Sparatic Misfire - Help!

red9535 · Oct 9, 2007

My Mustang build is almost finished. I'm having a misfire problem though!

It's a mildly built carb'd 302. Cylinders 4 and 5 miss at idle.. and as you climb the RPM's, they seem to start to fire, as it smooths out.

Here's the news so far:

All cylinders have good compression and the block has good vacuum
Camshaft lobes are fine, all valves open and close properly
Brand new spark plugs
Brand new wires
Brand new distributor
MSD 6A box is working fine (swapped a new one in and it didnt make a bit of difference)

The only thing I haven't replaced is the coil - but i dont see how a bad coil would cause 2 specific cylinders to not fire unless there's engine load, in whcih case SOMETIMES they fire.

Just strange.

red9535 · Oct 9, 2007

Just tried a new coil, didn't change anything.

ryclef331 · Oct 10, 2007

Ok...yours is carb'd right?

How old is your cap and rotor? If anything else, I'd say your idle mixture sounds fat.....

red9535 · Oct 10, 2007

Cap and rotor are brand new, with the distributor. And it definitey runs fat - but not that fat. The only thing it's going to hurt is my fuel economy - the engine should run fine.

ryclef331 · Oct 10, 2007

Too much fuel can and will foul out plugs REAL quick. I'd recheck the gaps on the plugs, replace if they're overly fouled, AND verify you don't have a vaccum leak somewhere.

red9535 · Oct 10, 2007

ryclef331 said:
Too much fuel can and will foul out plugs REAL quick. I'd recheck the gaps on the plugs, replace if they're overly fouled, AND verify you don't have a vaccum leak somewhere.

Just got brand new plugs and gapped them yesterday - dind't do anything. There is no vacuum leak.

red9535 · Oct 11, 2007

Figured it out - stupid mistake. I was using the wrong firing order. The one in my Chilton's manual has the incorrect one for my motor - It it says to use 1-3-7-2-6-5-4-8.. but my engine's out of a 92 Grand Marquis, which used the 289/302 firing order (1-5-4-2-6-3-7-8).

Now it idles and hits all 8 cylinders (what a difference!) however, it still stumbles a little bit under throttle and heavy acceleration. Its a timing issue at this point - my base timing is set at 10 degrees advance, and my distributor gives another 21 degrees at 4000 RPM. What should I set my base timing at?