UPDATED - Backfire under Low RPM / Load (4th or 5th Gear).....

...Look for a Vacumn Leak.

Also try opening up the 'Idle Mixture SCREWS', just a Teensy-Weensy.

Could also be a Glogged 'Idle Air BLEED', Poke a Fine Wire down ALL of Them and then shoot them with Carb Cleaner.

Just sounds to Me like it's 'Fuel' Related.

Thanks, Marlin -

I'll give these ideas a try.

Rocky

Chuck ,
Can you make out what number is stamped into the pump discharge nozzle?
Also, what color is the pump cam ? And which position is the retaining screw ,that holds the cam in place ,in?

Ron

Ron -

I can pull all that up (I hope) when I get back to my car in a couple days.

The situation is though, that the car was running great with this same setup. The performance degradation has come on (and worsened slightly) over the last couple weeks.

So I am thinking that it's some kind of issue with my ignition or carb...

Chuck

you can try to tune the stumble out but it may be a Band-Aid for a mechanical issue, you say it has just come on in the past few weeks and is getting worse...

besides the vacuum hoses splitting at the connections a failed carb base gasket can allow air to enter, also check that the base plate screws have not loosened, simply grab hold of the carb float bowls and see if the top half wiggles while the bottom stays put. it's common to all base plate carbs

Power valve?

power valve failure should result in a rich condition, it's a vacuum switch between the float bowl & the main metering system. there's some confusion as to whether a blown PV can affect the idle circuit, I'm not sure that it can but the main circuit / jets do affect just off idle in the transition circuit. if the stumble is in reality a rich condition it can be a bad Power Valve...

if the OP is using engine braking during the 'no throttle' period before the stumble, it's another point for it being a vacuum leak. engine braking can pull a lot more intake manifold vacuum than the engine can produce under throttle, a marginal gasket would fail under the most extreme conditions before it totally lets go

I wouldn't try to tune it out other than as a method of diagnosing the real problem. once the problem is found you'd have to undo any tuning done if it was right before

I want to thanks everyone for the suggestions. Am slowly working through them.

Got some pix, and did a little work today, but it's pouring, so the drive will wait a little...

Again - the carb is a Braswell (modified Holley) double pumper, 4 corner idle.

It looks like a "31" shooter.

Accelerator pump screw is in the "2" position... But I am not seeing the typical Holley Cam in there.

I put a couple of more pix below.

Tightened (slightly) the 4 base bolts.

Only vacuum lines I have are to the PCV Valve and to the Brake booster. No vacuum advance to the distributor. EGR port (or whatever) on the carb baseplate has a good seal - now has a clamp.

Timing is set correctly (16* at idle, with "all in" at ~2800).

Unless that air cleaner threaded rod and nut is the one that came with your carburetor I would get rid of it and possibly save your engine.

Need to have the proper hardened stud there and no nut or a nut that is part of the stud.

Mike

quote:

Originally posted by Marlin Jack:
...when the Choke is removed, it leaves a 'Live' Vacuum Port, uncovered.
It looks like You have a Plug in there, but how good is the seal. When I removed My Chokes I screwed in self-threading screws with sealer.

You might take a hose to the ear, and listen there, when the engine is running, just to be sure.

ALL of us, with recommendations, must Know.

1. The Engine ran great, Until this Problem came-on Gradually.
2. The Problem is Off-Idle, and Improves with Higher RPM in Cruising.
3. A Blown Power Valve would cause the Engine to Run Very Poorly, Flooded Out, the Fuel Bowl would be constantly EMPTY, and A Cloud of Black Smoke would be Pouring out of the Exhaust.

I have had TWO degress of PV failure. I can clearly document them with the PV tester that I bought.

The most common is the completely blown out, will hold NO pressure.

There is a second kind. One that I never suspected.

What happens with that one is that it has a slow loss of pressure that happens over maybe a period of four or five seconds. You would never even find this one unless you removed it from the car and put it in the tester.

This type two seems more likely because of the lack of black smoke and other unmentioned symptoms?

Sincely, Doctor Doug.

I agree with Mike on the air cleaner stud. Garth has his engine at the machine shop being sleeved due to air cleaner stud breaking and going through the brand new engine. A very costly result due to failure of a five dollar part.
Forest

I agree on the carb stud - I will work to get that fixed. Good call.

Here's the latest report.

1) Full visual inspection (OK)
2) Tightened Carb Base Bolts
3) Reset Ignition Timing (was 14* set to 16*)
4) Verified Float Levels in both bowls (OK)
5) Verified Hoses / Plugs / Caps Tight

Took it for a spin.

I seem to have solved the "stumble", but I can still make it "cough" (backfire) with low RPM, high Gear (4th or 5th), giving it gas.

I am thinking that I may have had a vacuum leak (since corrected), but this backfire is something else again?

Thanks everyone for the interest, and the helpful suggestions.

I pulled the plugs, to see what I can see...

It looks to me that I may be running slightly rich all the way around, but the rear 4 cylinders are much richer than the front four....

I think maybe I could go 1 plug hotter, too.

I can get better pictures, with the plugs reoriented, but even if this isn't my backfire issue, it's something I want to look at.

Would this be more of a WOT situation, or not? I understood the primaries are supposed to feed all 8 cylinders equally.

Thanks -

Rocky

Chuck,


This issue is something that just started and the car ran good before with the same configuration? What plugs are those?
Did you check the total timing as well as the base timing? And can you take a picture of the driver's side (primary) accelerator pump linkage?

Ron

Those plugs to me show a carburation issue. It could be as simple as just needing to index the spark plugs but it looks more than that.

The black on the plugs is what I would expect to see from a leaking power valve. What is keeping the engine running is that there is a very hot ignition system in there that is causing the white tips.

There is somewhat of a misbalance here between the plug temps, the fuel mixture and the spark heat.

Those plugs should be rust brown with a transparency with no white carbon build up on the plugs.

In short, they are a mess and there is a bunch of work to be done.

Thanks, guys.

The plugs are AC R43TS

The car ran well with this carb, then I spun a bearing in Sept. The car is back with a new 10:1 compression motor (Closed Chamber GA heads / flat pistons) and ran well during the break-in period (which I admit, the engine was treated pretty mildly) except this "off-idle stumble".

The "off-idle stumble" kept getting worse, and in the process of troubleshooting, I noticed the 4th/5th backfire. If I don't load the engine at low RPMs, I would not have noticed it.

Here's better pictures of all the plugs and the best pictures of linkage I have.

Maybe I need to pull the carb off, but I want to be as sure as I can before I get too serious.

More New News:

Last night I attempted to make some minor adjustments to the Idle screws. I could not get the idle to drop (reduce RPMs) by adjusting (turning in) the screws with the exception of the Passenger Side rear.






Primary Accelerator Pump Linkage



Secondary Accelerator Pump Linkage





Thanks

quote:

I agree with Mike on the air cleaner stud. Garth has his engine at the machine shop being sleeved due to air cleaner stud breaking and going through the brand new engine. A very costly result due to failure of a five dollar part.

Very expensive indeed - sleeved #8 cylinder and new piston & rings. For what it's worth, I was not using all-thread. I had a Mr. Gasket carb stud, which I believe to be little more than all thread.

Get an ARP carb stud!

quote:

an ARP

Got one on order.

On the carb stud: don't rely on a jam-nut. Use Lock-tite as well. What's even more fail-safe is using a bolt thru the top of the air cleaner and a nut below it, smashed so it fits very tight on the threads, instead of a stud & nuts. But this rig is harder to get on than a simple stud....