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

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?