SOHC.co.uk Forums > CB750

Carb Balancing - on the bench?

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steff750:
 :o i will throw my vacuum gauges in the bin if you get them right,as long has you have not undone the locking nuts and adjuster nuts on the carb tops you wont be far out , if you have removed the jets they might not go back to the original settings,the k series carbs are the best ,once set up they will never go out of sync unless tampered with,you will never ever be able to bench balance a set of k0 carbs   ;)

K2-K6:
That is funny, and will see what you've got in due course.

My reasoning is this;- if you are running a motor in competition then in the past you'd have done a plug chop at typical rpm to see the effects of critical jetting under load and adjusted accordingly and probably most critically at large throttle openings.

Now say you have one cylinder that causes a vacuum discrepancy at tickover on one of these SOHC motors.....so you adjust the carb to make it different to the other three.....that offset remains as they open more but with the increase in rpm is that still appropriate?.....hence my assumption that 1/3 openeing being somewhere the most used position in general riding it should be better to go with that from a healthy combustion point of view.

It's ok we won't inform on you to Mrs Lobo about the hoover... ;)

Lobo:
Steff - I might be stuffed!
I have stripped the carbs (inc plate set for re-chroming) all to component parts, and renewed all jets etc. So this set-up is from scratch, and my biggest worry right now is (lately) reading the DSS aftermarket carb kits are 'iffy'. Will be v.upset if so, as there is absolutely NO point is selling 'em if they aren't up to the job. Comments on what to expect welcome as not able to reinstall carbs on bike till late Dec.

Lobo:
Results are in... and whilst near, not spot-on. This is my first time at carb balancing, and what I did notice is that the comparitive vacuums vary - to a small degree - with throttle setting. The good old Hoover was used at a constant approx 1/3 throttle, whereas the 'engine balancing' was initially done at approx 1000 RPM. I only latterly noticed a slight change in balance with throttle setting, and so then re-balanced to a 'mean' value between 1000 & 3000 rpm. (but honestly, nothing too significant). Comments pls?
Regarding the subsequent mixture screw setting (from the 'factory' 1 turn out) I must have the ears of an imbacile, and could not detect any change in engine note / rom with the recommended +/- 1/4 turn. Comments again? (the carbs just overhauled with brand new DSS aftermarket jets etc.
No road test yet.... raining & bike just too pristine!
Cheers.

Lobo:
Morn OddJob!
Yup, appreciated, but simply carrying out what the Honda maintenance manual says - it does make it all seem black & white.  Moving my air screws +\- 1/4 turn makes no discernible difference to the idle....which is fine, but I would have thought, might be silky smooth, ....instead of 'smooth'.
Quite happy no leaks, clogs, jet issues as the carbs just been very carefully & thoroughly overhauled with new parts throughout. Timing & valves set correctly before this set-up, and the engine at operating temp < set-up.
Will the plug-chop indicate best air-screw position?, I figured this was more about main-jet setting.
Cheers & Happy NY to all,
Lobo

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