Carb balance Q

[Gary Schulz]

I was checking things out on my car today and decided to raise the idle speed a bit to get things a little more smooth. I checked the airflow using one of these-

20200711_105206.jpg

For reference, at idle I am seeing around 6 kg/hour flow rate. Just for the heck of it I wanted to see how balanced the flow stayed at higher throttle settings so at a fixed RPM of 1500 I wound up with around 7 kg/h on the right carb and someting like 12 to 15 kg/h on the left carb. As the RPM's were increased to 3 thousand or above it seemed that the airflow of both were closer matched but the right carb was still a little lower.

Is this a big problem at anything other than idle? You can see the left carb piston does come up higher at 1500 RPM than the right one. Is this a case where the carbs just come up based on demand (vacuum) for each pair of cylinders? Car has twin HS-2's and standard manifold with balance pipe...

Engine seems to run very smoothly at all RPM's so I am not really diagnosing an operational problem per se but was just curious.

[iain1967s]

Do both carbs have the same colour spring in them?

Try swapping the springs between dashpots, see if the situation reverses.

[imack]

I'd try swapping the springs around in the carbs and see if the higher flow moves to the other carb.
EDIT: beaten to it!

[Gary Schulz]

Both springs are brand new red springs. I can try to swap but I went with new parts since the old ones were unmarked...

[bwaminispeed]

Brand new springs are quite often not even close to being the same........

[Gary Schulz]

I will swap them around later today and re-measure. I suspect that since the rate on these springs is so low compared to the force generated by the large surface area of the piston that springs are of little importance really.

I hope I can prove myself wrong however...

[Gary Schulz]

Simple test and a good suggestion. Left carb still has substantially higher flow even with springs swapped. I guess I can swap pistons next?

The real question remains- does it really matter? I think the carbs are just seeking out equilibrium by maintaining constant "depression" or pressure drop across the carb. Not sure why one is acting differently than the other though...

[Spider]

The problem is more related to the Manifold and the huge coff coff balance pipe that has.

To perfectly match the carbs not just to each other but also to cylinders 1 & 2 and 3 & 4 would be a mission on it's own, so that they flowed perfectly the 'same'. You'll find that all twin sets on Manifolds that are factory items or that have big balance tubes will do this and with changes in temperature, it's possible that they may 'swap' where the now lazy one flows more.

[Gary Schulz]

The problem is more related to the Manifold and the huge coff coff balance pipe that has.

To perfectly match the carbs not just to each other but also to cylinders 1 & 2 and 3 & 4 would be a mission on it's own, so that they flowed perfectly the 'same'. You'll find that all twin sets on Manifolds that are factory items or that have big balance tubes will do this and with changes in temperature, it's possible that they may 'swap' where the now lazy one flows more.

OK so you are saying "don't worry about it"?! I guess as long as they are balanced at idle that is the best we can do to control the situation...

[Spider]

Sorry Garry, yes, don't worry about it, even at Idle, I can't see much point in agonizing over getting them 'balanced', if you had a Manifold with a small balance pipe or no balance pipe, then yes, definitely.

The will both start to come 'in to balance' with each other those, when (if) you get well up in the flow rate of the carbs.

[Gary Schulz]

Good to know that I am chasing my tail! Thanks for your input.

[abri]

Have you tried doing a drop test to see how fast the pistons drop relative to each other?

Could be that the dashpots got swapped around. If I understand correctly these carbs have matching sets of dashpot/piston combinations and mixed sets can give inconsistent results. Just something I read - happy to be corrected, but easy to check.

[Gary Schulz]

Yes, I had looked at the relative rate of drop for each dashpot and they both seem pretty much the same. As I think Spider has mentioned, demand for the engine comes from contributions from both carbs especially given the very large diameter balance pipe on the stock manifold. At idle where demand is low it looks like airflow is more "deterministic" so balance is trivial. As RPM's increase I think the issue becomes far too dynamic to even think there would be any real isolation in function between the two carbs. The more I think about it the more I am convinced that it is an exercise in futility to assume their functions are at all isolated.

As was said, if they were on separate and isolated manifolds the story would be different.

[Charlynsane]

You should try to check if your butterflies are properly centered in the carbs