Misfire, can't find problem and have tried everything. Please Help

i cant see the distributor it self being the cause of that. more likely the plug wire or the cap are causing the loss of spark energy.

 

Do a leak down test on that cylinder. If a valve is not seating correctly that cylinder won't fire.

Are you running adjustable rockers???

 

IF the spark will jump to the installed plug while running, you have enough fire power to light the plug... Rarely is there ign problem that's localized to one cylinder that isn't cap, plug or wire...

Has compression been checked???

 

Compression is good in all cylinders. It is a roller motor, hydraulic, rockers aren't adjustable. The rockers function as they should and the valves open and close as they should.

 

Far out thought, but I once had a 2.3L Mustang that did the same thing. I found a fender washer stuck in the intake port in the head! The car ran perfectly for years and one day started missing. That washer laid in the intake for God knows how long and finally dislodged and got sucked into the head port!

 

How did you perform the compression check?

Engine warm, carb blocked open, all plugs removed, ignition disabled, 3 to 4 revolutions per cylinder???

Wet or dry??

What were your numbers on each cylinder? Be specific.

 

I really do think that something has been missed here too. Especially after throwing all those new parts at it.

 

Best guess is that the plug wire is lying up against something and getting too hot when the engine is running, but when he's under the hood he doesn't notice it as much because he's busy fiddling with crap. It's a common problem you have to watch out for when using the Ford Racing pre-booted 9mm wires because they are sized for a 351 not a 302. Many other performance wires also have that problem, I just specifically know about the Ford wires because I use them, and have used them on several 302s now. I see you are using the MSD wires, which ones specifically? You might actually be undersized for the longest wire run too.

Second guess, heavy carbon build up in the threading on that spark plug hole, giving you a terrible ground. Use a high suction vacuum attachment and a wire brush to ensure you have a good connection. I've seen it on the rear side of a transverse engine with a valve cover oil leak directly into the spark plug hollow. The oil penetrated into the threading and then would bake off. Eventually it insulated the spark plug, but the plug never felt "loose", so there was no immediate indication of a seating problem.

 

I think he said it fires just fine externally and even the longest MSD wire known to man would not approach the resistance of a factory wire 12" long. lol

As for that second part.. I'm thinking that you you may be on to something there and I did have a few passing thoughts while working on other projects about a potential grounding issue myself. He did say that he changed plugs but that doesn't mean that their seated against good clean iron either. Also, some anti-seize compounds have also been known to dampen grounding and cause intermittent issues too.

IIRC.. too lazy to look back right now.. he said this issue started after the car was parked for a period of time? Could be thinking of another thread though. If that is the case though.. that should be a clue to potential grounding issues since they certainly don't seem to get better with age. Especially when they sit on concrete.

 

You missed using a vacuum gauge. Missed a running compression test. Vacuum gauge is my first go to if I suspect mechanical. Assuming the misfire is constant.

 

Hi Darren. Not trying to be facetious here and am just curious as to your idea on the matter.

Wouldn't the vacuum gauge fluctuation just reiterate the inevitable affect of a dead cylinder? And have you ever seen a good static compression reading show up contradictory readings during a running test?

Personally.. I think he needs to start over with more baseline testing because what he's indicated makes no sense in the scheme of all things engine related. Maybe even swap wires around.. even run it in the dark to see if the suspect cylinders wire is grounding out somewhere.

 

Yes, he said it fires just fine when he pulls the wire and jumps the spark to a good ground. That doesn't negate ANY of what I suggested. And yes you can have a wire that is too long, because the electricity only really flows on the external area of the wire, so if you have a low quality wire that is too thin, and too long, and gets too hot... It won't conduct enough electricity to jump the spark inside the combustion chamber, but will conduct enough electricity to jump a spark in ambient air pressures outside of the combustion chamber.

I had it demonstrated to me at the GM factory when I got my Automotive merit badge in boy scouts. They had about 200 scouts in for a 10 hour crash course on automotive basics. The coolest thing was when they popped open the hood on a brand new 1996 Northstar Caddy and cut hoses and wires until the engine stalled to show off how many computerized sensors they put in it. That's not important, what is important, is that your ignition wires can easily be a primary failure point.

My current ignition set-up can not even function properly on the factory OEM wires, they get too hot and have serious spark degradation, which leads to a rolling miss, and constant stuttering and choking.

 

Start your car in the dark and watch for spark scatter in the engine compartment. MSD puts out a lot of energy and will find any weakness in wires. Absolutely no lights on and rev the engine and watch for it.

SPark

 

I installed the new Distributor tonight. Did not fix. Ran the car in the dark, no visible spark jump. Threads are clean and plug is grounded. Pulled intake manifold to look for blockage or anything else, saw nothing wrong. I am still stumped and am getting frustrated. I have tried everything suggested and nothing is working. My mechanic who has built several sbf engines is also stumped. I don't know if I should pull the heads too or what to do next. It seems pointless when the valves seem to function as they should and there is no water or oil in the cylinders.

 

Have you checked the valve springs? I had a rough idle & when I got on it, it ran with a miss. Could tell not getting full power. I replaced all spark plugs, dist cap, rotor & rebuilt Holley street avenger carb. Did not fix problem. When I replaced the broken valve spring, runs like it should. Easy & cheap fix, if that's the problem....