SOHC.co.uk Forums > CB350/400

Carb sync

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K2-K6:
It's something I've thought differently about more recently. I've owned a set of vac gauges for 30yrs or more and used them on many things. As I've said on some other threads, I do feel more now that there's an element of show with this method of syncing that maybe inconclusive at best.

Over many uses for syncing other's bikes I've experienced just that variability that seems to fall either way with the engine not seeming to run or perform any better than before the gauges were used. Logically the smaller bore carbs would appear to need a very finely resolved adjustment mechanism to achieve a very close parity. If the above posted tolerances are correct, then this I would see as an acknowledgement by honda of the physics involved.

The thing that really made me think it through again is something I read (I thought by Hondaman, but can't at the moment find it. Please correct me if that's not the case) and it was to check the sync at 5000rpm. I first thought that this was really odd, but then working back through it I can see why this should be logical.

If you've built the engine well and it has a reasonable comparability from one cylinder to another, then it's further reasonable that to run it flat out (wide open throttle)  the slides should all be matched for height physically to supply the same torque from each pot. In other words, bench syncing accurately with a metal gauge in the carb should give you that.

Is there any reason to alter them based on a tickover that essentially has a method of equaling (the idle circuit and airscrew adjustment) dedicated to its refinement AT IDLE SPEED ? To me it just looks like if you've got all the other elements, timing, plugs,valve clearance etc then the main slides should be left alone and trim each carb individually to give even load to each of the cylinders at tickover with that very fine adjustment the idle screw allows you.

It should result in a better setup for the entire range of operation and overall smoother running.

I'm more than happy for people to critique this view, I'm trying to understand if it's a valid one too.

kevski:
K2-K6 you have pretty much summed it up, the two words reasonable comparability across all four is good but not absolute thereby giving us the need to balance the four carbs, unfortunately there is no magic formula to do this and it is just a process of trial and error, myself when i cannot get absolute identical readings at idle but within stated tolerances i then see what it's pulling at the higher rev ranges, or we could take our bikes to a dyno and they could do it all for us, but where's the sense of achievement in that.

K2-K6:
I'll respectfully disagree with you kevski on this.

If you've set the carbs on a bench to parity,  then when you run the engine at idle it shows a difference when you measure the vac levels, the tool for modifying that characteristic is definitely the idle circuit. This will speed up or slow down that cylinder and change the vac reading you obtain.

Just for a moment if we consider what's happening there, the idle circuit if it has any differences from one carb to another (whether manufacturing or cleanliness) will because of its overall size have quite an influence over the idle speed of that cylinder. This is the right tool for taking any differences into consideration.

If we go back to the method of adjusting the main carb slide to correct the idle setting,  then as you lift the throttle away from idle (and the idle circuit ceases to contribute to the mixture now used) that adjustment you've just made becomes a mistake from the terms of cylinder parity. The other end to this could,  as there are other questions about this on here, be why is one of my header pipes / plugs running a different colour to the others? Because you've potententially just built in an error to main running mixture while attempting to get the idle smooth.

I believe the two parts need to be understood for what they do, yes,  adjusting one will affect the other but you'll possibly end up with two errors that compromise the smooth running you're after in the first place.

kevski:
How can you respectively disagree with something you said that i agree with, in my first reply on this topic I made reference to the fine tuning via the air screw for the pilot circuit, regardless of bench syncing there will be small differences that may or may not be adjusted with a combination of vac gauges and pilot circuit adjustments, either way it's still trial and error until we get running how it should.

K2-K6:
I don't think you've got my interpretation quite right in what you're saying about it.

To make it clear,  what I'm saying is that having set the carbs to have the slides equal,  then adjustment of the slides should not be used to alter the idle balance at all. The idle screws aren't fine tuning of the main slide position, they are wholly and singularly the adjustment method to balance the cylinders at idle revs.
There is no conflict of what I've said so far.
It maybe trial and error, but I think to use the right adjustment is fundamental to achieving the ideal result.

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