Carburetor stumble

[Aron Gleason]

He says he has an afr gauge on a post.

how much is this car street driven or raced? Freeway or idle around at the cruise night like In Enumclaw? does it overheat? Lean overheats easier.

This year I've put 350 miles on it. And yes I've taken it out to Enumclaw a couple times. Mostly to issaquah and back to Renton.

 

[Aron Gleason]

It looks like you have been chasing this issue for a while. It might be time to get a good, wideband AFR meter to give you a better idea of what's going on.

A stumble like you describe is usually too much or too little fuel. Either condition can usually be resolved with a little carb tuning, but it's hard to tell which one you're dealing with. The AFR meter just lets you cheat a little. If the meter shows 9-1 when you stumble, the carb is too rich, if it shows 15 or 16-1 then the carb is too lean.

When it stumbles it goes lean. 15+ on the afr gauge

 

[Aron Gleason]

I hope the lobes are hanging in there.

It's got a good idle and pulls hard to 6300 rpms

 

[Aron Gleason]

Now you are on to something! The BIG clue it it does the same thing with 5 different carburetors.

What compression ratio?

how much power and torque on the dyno? 4.5 gears and 4500 converter are too much for 1/4 mile and that tiny camshaft.

With that converter and gears 15+ years ago it ran 12.16 @109. And it wasn't running half as good as it is now. Plus I've upgraded the fuel system with a return line. And many other upgrades, better ignition.

 

[Aron Gleason]

What is in this car?
cam
heads
ignition
converter
gears


if they all stumble then it’s probably not a carb problem.

Do I know you? Have I talked to you?

 

[RemCharger]

It's got a good idle and pulls hard to 6300 rpms

That's good, but bum lobes can be deceiving. Keep it on the back brain burner.

If you've tried all the pump shot tuning you can, it wouldn't hurt to *try* plugging the PV and square jet it.

 

[Bee1971]

What intake manifold are you running ?

Any carb spacer ?

 

[Aron Gleason]

What intake manifold are you running ?

Any carb spacer ?

M1 single plane intake with a 1/2 inch spacer.

 

[R413]

Do I know you? Have I talked to you?

No but I guessed you would go to enumclaw cruise, heard it’s the place to be.

 

[Aron Gleason]

No but I guessed you would go to enumclaw cruise, heard it’s the place to be.

Yes it is. If you're close, the Sumner cruise is the 15th. I'm planning on being there.

 

[Aron Gleason]

Are the ifr jets the same thread size as the power valve jets? And where can I find them?

 

[Aron Gleason]

In reading on pump cams. I read where the blue cam is the most aggressive cam. Would it be beneficial to put a blue cam on the primary and secondary accelerator pump? Then go up on power valve restrictors?

 

[Project66]

Sorry guys, I missed the AFR reference on my first pass through the thread.

It looks like lean stumble to me. I would make sure that the accelerator pump arm was properly adjusted. If it is, then I would go up a couple of sizes on the squirter (accelerator pump discharge nozzle) to see if that helps.

 

[Aron Gleason]

Done that. But I was reading about the blue pump cam is the most aggressive cam. And to go up in the pvcr.

 

[Some Car Guy]

up the squirter/shooter size a few steps and more aggressive throttle cams,,when stabbing the throttle without more gas it will stumble and/or backfire thru carb,,,,,Been there.

This right here. If it overall runs well and just has trouble with a transition, this is where to look. No need to overcomplicate things and go digging deep into a well tuned carb.

 

[Bee1971]

Even on five different carbs ?

 

[Bee1971]

Reason i asked about the intake manifold and if spacer - CARB SIGNAL

With that 284/484 Mopar camshaft and an open manifold plenum like a single plane - Had a few buddies that ran into this same issue on the street

When we finally switched over to an divided intake like a dual plane , or a divided or four hole spacer , the stumble was no longer an issue

Not saying this is your issue , but man five different carbs and the same thing

 

[Some Car Guy]

Even on five different carbs ?

Yes, the tuning of a transition would be expected on any carb in an engine with custom parts in it. Carbs are going to have similar baseline tunes and it’s expected you will work out what your engine needs. He’s having a lean stumble on transition, so I’d venture a guess a bigger shot of fuel is necessary. Sounds like the carb is otherwise dialed in.

 

[Geoff 2]

He is not getting a lean stumble on transition. He states in the first post that when he is cruising & he stabs the throttle, he gets the stumble.

With main jets reduced by 3 sizes, it is going to need a sizable increases in the PVCRs to make up the difference & avoid a stumble.

 

[Aron Gleason]

He is not getting a lean stumble on transition. He states in the first post that when he is cruising & he stabs the throttle, he gets the stumble.

With main jets reduced by 3 sizes, it is going to need a sizable increases in the PVCRs to make up the difference & avoid a stumble.

After reading up on forums in regards to accelerator pump cams, people say with a stumble like I have. A more aggressive cam such as the blue cam has solved the issue. Knock on wood that gets me in the right direction.