As many of you already know I’ve been battling a nasty missfire on my 300hp crate 360 magnum for months now and I can’t for the life of me figurenout what’s causing it.
Engine is low mileage, 30k miles, my problem cylinders appear to be cylinder 6 and cylinder 8.
Cylinder 6 is almost always carbon fouled, with what looks like dry sooty residue like what you’d see on the inside of a chimney.
I get 155 psi of cranking compression on all 8 cylinders with engine warm and carburetor wide open.
I thought it was ignition at first, ive put a brand new firecore 50 electronic vacuum advance distributor in it, new cap and rotor, new Taylor plug wires, new ngk bkr5e plugs.
New MSD Digital 6 ignition box, coil wires and trigger wires routed on opposite sides of the truck.
Pick up coil wiring is the correct polarity, I’ve triple checked that.
Timing is 18 initial and 34 total all in at 2500 rpm.
I’ve verified the accuracy of my balancer with a piston stop, it’s dead on.
I’ve checked timing with two different lights, both are the same.
I’m running a brand new crane cams lx91 e-core coil.
I’be pulled the valve cover on passenger side, rocker arms and valve springs and push rods appear to be fine, nothing is loose or broken.
I’ve watched the valve train with engine idling, everything looks normal.
I’m at a complete loss here.
The missfiring starts around 2400-2500 rpm in every gear, even in nuetral.
I can get it to clear up a tad bit on the highway if I stand on it pretty hard and then back off a little bit I think that’s because the plug in cylinder 6 gets cleaned off a little bit and starts firing better? Idk why it will clear up a little sometimes when you poor the coals to it.
Carburetor is a brand new Quickfuel super street series 680cfm vacuum secondary carb.
Intake is the Mopar Dual plane M1.
Could this be a fuel issue? Is it getting too much fuel and missfiring because of it?
Below 2500 rpm it runs super good and feels completely fine.
It isles nice and has 20hg of vacuum at idle.
If this is a fuel issue, why is it only affecting cylinder 6 mainly and sometimes cylinder 8?
I mean I believe it could be a fuel issue at this point, I just don’t understand why only 6 and sometimes 8 are being affected.
I don’t think my carb settings are all that crazy rich ether.
I live in Montana so I’m 3500 feet above sea level.
I’m running 65 main jets and 75 secondary jets, I’m running .031 idle feed restrictors and .065 idle air bleeds, I’m running .033 high speed air bleeds, a 9.5hg power valve with .049 pvcr’s.
I have the floats set at the bottom of the sight glasses, all 4 idle mixture screws are about 3/4- 1 full turn out from seated.
I get 6 psi of fuel pressure from my mechanical fuel pump.
I’m just at a loss here and have no idea what’s going on or how to pin point what’s going on.
Should I step down a couple notches on the idle feed restrictors and step down on the jetting a little bit and see what happens?
Last night I lowered my floats from being half way up the sight glasses to the bottom and that appeared to slightly make it a little better but it’s still missing at those rpm’s.
