I've got my engine running quite well now, cept for one little problem. Sometimes when I get off the throttle, my idle drops too far and seems to stay there. I gentle tap of the gas pedal clears it and then it's fine.

I've seen dashpots to slow down the carb return to idle position before, but not on a Holley, which is what I am using.

My idle mixture screws are set for max vacuum, so I'm unclear as to how the engine seems to be loading up with fuel when I get off the gas.

To complicate matters, my dist is actually a bit advanced at idle, so I'm wondering if that may be part of the issue, but seeing as how a quick stab of the throttle clears it, I am leaning towards a fuel issue.

I have one thought so far. Float bowls flooding under acceleration. Fuel flows to back of bowl, floats drop a bit, fuel flows into bowls, and when I let off the gas and car stops accelerating, resultant fuel level is too high and idle is rich. If this is the case, I'm not sure what to do about it as lowering the float level may cause other problems.

The problem does seem more common when I have accelerated up a hill, but I need to re-check that. And now that I think about it, I'm not sure, but I may have had this problem on and off before, but not as bad or as often. However engine is up on power (due to tuning) than it ever used to be, so car is probably a bit faster.

Oh...it's a twin Holley set-up.

Any thoughts?
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I wonder if actually your idle mixture might be a tad lean and the throttle is pumping a little fuel into the intake and that is enough to richen the mixture and stabilize the idle. Highest vacuum often means it is as lean as possible, with your combo it might be a tad TOO lean. You might also double check the throttle return springs, idle speed screws etc. for any chance it might be hanging slightly open the majority of the time.

Are the carbs vacuum or mechanical secondary?

 

Carbs are vacuum secondary. Linkages are all good. Could need a little richening, however when I do get that low idle, it sort of sounds and feels like it is too rich then.

These carbs are a bit different in that they have an externally mounted primary float bowl vent that is operated via a lever from the throttle bellcrank. I tired to find a net pic and provide the link, but no luck.

As the carbs are mounted backwards in a 2x4 situation, teh primary bowls face the firewall, so these vents are on the rear floatbowls. I notice that there are signs of evaporated fuel around these vents. So if that's due to fuel slosh under hard accelration, where does the extra fuel go from the front (secondary) float bowls?

I see Holley has some baffles, screens, whistles etc to help fix float bowl flooding, but they are mostly designed to work with the carb facing forward, not backwards like in the Ford 2x4 systems.

Anyway, no tinkering yet today. Must try a few things and see how many facts I can get to help figure this thing out.

 

You mentioned timing changes. If the advance plate is moving around, then it might be sticking UP a touch, then when you give the throttle a quick tap, it then allows it to fully return to the low stop and give you a slower idle speed.

 

It's sort of the otherway. Advance starts around 600 rpm, but I have my idle set about 850.

I managed 1 test today. Got the low idle after letting off the gas, and waited. After about 10 to 15 seconds (I'm guessing, but at least that long) the idle slowly crept up to normal. I would think lean would kill it, so I'm still thinking momentary rich after return to idle.

But I'll check out the mech advance again and make sure it's not sticking. Thank-you.