SOHC.co.uk Forums > CB350/400

The elusive surge

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Alexrayz:
Hey Dennis, thanks for all that sweet sweet info, I think I will do the vacuum gauge sync next time working on the bike. Seems like it will clear up more questions and hopefully solve my issues.

I was actually wondering if there is a proper methodology for the mixture screw settings. I know that the look of the plugs is a telltale sign but I was wondering how precise it gets without requiring an IR exhaust gas probe.

Cheers!

K2-K6:
Certainly it's interesting to gain the contributing views already expressed, and hopefully add to the picture.

Generally these engine don't "surge" as such with a std setup but would more indicate something amiss that has not been quantified.

The reference to bench setup in the tips section is to very particularly include the routine contained in establishing the correct offset between carb slide position and overall idle speed control mechanism. I'd encourage anyone to fully apply that in rebuilding these carburettor to prevent effectively unseen errors while using.
Without that,  you can build in a mechanical conflict that becomes apparent as you try to set the idle rpm speed with the adjuster as it's possible to force a shift in balance between the 4 carbs by doing this. It shouldn't be there, obviously,  but completing that routine will remove that potential.

Carb balance is not in reality set for idle speed. That may seem contradictory, but it's primarily to set parity of slide height for wider open settings with idle speed smoothness a side effect resulting from this adjustment. 

Honda gave in their manuals at the time a routine to then fine tune each individual cylinder's speed at idle to trim that point of running,  once the parity of slide has been established, which is micro adjustment of the airscrew individually.

I'd,  at this point in how it's running, argue not to set the sync using gauges. If you've a problem on one cylinder that's not fully identified, then setting by using vac guages can put a compensating error into your carburettor balance that's not needed. Yes it may sound better at that point, but it's wrong with other mechanical errors needing attention first.
Pre requisites for vac synchronisation are that all other parameters are within adjustment before that is employed.

Bench synchronisation avoids the above as it's independent of engine condition and running problems residing elsewhere, also perfectly good enough to run the engine accurately.  Guages don't hold the upper hand here and it may help to diagnose faults by leaving accurately "bench" set as detailed.

Listing order of work, bench sync as tips post on here, then assess what you've got without further changes. It may influence the condition, but you can only check that one way.
Then look at idle air setup in manual and complete that to assess effects.

K2-K6:
"Dennis

PS  Just thought I would add a small note to explain that for example in the same period (1970 - 1980 )that with a 6 cylinder car engine with 2 or 3 Carbs, to overcome a tendency for imbalance on idling the manufacturer would tend to put a small balance pipe connected across the manifolds so that all cylinders could suck Air/Fuel mixture, even though the individual Carbs may not be set spot-on."

That's generally used on SU, Solex type constant velocity carburettor and not essentially to promote mixture sharing but to give vacuum parity across the different intake tracts as it's the vacuum variation that would cause the fuel air metering slide to run at different heights in spite of each throttle butterfly being equal in position. 

These engine having a fixed relationship from one slide to another don't  manifest that variance, and so don't need it.

McCabe-Thiele (Ted):

--- Quote from: Alexrayz on January 10, 2022, 04:39:07 PM ---Hey Julie, finally got time to search for it, I knew it was buried somewhere in my mailbox.

The plugs are NGK D8EA and the caps fitted on it are XD05F and VD05F. The leads are originals, same as the coils.

Ted: I did do that, I have a daytona electronic ignition trigger, there is an LED that turns on to show when the sparks will happen and I'd say it is right on the dot. It was a static test though.
You'd recommend getting a stroboscopic light?

K2-K6 I have bench synced them yup. Some lad at work advised me to do a running sync to make sure everything is tip top on that front. Even with a bench sync you can get inbalances that throw off the whole thing.

Daz-J did you check float heights?

Thanks for your help guys, means a lot!

Take care,

Alex

--- End quote ---


The bonus of a strobe check is you will see the timing fluctuating - that said it would not get you any further forward as a carb inbalance causing a surge would probably bring in the advance/retard mechanism thus showing timing fluctuation. Someone here will know at what r.p.m the advance trips in.
Sounds like you have it right statically with your method.

I did my timing statically initially and then dynamically - there was a very slight difference between the pairs of cylinders,  after adjusting 2 & 3 it made no obvious difference though I was not able to ride it on the road to check for what it felt like in use.

Bryanj:
Should be on full advance by 2500 rpm

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