Using vacuum gauges to adjust carburators

[jagarra]

After I rebuilt the carbs I decided to try to adjust them using vacuum gauges. I took my old boots between the carbs and head and modified them for a vacuum port. Pretty easy, I used a small brass tube with a brass washer solder on the end. Fit them through a hole from the inside out and I had access ports. So I played with the set up, trying to see what changed when I tweaked the idle screw or the mixture. Did notice a difference between the left and right cylinder, moved screws, burned finger, but could not get them even, left side had more vacuum at idle then the right. Could it be valves, age, or just characteristics of the bike?
Another worry, even though all parts in carbs are new, jets, emulators, floats, needles, diaphragms, o-rings, right carb would leak when right petcock opened up, but just sometimes.

[Jean]

I will assume you have the OEM Bings: 64/32/211 - 212?
There ought to be a vacuum test port on the outlet side of the carburetor body, normally plugged by a small screw.
If you have a parts blow-up picture of a "64" it is screw Number 39. It's on the "outside", opposite side from the enricher.
Check if you DO have these ports. If you do and one of the screws is missing, it will mess-up your vacuum readings.
If one of your carbs is not an original, it might have the port and the other one not.

Gauges work better if there's an orifice in the line if you are not using the vacuum port. It smooths out the pulsations.
Likewise a homemade carb-stick will make the balancing job easier. It's not as sensitive to pulsations.

Where was right carb leaking? was it overflow or LEAK?

[Duane Ausherman]

This is from RA Foil and I totally agree, except that I don't like the shorting method at all. We tried all of the "magic" methods and found that tuning by ear was far faster and as close as any other and better than most.

With so many "unbalanced" aspects of the engine to consider, tuning by ear
will get you as close to balanced as you can ever hope to get. Those
unbalanced aspects include (but are not limited to) the differences in
compression between cylinders, shorter exhaust length of left side,
crankcase venting into right side, differences in spark strength (even with
matched plugs), etc. If your carbs are spot-on vacuum balanced, it only
guarantees that the engine will be unbalanced. Vacuum balancing may get you
in the ballpark - but plug-shorting / ear-tuning gets you closer to where
you want to be because you will NEVER achieve 100% balance at all RPMS at
all temperatures and atmospheric conditions regardless of method used.



A few years ago, I bought a hand-held laser tach to fine-tune my carbs
beyond what I thought my old ears were capable of. I painted a reflective
silver spot on my flywheel and went about shorting each side and measuring
the RPM from each cylinder firing solo. In doing so, I learned / proved a
few things:



1.. my ears were pretty dang accurate - usually getting cylinders within
10-15 RPM of each other
2.. what is spot-on balanced RPM at one set of atmospheric conditions
(ambient temperature, barometric pressure, etc) will be a bit off by varying
degrees when conditions change
3.. had I not had other needs for a hand-held laser tach, it would have
been a waste of my money
4..
I still use the hand-held tach to check myself after I have ear-tuned. I do
it more to justify the money I spent for it than to ensure accuracy because
my ear-tuning seems to be good enough.


RAF

[jagarra]

I agree Duane, when I tried to adjust them via the gauges, well I wasn't very impressed with the results. Wasn't even close to how satisfying doing a 4 cylinder bike. Ended up tuning it by ear, seems to tick over really nicely at idle, started right up. Changed the plugs from BP5ES to the correct BP6ES which is a bit colder, now it does not idle as well and I may have to crank the idle up a bit. The leaking carb didn't do anything the other day when I was playing with the adjustments, today I put the left petcock in the reserve position and the right carb started puking out gas. Must be an overflow condition or something close. Will have to pull it again.

Installed new air horns as part of my rebuild, had problems with the right air horn, just wouldn't fit right, seems too long. Figured out that the carb end was not big enough to slide over it correctly, had to take a dremel and sanding drum to it. These replacements were made in Italy, ID was uneven in thickness.

[chasbmw]

Some digital multi meters have a tach function as does the Grok Harmoniser which works well as a carb synching tool.

Your ear will tell you when you have the carbs properly adjusted at idle. However I think that the most important part of the process is making sure that the throttle cables are properly synched , I do this at around 2500 rpm and find that Vaccum gauges or a Harmoniser works well to get this right, Bings being pretty sensitive to this adjustment.

[ME 109]

Your rare will tell you when you have the carbs properly adjusted at idle.

Well done Chas.

[chasbmw]

Post now edited, I'm sure my rear is not that sensitive.

[Duane Ausherman]

LOL. This is mainly why I stay here. The humor is great, as is the humour.