Topic: Vacuum level for synching carbs?

[azuredesign]

The Clymer says synch carbs to lowest level or 8". I think Mike Rieck suggested I synch to the highest carb reading. Can anybody clarify this for my quickly as I'm awantin to get this done?

Thanks for your help,
Ben

[malcolmgb]

I use a Morgan Carbtune 2 to do my 400, set idle to about 2,000 then adjust up or down whichever is easiest, always seems fine after, just use the numbers as a reference.

[azuredesign]

Thanks, I'll bring the idle up some before synching. It raises a question for me, as I wonder at what engine speed the slides and needles start to open at. They should already have started to open at 2k RPM, shouldn't they?

Thanks again,
Ben

[gregimotis]

I synched my CB750F1 to the highest one which I seem to recall being 15ish.  I set my idle at 1500.

The Morgan carbtune instructions emphasized  that the number on the gauge was minimally important, it is the relationship of the cylinders to each other that matters.  Can't say that I understand the science of it very well, but it clearly worked.


A hint I would give:  make small changes (1/8 turn or so) and bump your throttle after every change, then give it several seconds to settle down before changing things again.

Use a fan or something to blow air over the fins and (obviously) work in a ventilated space.

[azuredesign]

Thanks Greg,

I think that what is happening is that maximum vacuum is attained when the slide( and needle) are seated, just the idle circuit is functioning. synching here should balance the idle circuit of all carbs. Bringing the rpm up should cause a bit of a drop in vacuum, showing where more gas and air are introdced as the slide starts to open. Balancing here should help to make sure the carbs are balanced at midrange. I think that checking spark plugs after running at revs consistent with the thee circuits is probably a good idea as well. I wonder what affordable instrument can be used to accurately check mixture while running the bike.

Best,
Ben

[edbikerii]

I don't know what bike you're tuning, but on the 77 550 K carb #2 is fixed and cannot be adjusted.  The other three are tuned to match it.