Carb sync

[royhall]

Does anyone have a simple and effective step by step way of syncing the carbs on a 350/4. The manuals all describe slightly different ways of doing it. They appear to be very sensitive to each other, a quarter turn on one seems to disturb the others a fair bit, so I'm chasing my tail with it. All was very simple on the 750's, but not on this. Think I may have missed something somewhere. Cheers.

[kevski]

I have also found that the 350s and the 550s are very hard to get spot on, my 350 is to within 1cm hg and then fine tune using the air screw, i have always found it easier on the bigger bikes and more modern bikes, i think it's just a case of plugging away and using either number 2 or 3 as the bench mark carb.

[royhall]

On the 750 I usually take the bottom of the air box off and the air filter out before syncing. Is it required on the 350/400 to take off the airbox manifold first, as its a royal pain to get out. Or can I get away with just taking the filter out. Cheers.

[K2-K6]

I don't have a reference for the tolerance on these smaller fours, but the 750 is given as 40mm (1.6 in) Hg.

[hairygit]

On the 750 I usually take the bottom of the air box off and the air filter out before syncing. Is it required on the 350/400 to take off the airbox manifold first, as its a royal pain to get out. Or can I get away with just taking the filter out. Cheers.

Not sure that's such a good plan Roy, the idea is to set the motor up to run properly as it will be on the road, removing the air filter to set them will be wrong when the filter is refitted, as with the filter out, all carbs get a straight supply of air, and when the filter is put back there will be resistance to airflow, but some carbs get a fractionally higher flow than others, messing up all your work!

[kevski]

I don't have a reference for the tolerance on these smaller fours, but the 750 is given as 40mm (1.6 in) Hg.

I think there is a parameter of 2hg on the smaller fours, around the 22 mark, but it can be any number as long as the difference between carbs does not go out of the 2hg parameter, the air boxes and filters should be left on.

[gtmdriver]

I had them off the bike for servicing so I synched them on the bench using a 3mm drill under the throttle slide on the engine side of the carbs When I fitted them and fired the bike up with the vac gauges connected only one of them needed a minor tweak for them all to be spot on.

Don't forget that balancing the carbs is the last thing you do after setting the ignition timing and the valve clearances. If either of these are out you won't get them to synch correctly.

[royhall]

It does say in the official Honda manual to remove the air intake housing before syncing. Haynes and Clymer don't mention this. Honda book states 3hg tolerance. Have been through the rest of the settings first but I think I will go through them all again just to be sure. Raising pressure on one cylinder is dropping the other three quite a lot, more than I would have expected.

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[kevski]

I find that odd as the air box stabilizes air flow to the carbs, maybe try without the filter at least then there is less resistance.

[royhall]

I agree Kev........ I would have thought syncing should be done as near to real operating conditions as possible. Thats really why I started this thread to try to find out how the pro's do it. This is the first time I have done this on the smallest four so need to ensure I am doing it the best way........ On the 750, airbox on or off doesn't appear to make any difference in the syncing (set them up with the filter and lower airbox off then rechecked with them back on)......Maybe due to the small size of the carbs I am making too big an alteration at one go. I will leave it for today and see what the replies bring. Thanks all.

[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.