SOHC.co.uk Forums > CB750

Carb Balancing - on the bench?

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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

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