Tuning ideas for 4776 DP on a 327.

[H.O.rider]

Thanks as always for the quick reply! Just to be clear, this happens on the 1850-2 and the 4777 DP. And only when 1) Idle air screws are set to less than 1.5 turns. 2) When accelerating with some spirit, the gas pedal is let off and the clutch pedal is depressed.

If shifted below 3000 RPM, driving more gently, this does not occur. Just when driving with more pedal. At 3000 RPM, my centrifugal advance is all in. When clutch pedal is depressed My vacuum advance will be all in, 8°-9° advance with my current limiter. That would give me 35°-36° + 8°-9°. Roughly 44° advance which gets me no trailer hitching or pinging.

I can eliminate this pop by turning the screws to 1.75 turns or greater even though max vacuum at idle is 1 to 1.25 turns. I feel the primary jets are a touch lean at 66. In no way does this contribute to my issue? I’m planning on trying 68s regardless. Rob

[Gaz64]

The main jet only limits maximum flow rate into the main well. The main jet at 66 is about 4 to 4.5 times the flow rate of the idle feed restriction. My bet is you could fit 98 main jets, and still experience this part throttle lean condition. The 98 mains would go extremely rich as the main circuit comes on. This is not the area of the fuel curve you're chasing. You've already proved this by opening the mixture screws. You should also make sure you have NO exhaust leaks. Gary

[H.O.rider]

I guess those exhaust leaks fix on their own when the idle air screws are turned out a 1/2 turn.

[Gaz64]

So you have exhaust leaks that you know of at collector joints, etc.?

[H.O.rider]

Ended up with 69s in the primary. Car runs cooler and smoother. I can run the idle screws at 1 turn and get no popping. Transitions through fuel circuits seem great. Really liking the 600 double pumper.

[Gaz64]

69s are stock in a 4776, 66s in 4776-2 onward. So yours could be a leaner version (-2, -3 etc.) than earlier version of original 4776. A bit small for a 327, I ran a 4779 (750) on a 308. Gary