408 misfiring at higher rpm
11-06-2021, 06:24 PM
#1
408 misfiring at higher rpm I just put a new motor in my car. 408 12.2:1 with ls3 Frankenstein m311 heads, 1050 carb, single plane intake, msd 6010 box and I’m having problems with a misfire right around 6000 rpm in high gear. Br7ef plug gapped at .035
It don’t brake up in the burn out box, it leaves clean of the brake at 4500 rpm, shift it at 7500 and it drops to 6100 or so and it starts braking up, tach starts jumping around erratically. I had a SBE 5.3 in the car before I put the new motor in with the same msd box and had no problem with the box.
Ive tried new plugs, checked the msd box ground, swapped the coil packs, ran timing anywhere from 27 degrees to 34 degrees all the same. AF is right around 12.2 at the moment but I also tried adding and taking jet out and still the same.
Any help would be appreciated
It don’t brake up in the burn out box, it leaves clean of the brake at 4500 rpm, shift it at 7500 and it drops to 6100 or so and it starts braking up, tach starts jumping around erratically. I had a SBE 5.3 in the car before I put the new motor in with the same msd box and had no problem with the box.
Ive tried new plugs, checked the msd box ground, swapped the coil packs, ran timing anywhere from 27 degrees to 34 degrees all the same. AF is right around 12.2 at the moment but I also tried adding and taking jet out and still the same.
Any help would be appreciated
Last edited by Rpayne41985; 11-06-2021 at 08:25 PM.
11-13-2021, 03:16 AM
#2
You have to do things methodically to diagnose a problem. You reduce the timing to eliminate that. 20 degrees will net you no power, but would eliminate a variable. Do the same with the plugs. .022 will not be ideal, but should eliminate spark blowout. drop the rev limit until you quit misfiring, etc., etc, until the motor clears up. However, before you do all that, check your float bowl and fuel pressure, because it sounds like a lean problem with a bigger motor. That is usually a fueling issue.
The following users liked this post:
G Atsma (11-13-2021)
11-14-2021, 05:44 PM
#3
You have to do things methodically to diagnose a problem. You reduce the timing to eliminate that. 20 degrees will net you no power, but would eliminate a variable. Do the same with the plugs. .022 will not be ideal, but should eliminate spark blowout. drop the rev limit until you quit misfiring, etc., etc, until the motor clears up. However, before you do all that, check your float bowl and fuel pressure, because it sounds like a lean problem with a bigger motor. That is usually a fueling issue.
When going to the front mount cam sensor does anything with MSD 6010 does anything else have to be done other than the ext. harness? My cam sensor has the factory pig tail harness on it.
11-20-2021, 05:12 AM
#4
Ive tightened the spark plug gap, I’ve added fuel I’ve taken fuel away, I’ve swapped coil packs, I’ve swapped plugs, I’ve swapped plug wires. I swapped out the cam sensor for another new gm sensor and the car wouldn’t start at all, put the old one back in and it fired right up.
When going to the front mount cam sensor does anything with MSD 6010 does anything else have to be done other than the ext. harness? My cam sensor has the factory pig tail harness on it.
When going to the front mount cam sensor does anything with MSD 6010 does anything else have to be done other than the ext. harness? My cam sensor has the factory pig tail harness on it.
12-13-2021, 07:10 AM
#5
I just put a new motor in my car. 408 12.2:1 with ls3 Frankenstein m311 heads, 1050 carb, single plane intake, msd 6010 box and I’m having problems with a misfire right around 6000 rpm in high gear. Br7ef plug gapped at .035
It don’t brake up in the burn out box, it leaves clean of the brake at 4500 rpm, shift it at 7500 and it drops to 6100 or so and it starts braking up, tach starts jumping around erratically. I had a SBE 5.3 in the car before I put the new motor in with the same msd box and had no problem with the box.
Ive tried new plugs, checked the msd box ground, swapped the coil packs, ran timing anywhere from 27 degrees to 34 degrees all the same. AF is right around 12.2 at the moment but I also tried adding and taking jet out and still the same.
Any help would be appreciated
It don’t brake up in the burn out box, it leaves clean of the brake at 4500 rpm, shift it at 7500 and it drops to 6100 or so and it starts braking up, tach starts jumping around erratically. I had a SBE 5.3 in the car before I put the new motor in with the same msd box and had no problem with the box.
Ive tried new plugs, checked the msd box ground, swapped the coil packs, ran timing anywhere from 27 degrees to 34 degrees all the same. AF is right around 12.2 at the moment but I also tried adding and taking jet out and still the same.
Any help would be appreciated
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Old Buzzard (12-23-2021)