Idle Stumble Issue '71 Corvette

[Kubs]

Last year I rebuilt the original 7041213 carb on my '71 Corvette 4 speed.  I bought a kit from Cliff and cleaned and/or replaced all items back to factory settings (meaning no jet/rod/tube changes).  I adjusted the idle mixture after getting it back on and it ran great.  A few weeks ago I installed side pipe headers and side pipes with no mufflers.  The car is otherwise all stock.  The car still runs and drives great but there is a small stumble off idle when you rev it, or are cruising below 2500 rpm and try to add any amount of throttle (whether you are getting into the secondaries or not).  It recovers shortly after and will rev or accelerate after the little stumble.  Based on a plug reading I think the car is running slightly lean in general, but I think think the stumble is from being too rich.  When cruising on the highway at 3000+ rpm and punch the throttle the car takes off.  No stumble on the transition or any throttle input and strong throughout the rpm.  Me being not much of a carb guy Im not sure where to look first. Would it be as simple as an idle screw adjustment, or am I looking at changing idle air bleeds?  Would increasing the primary jets to fix my lean condition make this worse?  Thanks for the input.

[Cliff Ruggles]

Most like a lean condition at light throttle. 

Several ways to cure that issue.  The APT system in the baseplate can be employed if you can get the factory screw out, we make an external screw to replace it.

A jet and metering rod change will help, but I like to have the APT system in place to fine tune after the carb is placed back in service, saves time pulling the top off and changing metering rods......Cliff

[Kubs]

Thanks cliff!

So your APT screw would add that function to my carb that didn't have it originally?  I'll have to check that out, and maybe try up 2 sizes on the primary jets?

[Cliff Ruggles]

The APT system is there, the factory set it and then put a cover over the screw.

We remove the factory APT screw here and replace it with an external screw for fine tuning on the engine without taking the carb apart.  It is a great feature, but that screw can be difficult to remove.

I use a little heat, and good fitting screwdriver, and plenty of penetrating oil.  Never try to turn the screw until it's ready to come out, they break easily.

Record the factory jet/rod numbers, and call the shop when you get the APT screw out, or even if you don't, we can correct the off idle issue with a better jet/rod combination......Cliff

[Kubs]

I ended up going up 3 jet sizes to 0.077 with 0.045 rods (over the factory 0.044) with the same 0.026 power tip.  Basically up 2 jet sizes at idle/cruise and 3 jet sizes at WOT.  After re-adjusting the idle settings the throttle is more responsive and pulls hard up to redline.  Stumble is gone!  Thanks for the input Cliff.  I figured I would try the jet/rod change before messing with the APT.

[Cliff Ruggles]

That's the only way to tune if you don't use the APT system, but the top has to come off and you don't have fine control of part throttle A/F externally.  We always try to remove the factory APT screw for this reason, but on occasion they will just not come out, so we have to basically do the same thing, change jets and metering rods and do some testing.....Cliff

[Kubs]

Long term I have another engine going in this car with a Holley on it.  I wanted this engine to run correct while Im still enjoying it but didn't want to invest too much into it, or take the carb back apart to do the APT mod. Taking the top off and trying some jets I already had took a small amount of time and cost.  If I were keeping it this way I probably would go the APT route for fine tuning.