OFF idle issue

[Jeffs68]

So here's the basic info.
Buick 7040244, converted to Pontiac primary linkage, Cliff supplied standard kit, with internals for a Pontiac 400 (stock, 16 heads, headers MSD ignition with an HEI)

Internal parts are:
42 Primary metering rods
Green power piston spring
69 Jets
Secondary rods DA
Float adjusted per Pontiac spec at 1/4 inch.

Starts and runs good, Idle mixture screws seem to be happy at 2 turns out.

Initial timing is at 13 degrees, about 36 all in.

When you cover the air horn with your hand to choke it OFF, it has very little to any RPM drop.

Also when you snap the throttle, it drops a few hundred RPMs lower than the set idle speed (almost to a stall) before returning to set idle.

When in gear and you initially give it throttle, it feels like it's loading up (gurgle sound from exhaust) with a slight hesitation, then it wants to take all you can give it, very smooth power delivery.

So, the APT screw is not installed due to the factory screw being completely wiped out (my fault)

I'm thinking, float adjustment? bigger primary metering rods to tame it down a bit?

Bowl plugs have been epoxied, no vacuum leaks, primary shaft bushed.

Ideas, criticism and snarky comments welcome 

[Cliff Ruggles]

Is the vacuum advance hook up?  Ported vacuum?  How much timing is it adding right off idle?.....Cliff

[Jeffs68]

Yes, it's connected to manifold vacuum.

I'd have to check to see the actual advance OFF idle.

[77cruiser]

Do ya think with the APT screw not in the rods are all the way down & it might be too lean?

[Jeffs68]

Do ya think with the APT screw not in the rods are all the way down & it might be too lean?

I suppose that's possible, I've never dealt with adjusting the APT before.

[Cliff Ruggles]

Without the APT functional you may have to install small rods instead. The "B" series rods were used in those carbs with the early APT system, and they really start to richen things up nicely as you raise them up and out of the jets and position the tapered section between the upper/lower sections further up in the jets.

I did that same carb number recently for a 455 Buick Stage one with a T/A cam in it.  We had the vehicle here, and I found that jet/rod combination perfect, but we did have the APT working and fine tuned it on the engine. 

In lieu of not having the APT screw out of the baseplate, another option is to bend the wire that the power piston post sits on up slightly.  It may take several attempts to find the ideal height, but still beats taking the top off the carb and changing parts......Cliff

[Jeffs68]

Without the APT functional you may have to install small rods instead. The "B" series rods were used in those carbs with the early APT system, and they really start to richen things up nicely as you raise them up and out of the jets and position the tapered section between the upper/lower sections further up in the jets.

I did that same carb number recently for a 455 Buick Stage one with a T/A cam in it.  We had the vehicle here, and I found that jet/rod combination perfect, but we did have the APT working and fine tuned it on the engine. 

In lieu of not having the APT screw out of the baseplate, another option is to bend the wire that the power piston post sits on up slightly.  It may take several attempts to find the ideal height, but still beats taking the top off the carb and changing parts......Cliff

What rods do you recommend?

[Cliff Ruggles]

I'd try a set of 39B rods

[Jeffs68]

So, I've got the rods coming, as soon as I cant get to it, I'll post an update.

Thanks!

[Jeffs68]

-UPDATE-

So, got the 39B primary rods installed today, fired her up and made some initial adjustments.

We're getting there, almost no OFF idle stumble, I think I may try to bend up the APT wire slightly per your original recommendation Cliff.

I need to get some other things sorted out, throttle cable, return spring and divorced choke. But we are making progress.

Was going to try going from manifold vacuum on the advance to ported at the carb, what do you think? Easy enough to try out.

[Cliff Ruggles]

I like ported vacuum for most set-ups, unless the compression is low and cam too big, and you find yourself needing a lot of timing at idle to make the engine happy......Cliff

[Jeffs68]

-UPDATE-

So with the new primary rods and bending the APT spring up a bit, I've eliminated all OFF idle issues. Very smooth transition all the way through, hammer down to about 5200 Revs.

Does seem a little fat at idle, but when I cover the air horn with my hand, no noticeable drop off in Revs.

On a separate note, the repro parts place throttle shafts for the Pontiac set-up are garbage. The primary shaft is fine when the engine is cold, but as soon as you get to operating temp, the shaft is expanding too much and binding up (not returning to idle)

[Cliff Ruggles]

I've not been happy with some of those re-pop throttle shafts.  We have had to remove some of the plating from them to get them to fit, and on recently wouldn't fit at all as it had a TON of plating material on it.....Cliff

[Jeffs68]

Just received an OEM one from Mr. Pontiac, live an learn I guess.