Hello all
I have a 1970 F-100 with a 360 and a NP 435 4-speed. I recently put a new clutch in, and put all new gaskets in the motor. Ever since I put it back togeather, the truck has not run right. It will idle ok, and at low rpm (1500 and under) runs pretty well, with an occasional miss. But as the truck revs higher (Especially when moving) it starts missing worse and worse. It is eratic, not a 'smooth' miss. I'm really not sure where to start. It seems like a simple problem but I am still at a loss. I checked the fuel pump, and with 3 or so cranks I filled a 12 oz. soda can about 3 inches, so fuel is definatly getting to the carb. The vaccuum advance works when I pull the line off the carb and suck on it (at least the advance mechanism moves inside the distributer). the points and condenser are new as of 2 months ago, and the wires and plugs are all new within the last year. I didnt make any ajustments to the carburetor or do anything with it at all when I pulled it off to pull the motor and put gaskets in it. Does anyone have a good starting point as where to go next? is it possible that the secondaries on the carb arent opening right? (its a holly 4 barrel) And is there a way to check this? Any ideas are greatly apreciated, as I am down at school and it is the only thing I have to drive.
Many thanks in advance
Garrett Schorran

 

I'm no expert but years ago I had a 69 Galaxie 500 with a 390. I had this same problem, took it to a mechanic, it was the vacuum advance.
They are cheap. I would swap it out and see what happens.

Thank you,

David

 

do they actually sell the vacuum advance pieces seperate from the distributer? Like I say, it looks like its working...I am not sure but I think it might be something else.
Thanks
Garrett

 

Take a look at the firing order. Make sure the wires are in the proper sequence on the top of the distributor and carefully follow each one to make sure it is on the right spark plug. It could be that you have one mixed up........Easy to check and no parts required!

 

Your problem is not secondary jets. They come in progressively, under heavy throttle.
Did you pull the distributor? It may not be seated properly, like a tooth off. Hand pull the timing mark up to top dead center and pull the distributor cap. The rotor should be pointed dead nuts to a node on the cap. If it is off you may be jumping fire to an adjacent cylinder.
Check to see that all the plug and coil wires are seated.
Yes, you can buy a vacuum advance unit separate from the distributor but since yours holds vacuum when you suck on the line, that is not your problem.
You idle okay. Under throttle, you misfire. Your problem is in the secondary ignition.

 

That sounds like a distributor cap problem. Misfiring and missing under higher loads means the spark is jumping off somewhere it's not supposed to. You could have some oil film or greasy fingerprints on or in the cap that's causing it to arc out and misfire at higher RPM's. Pull off the cap and thoroughly clean it inside and out with Brakeclean or some similar degreaser. Or, if it looks a little burned at the lugs inside, just replace the cap and rotor. Sometimes they can look good and have faults that cause arcing.

Also, check for some oil slime on the nose of the coil. That can cause the same kind of arcing. There's also the chance that you damaged the coil while you were rebuilding the engine. Did you drop it on the floor, by chance? An old coil with internal cracking can cause that same kind of misfiring.