Barfly

I've been reading about the lean backfire phenomenon and some fella's believe and some don't that a lean A/F mixture can cause a backfire through the carb. I have some of my own findings.

Since I got on the forum in May of this year I've been wining about this terrible and complete loss of power if I stomp the gas pedal from a dead stop. Rebuilt the carb, new distributor, cap, rotor, plugs yada yada yada..... Now I have the installed the "Performance" combination of jets, metering rods and step springs for my carb and I am getting closer, but still the stall and a back fire through the carb.

Now to what I have found.
Knowing I have the correct jets etc... I can adjust the A/F mixture with confidence and have found I can move the stall and backfire up in the RPM range to almost non existence.

Starting at two complete turns out on the mixture screws there is a definite stall and back fire almost immediately when stomping down on the pedal. By making the mixture richer I can move the stall/backfire up in the RPM's (takes longer to happen) and the stall is less severe. The richer the mixture the backfire changes from "wooooosh pop" to "POP" , the richer the better it seems.

Keeping in mind all of this is not under load. Tomorrow I will take her down the road and see how she does out of the hole.

I know from my day job that when trouble shooting, if you are able to make the problem repeatable, predictable and adjustable the solution is very close.

Joe.

Polaradude

Joe I am just curious...are you sure you have no vacuum leaks anywhere ?

78D200

If you are not sure, you can always spray water on each hose and connector. If there is a leak, the motor will stumble if not shut down. My dad did this to my grandmother's 64 Fury back in the day and found leak that when she would drive through a puddle, the car would stumble and most of the time shut down.

Polaradude

Propane is the way to go.

Barfly

I can't be 100% positive there is no vacuum leak, but I do have a gauge and it is solid on 18 hg at idle. Doesn't even flicker.

This does lead me to another idea I been having though.
The intake is a Torker 340 which is single plane and the plenums
are short, straight and dump directly into the ports. And from what
I understand this is not the best intake for take off or the carb I have.
And the fact it was discontinued makes me wonder as well.

From the guys here and what I have read it sounds like a duel plane intake
would be a lot more responsive and restricts the air flow somewhat with smaller and longer plenums. Even this though would relate to A/F mixture.

All that being said, I have had that carb apart, well lets say 15-20 times over the last couple of months, the gaskets could be a little worn

78D200

The Torker 340 single plane intakes were probably discontinued due to lack of sales, not because of issues.

You really should look into getting a dual plane intake (idle to 5500 RPM or 1500 to 6500 RPM) and a 650 carb. This will help cure a lot of your issues I believe.

Sangetsu

The Torker was an outstanding intake, and contrary to what you might think, it actually delivers excellent lower-end horsepower and torque (hence the name "Torker"). Sales of the Torker did decline, but only after Edelbrock released newer designs. I used a Torker intake on a 340 engine, and it worked very will with stock compression, trans, and gears. The only other additions to the engine were headers and a Holley 600 carburetor. There was enough low-end power to leave plenty of rubber on the road.

In my experience the vast majority of carb backfires are due to ignition problems, and have little to do with the carburetor. An early or erratic spark is the most common problem. Timing is an easy fix, but an erratic spark can be caused by a faulty cap or rotor, cheap or poorly routed wires, or a worn distributor. Nowadays a worn distributor is very likely cause as most of these original distributors are quite old. If you remove the cap and grab the rotor and try wiggling it from side to side, there should be almost no movement.

Adjusting the mixture can change the backfire as a richer mixture can drown out an erratic spark, especially at lower rpms when spark energy is lower.

It is difficult to know which jets and such are "correct", as correct is a relative word. If you car has an aftermarket intake, exhaust, and cam, the only way to find the "correct" jets is by dyno tuning, or trial and error at the track.

The most important thing to check is that all of the hardware and parts are functioning properly. Next, you must make sure than your timing is set to a baseline. If there have been other modifications to the engine, the factory timing settings are probably not going to be the best. Start with the factory setting and make sure it remains at that setting as you change the jets and make your AF mixture adjustments. Once you have the engine more or less running good, you can add a degree or two of advance and see if it runs better. The idle will speed up as the timing is advanced, so that will have to be compensated for by adjusting the curb idle. You want to get as much advance as you can without pinging. If the engine pings, back the distributor off a couple of degrees. There will be a sweet spot where the engine runs best, but you need to use a dyno or run it on the track to find it precisely. You need to keep an eye on your plugs as you make your adjustments, the color of the plugs will give you the best idea of how well your carb is jetting and adjusted. Black plugs mean you are too rich, white plugs mean you are too lean, slightly gray (with or without a touch of brown, depending on the type of gas you use) is where you want to be.

It takes time to get it all right.

67 GTX

Barfly, when are your four-barrels opening?
sorry if i already asked you this

what rpm are you leaving at?

67 GTX

you could also check the color on the spark plugs?

Sangetsu

I mentioned that as well, you'll see it at the bottom of my post above.

Barfly

Thanks for your replies fella's.

Sangetsu:
Much like having a base line for the timing I wanted to have a base line or a good starting point for the jets, metering rods and springs as I had no idea if what I had was causing my problem.

I found in Edelbrocks advertisements for the 1405 Performer carburetor "Tuned for Economy" and "Tuned for Performance" where they give the specs for each. So I have ordered and installed the tuned for performance parts.

I know it can vary greatly and for that matter could get down to just preference, but this at the very least gives me a good place to start.

When you say ping are you talking about hearing it at idle or highway speeds?

78D200:
After reading that extensive article about Single and Duel plane and the differences, I gotta believe this is a big part of the problem as well. I mean it really all kinda clicked to me. Large straight plenums seem like they would suck alot of air and require a lot of fuel to keep the mixture correct and would be less responsive at lower rpm's I mean it just kinda makes sense. And there was something about the plenums be routed in a fashion that the pulsing in the carb is 180 deg out rather than 90 however I did not completely understand it all.

67 GTX:
I went back to the carb rebuild kit and made sure that the secondaries are opening at just a little more than 2/3's into the primaries. It's definitely when the secondaries open is where the problem is. I mean I can punch it down say half throttle and take off just fine, but that's not going to win any races. If I power brake it up to around 1500 rpm I don't get the backfire at take off, but it is a slow and labored take off just the same.
Normal idle is set at 650-700 rpm.

Tonight I played with the springs I got. The Edelbrock ad said they used Orange which was the second to the largest, more or less the same results as what I had. I threw in the largest ones (Red) and BIG difference!
The larger the springs the higher the metering rods are held out of the primary jets at idle allowing more fuel to pass.

This additional fuel and a little rich A/F mixture and I spun both tires at take off for the first time with this truck.

I sure appreciate all you guys thoughts
Joe.

67 GTX

maybe try to have the secondaries open a little later
i am having an issue with my secondaries too, and i was advised to allow them to open later (around 4k)
not sure if that setting will work, but it is an idea that I will try