Topic: Bench Sync to gauge sync

[Chester345]

So I'm about to be ready to carbtune sync my carbs, and I took them apart 1 last time to clean.

My PO claims he 'syncd the carbs' before the sale, and it ran pretty good so I assumed he did. But, every tuning thing I've done has improved the running/starting of the bike a little.

So now to the meat of it, visually, my carbs look bench sync'd with a 1/8in bit, they are EXACTLY the same, all 4.

So has anyone put guages on their carbs after a bench sync and not had to adjust them AT ALL?  They were 100% good to go?

Just curious, I'll be hooking them up regardless this weekend.

[heffay]

So has anyone put guages on their carbs after a bench sync and not had to adjust them AT ALL?  They were 100% good to go?

never... a bench synch is a guide in a perfect world... manufacturing inconsistencies and cylinder compression, etc. make for large differences when you're talking about minute amounts of fuel.  

[bryanj]

Be more accurate if you used a smaller drill bit

[TwoTired]

A mechanical carb sync will only be close if each of the four individual cylinders had the exact same compression and breathing characteristics.

Not bloody likely on your average engine.

[Chester345]

Yeah, I suspected, thanks!

[Deltarider]

So has anyone put guages on their carbs after a bench sync and not had to adjust them AT ALL?  They were 100% good to go?

Yep, me, more than once. After a bench sync I've doublechecked with a manometer and on all occasions carbs vacuum was within 2cm Hg, which is within factory specs. I've never bothered to use my set of manometers afterwards. So, it can be done, but take your time and do it very precisely. I agree with Bryanj. I've used a small allen key under that little cutaway section at the front side of the slides.

[Old Scrambler]

The bench sync is only to assure that the throttle is opening the carb slides at the same time.  The vacuum sync allows you to adjust air and fuel mixtures to maximize the performance of each cylinder.  That assumes you have nearly equal compression and ignition timing across the motor.

I use thck string (4-lengths) attached to a pencil or a piece of cardboard for a bench sync.  Place the end of each string in the carb opening at the edge of the slide. With the throttle closed, you should be able to tug on the string-bar to pull them all somewhat tight.  Now slowly open the throttle. You can feel and visually observe if any one string is first or last to be released.  This will get you much closer to an accurate starting point for a vacuum sync.  

[Deltarider]

The vacuum sync allows you to adjust air and fuel mixtures to maximize the performance of each cylinder.

I doubt it. That's to say: I've never managed. But I like to learn.