I just recently installed a rebuilt 300 in my 82 F150. I completed the intial 20 min. break in procedure. After lowering the engine back down to idle i noticed a misfire. I let the engine cool over night and restarted it the next day and the misfire is still present. The timing is set, all the plugs are all gaped, burning the same, the wires have all been checked as well. The misfire is present at all rpms (more noticeable at lower to mid rpms).Engine also starts fine and revs quite freely. Any ideas? Thanks, Barry

 

Vacuum issue somewhere maybe? Kind of an out of the box approach..hmm..what cam did you get cuz it could affect how the engine is running

 

seems to have a good amount of vacuum at the pcv and choke. I can also see the armature moving for the distributor advance so that seems ok. As far as i know there is nothing major with the cam (reman engine from advance auto parts). Thanks for the reply

 

Okay..gotcha. I brought up vacuum and advance cuz I know some guys like to swap in radically big cams in these things. While some cam is good..too big is not..you start having vacuum issues that could in turn affect advance..etc. is it a stock carb? Usually I see guys run the advance vacuum line from their carb to the distributor..maybe somewhere? Or it is likely not a vacuum isue..only other thing i can think of is the timing is off(obviously or maybe your cam is degreed wrong? That would make sense if it was in all rpms..hiatus trying to get the thinking juices flowing

 

Hmm. I am running the stock carb, and i did time the engine with a timing light, i ran it with the vacuum advance line off and plugged and it ran the same, so im not thinking its a vacuum issue. The truck ran with these parts in less than a month ago. The only think i did different was the efi manifolds and put in a set of aftermarket gauges, coolant temp and oil press, so i am running with the stock wires for those 2 disconnected. I dont see that causing any interference with the ignition though?

 

It may be the carb. The YFA's I ve seen don't like being moved around. It isn't too hard to knock the float level off. Another thing with the carb is,,,maybe moving it around dislodged a peice of corrosion inside the carb.

 

im having the same problem with my buddys 4.3 85 chevy found out it was the ignition control modual double check your dizzy

 

Ok I will check these things you guys mentioned after work. Could it possibly be the ignition control module? Thanks for the replies guys!

 

Do you have a vacuum gauge on the engine?
What's it doing at idle? Steady? What's the reading?
If not, I highly recommend getting one.

A lot of auto parts store (Autozone for one) will check your ignition control module for free. Might as well take it down and have it checked. At least you'll know if it's a culprit.

 

AB, I dont have a vacuum gauge, at idle it stays steady, but really shaky for quite so time then eventually dies. I live in a border town (Canada/USA) so i dont get across that often, especially when the truck is my daily driver. haha

 

I'd highly recommend getting a good vacuum gauge. Since your engine is basically a big air pump, NO better tool can diagnose it like reading manifold vacuum. Every little hiccup or wiggle of the vacuum needle can be used to diagnose symptoms.

 

it could also be the pick up coil in the dizZy just acouple things that i know that will cause that my buddys truck was idiling rough with acouple of hiccups when you revved it up it missed and back fired fixed the ign contol model and now it runs perfect, you may be better if it turns out bad is replacing the whole dizZy

 

i purchased a vacuum gauge yesterday and also changed the cap and rotor, no difference yet but i do have another distributor pick-up that i will try after work.
Thanks again for the replies

 

may sound really dumb and stupid but make sure all of the spark plugs are fully tight, if you used autolite plugs for some reason they like to loosen in 300 after being run a few times, i chased my butt for a month on a miss fire and all it ended up being was number six plug not tight.

 

So I changed the pick-up assy in the distributor, re torqued the intake and made sure all of the spark plugs were tight. Still no difference. The only thing that has yet to be changed is the ignition control module. Could that be a probable cause? Thanks.