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....

Thanks for the advice on the carb stud replacement.

To get back to the questions that I have been asked -

I pulled the Braswell (Holley 4150 style, 700 CFM) carb off, and took a good look at it.

On the Primary Metering block it is marked:

PRI 5882
4776 (I think this is a metering block #)
39 59

SEC 6502
4776 (I think this is a metering block #)
40 67

Under the carb, I found a little vacuum port that was capped off. It looks like it goes to manifold vacuum, but I have to find out for sure. The cap was very degraded, and I replaced it. If it's ported vacuum, I could use it for my vacuum advance.

The primary has two screw in Idle Air Bleeds, both marked "73".

I used a little tiny wire on the smallest holes, and a larger wire on the bigger ones. They all seem clear, I squirted carb cleaner down there too.

The shooters are: PRI 31, SEC 28

The cams look like they are white, or a clearish, yellowed plastic. I think they are the same color. Both are configured with the screw in the "#2" hole.

Both Accel pumps fire as soon as the throttle is opened, or the secondaries (mechanical) are opened.

There seems to be a little leak near the primary accelerator pump.

I am planning to take the carb over to Braswell to have them take a quick look at it tomorrow. I assume they can check / change out Power Valves if needed.

Thanks for the advice, and any suggestions also appreciated.

Rocky,

Just read through your problems and the suggestions. I am not a carb expert by any stretch, so I'm thinking in other directions. A few of my thoughts:

Verify your crank / cam timing has not shifted due to worn timing gears & chain, jumped tooth, sheared pin, etc. Do you know what is in there?

Verify the ignition timing mark on the damper is true TDC. (I am assuming that is what you are using).

What are the details of your ignition set up; stock or not, points, EEC, or what?

Failing to find any other issues, you might want to do a compression and / or leak down test to check for valve problems.

Good luck and let us know what you find.

Thanks, Rodney -

I really believe (and am hoping) that none of the things you are describing are where my problem lie. I just put this motor back together about 1000 miles ago, after a spun rod bearing.

All this stuff has about 5000 miles on it...

Rollmaster Timing Chain & Gearset

Pioneer Damper

Timing matches on my balancer and degreed flywheel.

Ignition is a Duraspark, with a electronic pickup.

I really don't want to do the compression / leakdown test.

At least for now, I'm sticking with the carb as the culprit!

Thanks for thinking about my issues, though!

Rocky

Sorry, Marlin -

I am not really clear about what I need to check. I didn't take the choke off, the carb came that way.

I'll take a look again, as well as ask the Braswell guys.

I'm just trying to bumble my way through this...

OK on your set up so far. Since you obviously have the carb off, it might be worthwhile to swap it with another, and see if this one particular problem goes away or not. Then you see if you should continue on the carb path, or look elsewhere.

When you say "backfire through the carb", and "seems to be getting worse", that's what gets my attention.

Good luck, hope you find the (simple) problem soon.

Thanks Rodney.

A buddy of mine has wisely suggested I might be happier with just a standard 650 CFM carburetor. If only I had one to try out!

But then I would be less of an awesome person if I wasn't running a full race carburetor on my Panther! I know what is important. Ultimate coolness !!



And honestly I have had good luck with this carb - it ran great for me before.

PS. ..... And contrary to the impression you may get by reading this thread, this is really a minor tuning issue. My engine is not coming apart. The car drives fine - I just drove it 60 miles two days ago. But it a tuning issue I would like to solve.

quote:

Originally posted by Rocky:
...But then I would be less of an awesome person if I wasn't running a full race carburetor
... And honestly I have had good luck with this car it ran great for me before.
...this is really a minor tuning issue in my opinion...

Could it be that with the fresh engine your driving technique has let you lower the operating range and it is just the fact of putting air/fuel in at too low of rpm with your looping cam?

Get a ohmmeter and check the resistance of your ignition wires. Also take a look at your distributor cap and rotor button. If either is suspect in any way replace them.

There is something causing those 4 rear cylinders to be so black.

Definitely check the power valves and I would suggest try some other power valves.

Your problem is something that was good but is now degrading, so look at the components that are the most likely to degrade in a time frame similar to what you have experienced.

Thanks, all -

Took the carb out to Braswell, walked in and had it professionally gone through.

2 problems found:

1. "Leaky" Secondary Power Valve (Doug was right on!). The valve was definitely no good, would respond to vacuum, but would leak out over 4-5 sec.

I suspect that is the cause of my richness, black rear plugs, and inability to adjust the idle.

2. "Leaky" Primary Accelerator Pump Diaphragm.

But we will let the data (results) lead us.



That place (Braswell) is amazingly great. I walked in, got a very good lesson in carb rebuilds by a pro (Chris), got all my questions answered, got my carb reworked, with no appointment, and no hassle. Also picked up catalogs and cool Braswell stickers.


Plus you should see all the "stuff" they have in there! You should see the size of these racing carbs!

Glad it is fixed. Now you know the symptoms.

Unfortunately these crap power valves can strike at anytime? There is something wrong with the "rubber" material they are using.

Maybe it is the ethanol effect?

I'm not exaggerating. I replaced about 10 (I think) in my cars in the last 4 or 5 years.

Webers don't have that issue fortunately.

The "type 2" is a sneaky one and can be difficult to diagnose without taking it out and putting it in a tester.

I have the little Moroso test kit. It has been worth it's weight in gold so far.

Yup. Those "Dominators" are monsters. Two of them sideways on top of the engine are amazing. I could go for that.

Happy trails.

Been watching all along. Braswell sound like good folks. Glad you've been taken care of.

so it was pig rich the whole time ...

that is great service from Braswell

That's my belief. I have to get my wires done, and drive it. Here's pix of my carb, and the Braswell Operation.







It's a little hard to have a sense of scale on these, but look at these vs. the size of the desk blotter....

Anyone here have a pair of Boss 302 Trans-Am type Dominators? I can trade you for something right now. How about an electric lawn mower?
No good?

Yeah - We're all pretty green around here....

Everyone wants one of those "coal powered" lawnmowers!

Yea but an electric mower starts every time!

I had to re-read this thread:

http://pantera.infopop.cc/eve/...562/m/6231082456/p/1

In doing so, I hope the urge has passed?

Oh. I forgot. You wouldn't need a lawn mower, maybe just a good shop vac to pick up the sand from the evening winds right?

Hey? I've got some good cow bells? You could hang them out in the wind?

In Viet Nam all you needed to do was to hang tin cans on the trip wires. Then when the gooks tried to get into the compound they usually would rattle the cans.

That's if they were lucky. Most of the time they would set off the Claymores?

No more 'boom-boom' for them?

The standard procedure was just the tin cans on the wires and the enemy would rattle them but I hate being predictable and run of the mill. I like to innovate and heck the Claymores weren't costing me anything?

...cowbells are just SO obsolete. SO yesterday.

https://www.youtube.com/watch?v=TklM2-lSby4

That was another person. "I was so much older then, I'm younger than that now".

That was just a training mission right? A STO (in new speak).


Oh, the urge? I just re-read that thread and reminded myself but it's a really good thread.

Lots of great pictures like in Playboy..."they sent us Playboy".

For one thing I'd have to start by having the ports on the Weiand manifold welded up (I don't do 'luminum) to match the smaller A3 heads.
Then probably have the ports rifle drilled so they could be tied together for vacuum...
So far, yes, I'm over the urge.
It was probably just gas anyway? I had beer and chilli last night?

Did deimh ever get his engine built? Ah dam...I scared him away? Oh no? Not again?