Engine Missfiring ?

[BroncoBlues]

This '73 302 has run sharp as tack for years, at 209,000, second owner. Recently, it began running rough, and got worse and shut down, and towed. But, it restarts fine after sitting. Tuned-up, rebuilt carb just now installed by mechanic as thought that might be the problem as getting overload with gas. But as usual, idles fine, sharp, says mechanic, but as soon as accelerates or load on, its gets really rough. If accelerating further at idle it seems to smooth out a bit once past the roughess point of running. It is more noticeable on the road and a climb, and it seems the longer it runs the worse it seems to get. Could it not be firing or misfiring on a cylinder? Gas building up in one cylinder and not being burnt. Compression tested 170s, 155s, and a 135, prior to installing rebuilt carb. Coil okay, wiring seems fine. Not certain if distributor bad, timing chain off a bit, or what? I was thinking bad fuel pump? Mechanic doesn't think so. I have worked with external engine repairs, ei. tune-ups, etc., for years, but can't quite figure this one out, nor have had a vehicle ever act up like this before. Changed point, condenser, button, and cap, again. Has 7mm wires solid wires. I had a for sale sign in the window, but not at the moment until I fix this. Thanks for any help!

 

[STLKIKN]

Try disconnecting the vacuum advance to the distributor. (don't forget to plug the hose)

After the engine is warmed up take it for a short drive. If the problem is slightly less noticeable, check the distributor breaker plate for wear and the springs for loss of tension.

I had a problem that sounds VERY much like yours, it had me baffled for a couple of days too.

Turned out that the plate was worn.

good luck

 

[Broncoholics]

Do you have points? If so check the little modulator (little silver canister) in the distributor. It could have failed. Its usually the case when running points (runs well when parked but runs bad under load).

Also **** on the hose that goes to the distributor (vacume advance) put your tounge on th end while sucking and see if you tounge sticks to it. IF it doesn't then you need a new Vacume advance.

 

[Guest]

when you did your comp test was it cold or hot?

according to your #'s you have an internal problem.

redo your comp test again when cold and notice what your needle bounce is on the first bounce. compare this with all the cyl. this will tell you if you have a bad valve. the fule pump is not the problem.

it is either in the igniton syst. or a vacum leak.

you must determen wich cyl. is not firering first to fix the problem.

have you checked the firing order?

 

[Toby the Dancing Bronco]

I agree that the vacuum would probably be the cause of your rough running engine, and the vacuum advance is easy enough to replace. After you re-check your firing order, watch the engine run in the dark and look for arching from the plug wires to the exhaust/headers, etc, and replace wires if necessary. It may be cost effective to upgrade to an electronic system, either aftermarket or the Dura-Spark set up that came in the 74's. They are easier to deal with in bad weather, too. However, if you have over 200,000 on the original motor, changing the timing set is probably the best way to see another 100,000. Get a good double roller set up and throgh it on. The original link-belt can stretch pretty far, and will always cause a problem with timing when all that slack comes around. Timing sets are real cheap insurance, too! When you pull the distributer, check the bushings for excessive play. Time and miles all wear these things, and tweaking little things here and there will not make up for worn parts. And, it will be fun!