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Tough Tuning Question


240260280z

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L26 w round tops identical and new needles... jets ok... hoses to jets a bit suspect as they bent a little... pistons moved freely ... no choke cables attached during tuning .... nozzle in correct locations... linkage working fine.

Initially the back carb was doing most of the work and it balanced better by adding more fuel to the front carb at the same air flow.

After some exploratory work it was discovered the PO did pass the car along with a good valve adjust. The front 3 cylinders were ~ 100psi and the back ~ 150psi. A valve adjust brought all up and mostly ~170psi.

After the valve adjust, it balanced better by adding more fuel to the rear carb at the same air flow???? The problem moved... but remained...poor balance

Fuel bowl levels were OK. Timing was ok.

Any idea?

It was interesting to see how valves affected carb tuning.

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If adjusting the valve clearances increased compression, there must have been a valve not fully closing. The fact that both carbs were flow balanced would suggest a leaking exhaust valve.

Enriching the mixture on the front carb to increase power would indicate initial mixture was lean. This is why you enriched the mixture in the back carb after fixing the valve problem.

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Thanks! I guess I should add more detail: The power now balances with ~4 extra turns down on the rear nozzle, it was the reverse before the valve adjust. All SU's I tuned to this point power balance within 1/2 a turn difference.... something is amiss. I'm wondering if the odd fuel lines between the nozzle and fuel bowl are causing the grief. JFA had a problem recently that is making me think it could be in common.

FYI most intake and exhaust valves were tight and the plugs had a crispy coating. Another guess is that the re-lashed valves may have beat off crust and carbon upon their restart to rise a little and reduce the lash again.

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Wow, that's a big change in pressure.  Pretty good example of why valve lash is the very first thing in the Engine Tuneup chapter.

 

You can get low compression also, if the intake valve doesn't open soon enough.  It's the air that pulled in on the intake stroke being pressurized that causes the cylinder pressure.  Less air to compress = lower pressure.

 

So your adjustment caused the front three to pull more air through the front carb.  I know very little about carb tuning.

 

 

Very little.  It's like more vacuum.

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I don't believe that timing light. 32 degrees at idle would surely ping off idle.... or the timing marks are off... we did not verify so the PO could have left us another thing to fix like the cam oiler.

As well timing would affect all cylinders.... it is a mystery for the moment.

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My timing is around 32 degrees with the vacuum advance connected. Drops to about 16 degrees without.

I would look for a vacuum leak. Balancing the air past the throttle plate with one carb richer than the other means extra air from somewhere.

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I'd say you are fighting multiple issues.

If that float hose is bending replace it or it will impede flow.

As far as the flow balance, don't forget these manifolds have a balance bar that will attempt to even out vacuum issues between carbs. It's a rather large pipe too.

I don't know what to say about the timing. So at idle 32 degrees- really?

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Thanks for the ideas.

We sprayed the intake, balance tube and carbs thoroughly with starting fluid... no raise in rpms so no apparent air leaks.

The observation that the front carb initially worked better with more fuel and then after the valve adjust, the back carb needed extra fuel to balance would mean an air leak jumped. We did not do anything to the carbs or intake after the valve adjustment so we could not have caused an air leak.

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Well the timing light showed 32 at idle but it did have an adjustable pot as part of the circuit and it had a flakey switch so it may be out of calibration...especially since that was with the distributor in the middle of its adjustable range. I swung it to max for a run and no ping. It just lost power and the typical contrary starting was noted. I also tried minimum range and a loss in power. it worked OK in the middle so I left it there.

We did not have time to check that the timing marks on the crank pulley and scale were aligned with TDC so the weird 32 number could have been due to the light out of calibration or the scale being off or a different pulley on the car.

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Can't tell whose car is what, but if you have full time vacuum on the advance canister, you could see 32 at idle, which will drop to 32 minus max vacuum advance (the 16 djwarner saw) when you open the throttle.  Most people run ported vacuum to the vacuum advance canister.  Or disconnect the vacuum hose when checking timing.  16 initial isn't unreasonable.  That would give 34 total at open throttle, high RPM, with 18 degree (not uncommon) weights.

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