Vacuum adjustment problem

[Dirty Dave]

Anyone ever had an adjustment screw on a carb not react when synching carbs. My #4 carb vacuum screw does nada while all others (except 3 of course) adjust well. It (#4) reads higher than the others and doesn't give a smooth reading, kinda jitters...............

[SCain]

Never had that happen, the only thing I can think of is check to see if the small spring is there behind that carb sync screw, also make sure that the nipple on the #4 intake is clear and your getting a good vacuum.
Good Luck

[Kingbee]

Are you sure you're using the proper screw to synch?  The "air adjustment" screw is not for synching the carbs, but for adjusting the idle/slow speed fuel mixture

[Patrick]

 What are you using to take the readings?? A vacuum gauge needle will ' kinda jitter ' if its not dampened enough,but, if one 'jitters' all should 'jitter'.. Actually the needle should 'jitter' slightly on all cylinders, don't dampen too much..Make sure you're checking [pulling vacuum] from only one cylinder..Its easy to end up pulling vacuum from 2 cylinders if you do this job the way I do..
  You are attempting to adjust the phillips screws with the blue painted heads,, had to ask..
  All your connections are good? No split or damaged lines? Again, had to ask..
 

[Ricky-D]

Those screws actually open or close the butterfly valves inside the carburetor.

One reason you may not be having a reaction for that carburetor is the linkage may be sticking. It's always good to spray all the linkage with a cleaner/lubricant to insure the parts move when you turn the screw.

Another reason you may not be seeing a result can be the linkage itself. A close inspection will reveal that the linkage on the 2-4-6 carburetors is designed where the adjustment on one carburetor will limit the adjustment on another. For instance one carburetor could be "bottomed out" and severely restricting the adjustment of one of the others.

Just remember that small incremental increases or decreases of the screws are the best way to go and always adjust the idle back to the baseline.

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[Dirty Dave]

Am using a Carbtune Synchronizer. The spring is in place on the philips screw and I have done the adjustments before with the same results (#4 wonky). Valk seems to run great regardless but I
always want it 110%. Gas mileage is great. I'll check to see if the vacuum nipple is clogged. I can adjust 2 & 6 with no problem (as well as 1/3/5). Will also check all linkages (already sprayed at the time) to ensure proper function. Thanks for the hints. I feel honored to have a unique problem.

[Dirty Dave]

Oops. I realize #3 is my reference and not adjustable.

[Dirty Dave]

No blockage in the nipple. All parts move when I turn the screw. Could an imperfection with the butterfly valve, a scratch or nick cause something like this?

[Ricky-D]

No blockage in the nipple. All parts move when I turn the screw. Could an imperfection with the butterfly valve, a scratch or nick cause something like this?

That's very doubtful!

It's been some time since I last synchronized mine but I remember the same difficulty on the left bank and it's definitely related to the way those carburetors are hooked to each other by way of the linkage.

A close look at that singular area will reveal what I mean. If you adjust the screw for #2 then the result will be an adjustment to #2 only, but the other screw will adjust #4 and change the adjustment for #2 at the same time!

Like I previously stated, the most important thing is to keep the idle speed constant . The recommendation I seem to remember is to use a more sensitive tachometer than the one that is on the bike. Obviously this makes better sense with the emphasis being on idle rpm maintenance while making adjustments.

I feel this point (maintaining a uniform idle speed) is not given the importance it deserves thereby affecting the completeness of the synchronizing experience.

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[Dirty Dave]

Thanks Ricky-D. Point well taken.

[Patrick]

Dave, are you familiar with our old friend and member Normand [Quexpress] ??

[Dirty Dave]

Yes Patrick, he lives close to me. Only met him once. We were supposed to get together to take
measurements of 97 vs. later carb needles but it never happened. I didn't want to be a pest.......