw setting and made right cylinder plug read leaner. BTW, your jetting is probably too fat. Good luck.

rshelbert, reading the smoke ring is only useful off a full-throttle chop on a plug that has not been used in mixed riding. Tune the midrange by function. If your eyeball won't show you float height against a rule, use a caliper, and if you don't own one either correct that deficiency or stop screwing around with old bikes. 1/2 mm. is too wide a tolerance.

 

[Kathos]

When I say sync, I refer to cylinder afr and vacuum -_-
Sorry for trying to help

 

[grizld1]

Synch can be set by measuring vacuum, which is controlled by throttle position. Air/fuel imbalance between the carburetors can still be present with vacuum equalized. The 1/2 turn difference between mixture screw settings and the off-idle lean mix on one jug was the giveaway here. Keep laughing.

 

[Kathos]

Synch can be set by measuring vacuum, which is adjusted by throttle position. Air/fuel imbalance between the carburetors can still be present with vacuum equalized. The 1/2 turn difference between mixture screw settings and the off-idle lean mix on one jug was the giveaway here. Keep laughing.

Fixed. There are plenty of other factors that contribute to engine vacuum. Throttle sync is merely the adjustable way of creating even vacuum between cylinders. Nothing else that you said goes against what I was stating.
If you read my previous post, I never talked about the floats versus the throttle sync. Which is why I stated that when Im talking about sync, I'm reffering to the cylinders in terms of vacuum and afr.
And my laughing was not directed at you. I was joking that weekend rider was referring to me.
Go back to bed.

 

[weekendrider]

Kathos I made no reference to you or your posts.
Don't try to drag me into your bullshit.

 

[rshelbert]

Thanks for the information again. I will do as instructed and let you guys know what results I have. Going to work on it now.

 

[rshelbert]

OK I changed the floats to 22mm. I do have a digital caliper and its right this time. I reset the air adjustment screws with the dead cylinder method, The left one changed slightly to about 1 1/8, The right one is 1 1/8 as well.

When running on either cylinder it runs very smooth and even, when running on both it is a little more lumpy with a little more air being pushed on the right side so I assume my sync is a bit out. I have the tools to sync it so I will do that this afternoon.


I started the bike and it did not display the usual popping until warm like it normally does.

Usually takes choke for about 10 seconds then i can take it off. About the same here.

I let it warm up to normal running temp and took it around the block. It seems to run fine, very responsive there is no hesitation, no lagging, no throttle hanging or bogging and it will drop to idle nicely when coming to a stop. It will idle as long as you let it in front of my big fan and does not bog down. It does pop a little when you decelerate.

If a mid range plug chop will not tell me anything I do not see anything wrong with the way it runs. I did look at the plug the ceramic has a very slight tint of brown on the ceramic, and the ground is just a bit grey The rest is like new with slight traces of fuel on them. I did not cut the plug since it may not tell me anything anyway.

Unless you guys have any suggestions on determining more I am going to leave it like it is and move on to the main.

Thanks again for all the replies and help.

 

[grizld1]

Exhaust popping under engine braking is usually caused either by exhaust leaks, OE double-wall pipes introducing cold air into slip-on mufflers through the vent holes in the outer layer of pipe, or slight leanness in the mix screw setting. If your exhaust is good try turning the mix screws in 1/8 turn.

 

[rshelbert]

I had some pretty significant exhaust leaks where the inner pipe separated from the outer right up near where it connects to the engine. I welded them up but I fear the air is still leaking from the inner pipe to the outer pipe a little bit. Possibly this is causing the slight popping on deceleration.

I tried to set the mains today but other than the interstate I cannot find a place where I can open the bike up for any length of time. What little I could it seemed to pull fine with little hesitation or flat spots. I am going to leave the carbs as they are for now and finish up and get the loose ends buttoned up.