Help with a new off-idle stumble.

[skidd]

My car has been running like a champ since January. About 2000 miles on the clock so far. So, this is a new issue. In fact, this stumble was something I noticed in a single day. On my way home one day, stopped at a store for a candybar. Got back in , fired it up, and it just felt different. Off idle stumbled, missfires and more than usual black exhaust. Drove it home no problem, and parked it until today, (was 2 weeks ago). Keep in mind, this is very much a recent thing. The stumble is not a a big one, it's pretty subtle. No backfires, or major coughing, just a not-as-smooth as before stumble.

Engine Details:
1999 Explorer 302. GT50p heads, E303 cam, Weiand Stelth Intake, Summit Racing 600cfm VacSec carb. Echilin E-Core coil; Echlin Chevy HEI Ignition Module, Bosch Iridium plugs and 8mm wires are new when installed 2000 miles ago.

Today.. trying to diagnose the problem.
Went for a short around the hood drive to warm it up, and get a feel for it. Stumble is still there.
Though, she idles perfectly fine at about 800-900 rpm. No indication of a problem.
Very slowly in Neutral, roll up the throttle, and she spins up nice and fast and smooth.
But, stab the throttle and it boggs and sputters.

1st thing I did was swap out ignition modules. I have a Duraspark+HEI conversion. I have a 4-pin chevy HEI module tied into my 1985 Vac Duraspark dizzy. I swapped out the higher quality HEI module for a backup I keep on hand just in case. No change. Off idle stumble still there.

Float Bowls are adjusted correctly and getting fuel. Pressure is at 6psi.

Pulled my PCV hose off and plugged the carb port, looking for a vac leak. Nothing. Did the same for the Distributor Vac. Again, nothing.. though.. pulling the vac sure did make it run like ***. I'm not using timied vac, full vac.

Checked my timing, base timing was a bit retarded I think (8deg), so I bumped it up to 12deg. This actually seemed to make it feel a little better.. like.. the stumble was less.. but.. still there.

Hooked up my vac gauge, and fiddled with the idle circuit jets a bit. One jet was spot on, the other I opened up by 1/4 turn. Went from 15inHg to 16inHg. This too seemed to help, but not cure it.

Jumped in, and went for a good spirited drive. And one thing is for sure, she goes like a bad-out-of-hell when the throttle is being used. I can romp through the gears at 1/2 to full throttle and she'll happily spin up past 6000 rpm and not slow down at all. Sounds great, and pulls like a freight train. Even modest cruise seemed to do excellent with a good stab of the throttle. Slow cruise roll on in lower RPMs.. perhaps a small stumble, but.. most likely due to the E303 and the low RPMs. Needless to say, when driving hard, she goes and goes and goes!!

Pull back into my garage. She idles perfectly. Pop the hood, stab the throttle with my hand, and yeah, stumble.

My best guess was it seemed to be getting more fuel than it could handle. I checked my accelerator pump, and it was a bit tight. Manual says 0.015 gap, there was no gap. So, I adjusted it. Didn't expect that to do much, and it didn't.

Here's one thing I did that seemed to do the most though. I grabbed my vice-grips, and locked "gently" to the accelerator pump arm. Stabbed the throttle, and held the arm in place. Basically, eliminating the pump from being actuated with the throttle linkage. Now, when I stab the throttle with my hand, there is almost NO stumble at all. Hard to say for sure what it's doing, but it's far far more responsive now. I can see some fuel being drawn out the pump nozzles when I look down the throat, but at a far less rate. Almost no bog or black smoke. So, that leads me to think it's somehow related to my accelerator pump. But how? Used to work great and smooth... then just didn't. Of course I didn't drive it with the "vice" grips, so this is just in my garage.

Any ideas what I should look for? More diag ideas? Things to try?

Cheers

[BadAsp427]

While I do not know how often you get to drive the car, I think you may be on to the problem with your accelerator pump. I'm guessing that the temps are much hotter in SA now than they are in January when you first started driving it. Of course hotter air is much thinner than the cooler air and thus you could be just giving it a much richer mixture than it likes now in the hotter temps. You may want to try to put on a more appropriate accelerator pump that is a little more friendly. I remember back in my drag racing days, we were always having to play with those to get it right. Good luck, I'll be watching as my build that I just started will include a 347 Carb motor.

[skidd]

Thanks for that.. yeah.. it's been pretty hot here the last few weeks, and I have actually been driving my car a lot. I commute in it! :)
It's a little more $$ than I would have thought, but.. Summit makes an Accelerator Pump Tuning Kit.
https://www.summitracing.com/parts/sum-m08040
I think perhaps it's worth a try.

[johnnybgoode]

Low speed tip in is usually a lean condition. If it was running well originally and then all of a sudden the problem started then I would start by checking for dirt in the carb/fuel bowls/needle/seat. It's fairly easy to disassemble and blow out all of the passages with compressed air. After that if the problem persists I'd try richening up your idle screws 1/8 of a turn and see if that improves the throttle response or not. If the problem gets worse then go a 1/8 turn leaner. It's also easy to change the squirter to a larger or smaller size depending on what you find through the idle mixture adjustments. A couple of quick thoughts for what it is worth. Scott

[NAZ]

skid, the condition you're describing would normally be attributed to too much fuel from the accelerator pump circuit (black smoke = too rich). But what's changed to cause this condition?

If this was a Holley I'd first have you check for loose squirters and even a loose power valve as loose can cause leakage around the seals. A blown out PV diaphragm will cause a rich condition so bad you'd have trouble adjusting the idle but a small leak at the seal may cause a slightly rich condition if it allows a small amount of fuel to flow through the PV restrictors. Once you've checked the basics then you may be able to tune out of this condition with squirters, pump cams, or other adjustments the Summit design has to offer. If it were a Holley I'm very confident you could tune out of this condition. At any rate, I believe you're on the right track looking for a rich condition caused by the accelerator pump system.

[skidd]

That's my exact question.. what changed to cause this condition. I'd been driving in days every bit as hot, if not hotter with out any noticeable issue. So.. it could be the temps I guess.. but.. dunno.
I can't say I love the idea of dropping $50 on some parts that "might" fix the problem. but.. then again.. if they DO fix the problem.. I win!

[Railroad]

You might cure the problem by just disassembling the carb, less any torn gaskets.
You should not have any gasoline dribbling out of the accel pump nozzles, unless the pump arm is moving.
I would check for trash under the ball check valve or check valve weight located under the nozzle retaining screw. Next would be the ball or umbrella check valve in the pump housing.
I do not guess you have any blockage on the bowl vent tubes?
I have never had a power valve go bad, but worth checking. It could let you be running too rich and the accel pump shot put you over the limit.
With your idle mixture screws being relatively close, probably not the problem.

[j.miller]

I'm betting vacuum leak on a ported vacuum line or crack in a rubber plug... I lay 3/1 odds....any takers...(and they wonder why my legs keep getting broken)...da bat

[CraigS]

Do you have any friends w/ a wide band O2 sensor. It's a lot easier to diagnose if you know whether it's rich or lean.

[seagull81]

You have a PM

[scottiec]

Do you have any friends w/ a wide band O2 sensor. It's a lot easier to diagnose if you know whether it's rich or lean.

This ^

I was able to solve a lot of problems with one.

[skidd]

Do you have any friends w/ a wide band O2 sensor. It's a lot easier to diagnose if you know whether it's rich or lean.

that's a good point. I have an Innovate LC1 in my Subaru.. but.. way to hard to remove just to hook into my cobra for a bit.
But.. I'm a member of the local AutoX .. so.. I'll post in there to see if anybody has a more portable one. I'm sure someone does.

[scottiec]

that's a good point. I have an Innovate LC1 in my Subaru.. but.. way to hard to remove just to hook into my cobra for a bit.
But.. I'm a member of the local AutoX .. so.. I'll post in there to see if anybody has a more portable one. I'm sure someone does.

I use the innovate lm2. I used to clamp it to my exhaust, but that got tiring. So I had a bung welded to the collector on my side pipe.