truck begins to backfire over 3000rpm?

[BrutonLS1]

I've got a 5.3 bored to 5.7. I converted it to carburetion and I'm using the edelbrock performer rpm intake, with a 650 thunder series edelbrock carb. It's got the edelbrock/msd Ls1 timing control module with the 6 different timing pills. I've tried all the pills, and the backfiring still happens? When I'm parked the engine runs just fine, I can hit 5500 rpms no problem, but when I'm driving and get around 3000 rpms to go faster, "pop...pop...pop pop pop!!" I'm not sure what's going on here guys? How can I adjust the carb differently? or what else can I do? Thanks in advance

[68gmsee]

ttt

Will it backfire when driving if you stay below 3K rpms?

If it drives fine below the 3K rpm's and start popping at the higher rpms, I would go with ignition problem first. I'm not familiar with the Edelbrock msd control module but can you install a regular HEI for trouble shooting purposes? Local junkyards sell them for around $20 to $30.

[airdale94]

Is it poping out the carb. or tailpipe? If the carb. it's carb. adjustment. If tailpipe, timing. Some have to run fuel press. regulators with the Edelbrock carbs. They can't take to much press.

[BrutonLS1]

The carb is getting 4-4.5psi. It's coming out of the tailpipes for the most part. It only back fires out of the carb when I push on the gas as its popping, at 3000rpms driving down the road. I've done adjustments on the carburetor, it seems to help, but doesn't solve the problem. I would have thought edelbrock would've done their homework on a product that's so crucial to an engines performance; so I'm trying to figure out what else it could be, but I'm running out of ideas.
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[airdale94]

4psi sounds kind of low.

When you adjust it and it helps are you richening it or leaning it? You might need to get a adjustment kit.

[BrutonLS1]

Adjust the Gas / Idle Mixture Screws on Your...

I'm doing exactly what this guy is doing, I did it 3 times.
could Premium 96 octane gas be the culprit?

[piecesparts]

Are you sure you don't have a valve that is floating on you?

[68gmsee]

Questions again: (these are so others can maybe make a suggestion)

You didn't anwer my first one- Does it pop at all while driving and staying below 3K?

Has it always had this problem or only after you installed the upgrades?

Does the edlebrock msd module require exact 12vdc?
If it does, make sure you're not using the original resistor wire. It needs a new wire installed from the bulkhead on the firewall.

Do you have access to an HEI distributor for test purposes?
This would be a quick way to determine if it's ignition or not.

[AirSpeed]

One of my trucks did that, it turned out to be my timing chain & gears. It had nylon gears and the teeth had broken off the gears. It would idle fine but as soon as I revved it up or put it under a load it would backfire like crazy.
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[hugger6933]

68GMSEE we are looking at his problem as a gen 1 motor and in his fisrt post is states he has a ls1. So the hei that you asked about in your first response won't be an option here. Let me ask this question is there a rev limit switch wired into the msd box anywhere? MSD rev limit switches come in pills similar to the timing pills that you plug into a box or in some cases into the side of a ignition box to limit the rpm and the lowset they normally have and atre shipped with for testing purposes are 3000 rpm's, but that would not explain the no load rev in park. I have seen what you are experaincing on some guys putting together a racecar and it would not rev over three grand and it kept sounding like it was against a "chip" and I asked the question about the box and the low side chip and all that I have laid out here and that was it.So anyone of you been to the track when the bracket cars come to the line and the lights start to drop and the cars are popping and chattering like that is what it called up against the chip. Most of the cars have a box that is a step above the RPM limit switch and it is called a two step. It has a low side and a high side the low side is activated with a lower rpm chip say 3000 when the transbrake is activated and the sw keeps the car from revving past that point until the brake is released. Then on the high side there is a chip to keep the car from over revving on the big end say 6600rpm once in the high end [or any time you rev up that high] it will make the motor misfire causing the popping sound that we are talking of.

[BrutonLS1]

Found the timing curves. Those curves seem pretty high? I also read that the edelbrock ls1 box does not have a rev limiter.
-I want to thank everyone for their response, you guys rock! Love this site.

[BrutonLS1]

Originally Posted by piecesparts

Are you sure you don't have a valve that is floating on you?

How would I check this?

[scorchingeng]

Hi,

If it's running well up to 3000 rpm I would not suspect fuel delivery at all.

It is either a timing issue ( but yours looks good to me...) or valve overlap. How about rechecking your valve lash, ensure your rockers are tightened evenly. I have no idea what heads you are running, hydraulic lifters, etc. But it's an idea.

By the way, you can check for floating valve, and narrow down a bank, by using a wideband O2 tailpipe sensor. If they are uneven, I think you can figure it out from there. Obviously if they are uneven, it's not your carb or intake.

Hope that helps you at all.