: 425 misfires (hesitates) under load

10-02-10, 09:21 PM
Hey, my ’79 FWB runs great, except under load like getting onto the freeway, then it cuts out like it has a fuel delivery problem. I recently had the carburetor rebuilt and I installed a new fuel pump and it still has the same problem. After doing some research, it looks like a faulty ignition system has the exact symptoms. Today I picked up a rotor, cap, coil, plugs, and wires. I bought some dialectic grease too. Has anyone experienced this? Did I overlook anything that should be checked or replaced? :hmm:

10-02-10, 10:13 PM
Fuel filter?

10-03-10, 09:21 AM
From what you described, I'd check the centrifical advance, (the flyweights and advance plates under the rotor) which sometimes get gummed up and freeze. If your not getting a stumble off the accelleration from idle, your vacuum advance is probably working alright. Check your vacuum lines for leaks anyway. Tthe miles of vaccuum lines and the various vaccuum motors are the biggest power robbers for this engine. don't forget the vaccuum units in the cab area, i.e. climate control door actuators, power brake release mechanism, etc. Is your engine properly tuned?
You didn't say how many miles were on your car, or if it had been regurally serviced. The only other thing possible is your timing chain. While the geometry used on this engine doesn't cause premature wear, as some of the other GM units, it still uses the nylon gears which are prone to failure. Check your timing. If you find your distributor is knocking against you valve cover, with little adjustment left, there is a good chance your engine is a candidate for a new timing set.

10-03-10, 10:39 AM
Thanks for the good tips, no off the line stumble, so I agree that the vacuum advance is probably working. I pulled the left bank spark plugs yesterday and besides one of them only being in finger tight, they all were simi fouled with some carbon build up. The spark plug wires look ok, but the boots have been carved on (trimmed) so I don’t know it there was a problem with arching. The factory wire looms are broken so they are not routed properly. I have had the car about six months and am slowly bringing it back up to speed. The seller told me that a tune up had just been performed a month prior to me buying the car, after seeing the plugs, I really doubt that. The car has 40k on it.

10-03-10, 01:07 PM
I'm hardly a mechanic, and I'm not sure this is even related, but I've noticed a big difference when I fill my car with gas that does not contain ethanol. My car has stuttered, like it had a miss, when burning fuel with ethanol, and without the ethanol, which is what the car was made to run on, it is fine. Good luck! :)

10-03-10, 08:13 PM
Well, replaced the plugs, wires, cap, rotor, and coil today; what a difference! Took it for a really hard test run (= FUN) east of San Diego through the mountain passes and still loses significant power going up grades. No more engine misfire though. My feeling is the exhaust is restricted. I had a resonator recently replaced and I think its choking off the engine. It ran fine up until then so I am having it removed next week. The exhaust sounds like a vacuum cleaner under hard acceleration too.

10-06-10, 01:25 AM
If the resonator is the right one for the car, it shouldn't make a difference. My 77 has a resonator and I have no problem shooting up the Santa Rosa grade out of Camarillo on the 101 at 70+, and it's like an 8% grade IIRC.

10-06-10, 03:16 AM
are the parts you replaced performance parts on the distributor, rotor, cap, coil and wires etc...
if so, what are they; company or where you buy them at, I've seen a few kits at PepBoys??
I've had my 80coupe for 12 years and never replaced anything in this area though car runs fine and I can haul ass with no hestitation. It has a 6.0 368 with 222,xxx miles on it. (gone over twice) Should I still replace?