Carb balancing - what affects balance

[AndrewSlater]

After my 912 engine rebuild I checked the balance of my Solex 40pii-4 carbs using a synchronometer and found that both sides were well balanced but on both banks the backs were running about 7% higher than the fronts.

Now that I have run the engine in enough for its first oil change, I thought I would try to reduce the front / rear imbalance by trying the technique of slightly tweaking the butterfly shaft appropriately.

Before I do this I just wanted to check what other factors might alter the balance on the carbs, in case there is something else I should check first.

To my mind other than the throttle butterfly position and inlet valve operation the only other factor would be the temperature of the individual cylinders, but I'm not sure how much this can affect things. The hotter the cylinder the lower the mass of air it can draw into a fixed volume as the air will heat and expand on entry.

So the question is if my mixture was slightly different between neighbouring cylinders could it show 7% of imbalance on my synchrometer or would the affect be a much lower percentage?

The car runs well and I believe valve clearances and timing are all correct, but possibly I could go around the buoy again on the carb tuning although I'd be surprised if it was far off optimal.

Any thoughts welcome.

[KS]

Bear in mind that the cam lobes are shared by opposing cylinders. If an inlet lobe is worn then it will affect opposing cylinders in equal measure - worn lobe means less valve lift. Even if the valve clearances are correct, there will still be a disparity compared to the other two cylinders which use the other cam lobe.

[neilbardsley]

I'm interested to know the answer to this question. When I've balanced my Webers I used the air bypass screw to balance them. I like you have thought it means one butterfly is opening become the other but wondered is cylinder compression can contribute to how much air is draw in?

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[Mike Smith]

After my 912 engine rebuild I checked the balance of my Solex 40pii-4 carbs using a synchronometer and found that both sides were well balanced but on both banks the backs were running about 7% higher than the fronts.

Now that I have run the engine in enough for its first oil change, I thought I would try to reduce the front / rear imbalance by trying the technique of slightly tweaking the butterfly shaft appropriately.

If you have the correct Carbs for a 912, the Butterfly`s should be individually adjustable between the back and front throats
Look at the shaft between the throats and you will see the adjustment screw
There should be a small square Rubbery/Plastic block also at this point - It is often missing

See
http://www.356carburetorrescue.com

[AndrewSlater]

Hi Keith, thanks for that info - though having stripped my engine and checked my cam it did appear to be within spec so don't think that is the issue.

Hi Mike, I'm guessing you are referring to the split shaft Solex carbs - mine are the original solid shaft ones so no balance adjustment that I am aware of, only the procedure to tweak the shaft.

I could leave the misbalance at 7% at idle as the percentage imbalance drops considerably at higher rpms - but thought I could final tweak as it was back in my workshop.
Thought I would ask what the largest contributing factor was likely to be.
I could deliberately change the mixture on a cylinder to check if I can see any effect on the synchronometer.

Thanks for the feedback so far.

[neilbardsley]

Andrew what are using that is letting you know the flow difference in %?

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[Ashley James]

I’d remove both carburettors and use feeler gauges to establish both butterflies are shutting completely on each side. I had a similar problem because someone had already twisted the shaft. It’s quite difficult to get them back to where they should be, so I’d check the butterflies are centred correctly and that the shaft isn’t twisted already.

[AndrewSlater]

Andrew what are using that is letting you know the flow difference in %?

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Hi Neil,
just a synchronometer, the fronts read 3.5+ and the backs 3.8 ( so <0.3/3.8 about 7-8% ) - I'm assuming the synchronometer scales are linear with airflow.

Ashley - I rebuilt the carbs a while back and they had new shafts fitted so pretty sure they still straight, and the butterflies were adjusted as well as I could at the time to sit central to the bore.

I'm starting to think I might try to reset the mixture to the new engine build and see if any of the flows change. If it is still 7% out at idle I might just put up with it.

[Ashley James]

I’d have the carbs off and use a feeler gauge to make sure the butterflies aren’t over to one side. It takes ages to settle them properly.

[Bruce M]

If you remove & back off the idle stop, you can check with a torch to see if there is any gaps. Depending on linkage & carb design, the shaft can be twisted if the cable is tight and at full throttle you have tension on the carb shaft.

Have you got the mixture screws adjusted to a clean idle. If not get your rubber hose out and use Iain Tyrrell’s method

[neilbardsley]

If you remove & back off the idle stop, you can check with a torch to see if there is any gaps. Depending on linkage & carb design, the shaft can be twisted if the cable is tight and at full throttle you have tension on the carb shaft.

Have you got the mixture screws adjusted to a clean idle. If not get your rubber hose out and use Iain Tyrrell’s method

I've tried the rubber hose and it didn't help me but then again I struggle to tune a guitar by ear too

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[Bruce M]

What were you listening for?