Carb Balancing - on the bench?

[Lobo]

Hi all,
Lateral thinking or won't work?  Have just torn down / rebuilt the carbs of my K2. Last night had a brainwave (??!) and balanced them on the bench via the Hoover (don't tell the missus). With a steady vacuum got nice / repeatable stable readings across the bank, though of course not in the 16-24 cm Hg range (which I guess is simply that around 900rpm idle). Point is, the carbs are 'balanced' spot on.
Or are they?  In this exercise are you (a) trying to match the throttle openings precisely (which I have) or..(b) are you trying to match the vacuums taking into account each cylinder performance. eg - one worn cylinder would throw its carb slide out of matched position when balancing the vacuum.
Cheers, Lobo.

[Bryanj]

sorry matw the answer is B. BUT what you have done will be close

[archmill]

I set carbs by eye on the bench then use the carb balancer with the engine running. To tell the truth they are never that far out when using the carb balancer. Like the idea though 

[K2-K6]

I like that for lateral thinking Lobo and I think you'll be very close in practice if you measure them again with the engine running, be interested for you to let us know what readings you get.

I understand what bryan is saying and can see that it will have some effect, but equally if one cylinder requirment is that much different then the linearity of throttle opening given by the linkage could be equally wrong as the thottles are opened more.

I guess if you thought it through logically then a case for setting them equal at about one third opening and accepting some error at fully closed and fully open would best suit the engine operating range.......You may have created a way to do that Lobo...! if you put a block under the idle stop to simulate 1/3 opening before checking the vacuum readings.

[Lobo]

We'll see,...will report back on actual bench set-up versus engine set-up, tho' likely after xmas. Funny old thing K2-K6, I DID set them up at 1/3 throttle - only because the bloody Hoover kept cutting out! (overheat!)
By hoover (!), by eye, they are perfect,  fingers crossed...

[steff750]

  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

[K2-K6]

Happy new year to you also.

I'd read that as a success then for your methodology as it seems only minor tweaking after installation shows that it's pretty close/close enough to be ok for operational purposes.

I don't feel that these motors run "really smoothly" on tickover if they are in good condition and running efficiently. To me, the effects of decent compression and crank weight not too heavy seems to give a running fluctuation that can be heard as variance when ticking over....they seem to smooth out a bit when more worn and I'd take this as an indication that a lowered compression exerted less influence on the balancing effects of the crank journals.

Interestingly I was just reading something about setting up carbs on a Honda race motor and they also did the balance befor fitting to the engine.

The idle screws I'd go toward it like this.....If you accept the minimum of one turn out....then turn them to two turns out to see if you get a clear change....then if that's the case it shows you they are able to make that graduated shift.....also that your final setting (within reason) should be somewhere between the two. The final position is going to be influenced by your own preference specifically at the point when you are riding it and you have closed the throttle and coasting...then just as you try to feather it open again, then the transition at that point is more an indication of what you are trimming....too lean and it comes back in abruptly.....and bringing the screws out a bit allows a softer more controlled progress.

If the screws are too far out then it will hunt at idle when warmed up, indicating that you need to go back in maybe a quater.

I've alwas associated plug chops with higher rev jetting than tickover same as you.

[Lobo]

Thanks K2-K6 / OddJob. Will go back & try a 'full 2 turns' on each air screw & note whether a difference is now apparent. (should have thought of that, doh!)

Thanks K2 for the comment re 'silky smooth' unlikely; the dear old girl is 40 yrs after all & should perhaps be allowed a bit of leeway. It was semi-restored prior to me getting it last April (23000 miles on the clock).... but no idea whether the motor's been opened up - although I doubt is as no tell-tale scuff marks, rounded screws, broken fins...anything to suggest. (which pleases me)

OddJob....I've also a 400F whose engine is a little tired at 50Ks, and due nil time (assuming I want to ride it < I croak) will hand it over for professional restoration. On your advice I'll ask that all you mentioned simply gets renewed - thanks.

Cheers gents,
Lobo.

[K2-K6]

It's good to work through these things at a considered pace and you seem to be getting to where you want, so not bad at all really.

Although it's got some years on it 23000 is fairly low miles and I'd expect it to be reasonably crisp in terms of combustion if that makes sense.

It's worth mentioning for others reading this that (and I've heard this quite often) if you were to compare it with an ariel square for for example it sounds like a bag of spanners! but then you are comparing something with heavy crank, low compression, mild cams and not very high revs as the Honda has by comparison essentially the opposite and that slow running nature is not there to the same degree and with cam timing that allows it easily to spin to 9000rpm, then you can see a clear compromise at those low speeds. This doesn't mean it's wrong but that you'd recognise different characteristics in a healthy example of both.

Comparisons also made with it's Kawasaki contemporary....that has gear primary drive with the clutch spinning in the opposite direction of the crank to maybe damp crank inertia....again, they sound nothing like each other.

As an extreme example......the Honda 250-6 GP bike of the sixties (extreme cams, ultralight crank etc) won't even run below about 8000rpm....think the tacho starts at about 6000!

Accepting that you have all the parameters well adjusted, then i've always focused on making sure it runs as well as possible when using it even at a slight compromise to slow/smooth tickover.

Looks a nice bike though and hopefully we get some nicer weather this year as you seem to have it ready to get out and about on.

[Lobo]

Thanks K2-K6, when put into that perspective it all makes sense; I well remember messing around with high revving / light 2stroke model aircraft engines - never, ever getting them (obviously!) to idle smoothly. A far cry from the CB750, I know, but in essence the same when comparing to heavier low comp engines.
As I've said before, it was 30 yrs ago since I messed with bikes - been too spoiled with modern (auto) engine units and their controls / instrumentations and lost sight (a bit) of what was the norm.
Cheers,
Lobo