Misfire Problem
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Joined: Nov 2001
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Misfire Problem
I have a 302 with a little misfire problem. The engine idles fine and on slow acceleration it's ok, but when you accelerate fast or once the RPMs get up a little it starts to mis and back fire out the exhaust. I did notice that the exhaust smells rich even at idle. I have replaced the rotor, points and fuel filter. I have checked the plugs, wires, fuel pressure and cap.
This engine is in a '73 F100, but it has '78 heads and intake manifold. From what i understand the '78s came with electronic ignition and not points, but mine has points. The plugs for a '73 302 are bigger and hotter plugs than the ones recommended for the '78 (which is what I have). Could running hotter plugs solve my problem. I am still checking things out, but some help would be appreciated.
This engine is in a '73 F100, but it has '78 heads and intake manifold. From what i understand the '78s came with electronic ignition and not points, but mine has points. The plugs for a '73 302 are bigger and hotter plugs than the ones recommended for the '78 (which is what I have). Could running hotter plugs solve my problem. I am still checking things out, but some help would be appreciated.
New User
Joined: Jan 2008
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Misfire Problem
Sounds more like a carburetor problem to me.
Has the carb been worked on ?
You didn't mention it .
Dennis
78 F-150 429CJ C6 ,Silver w/Explorer Pkge
641/2 Mustang,Pre-World's Fair Car #8092
64 Fairlane S/C waiting for a 390-4spd.
68-Mustang.Sunlit Gold 80,000 miles
Has the carb been worked on ?
You didn't mention it .
Dennis
78 F-150 429CJ C6 ,Silver w/Explorer Pkge
641/2 Mustang,Pre-World's Fair Car #8092
64 Fairlane S/C waiting for a 390-4spd.
68-Mustang.Sunlit Gold 80,000 miles
Moderator
Joined: Jan 2001
Posts: 56,950
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From: Virginia
Misfire Problem
Does it miss and backfire only when you stab the throttle, or all the time when you are holding the throttle steady at high rpm? If it's only during the transition when you stab the throttle, it might be your accelerator pump. Also check the timing with a light.
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Misfire Problem
Thanks for the input. No I have not done anything to the carb yet, but that was my next target. Do you recommend replacing just the accelerator pump or should I rebuild the whole carb?
Posting Guru
Joined: Jul 2001
Posts: 1,613
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From: Gainesville, FL
Misfire Problem
Smurf,
Backfiring is not caused by the carb. You have an ignition problem. Most likely, it's your plug wires. Spray them with water in the dark and see if they spark. I use a old fantstik bottle with water in it. Again, backfiring is not a fuel related sypmtom. How did you set your timing and and what degree? Check yopr distributor shaft and see if the center shaft bushings are shot. This is another classic for your vintage vehicle.
Your head swap should have no effect at all.
Good luck, Have Fun,
KingFisher
Backfiring is not caused by the carb. You have an ignition problem. Most likely, it's your plug wires. Spray them with water in the dark and see if they spark. I use a old fantstik bottle with water in it. Again, backfiring is not a fuel related sypmtom. How did you set your timing and and what degree? Check yopr distributor shaft and see if the center shaft bushings are shot. This is another classic for your vintage vehicle.
Your head swap should have no effect at all.
Good luck, Have Fun,
KingFisher
New User
Joined: Jan 2008
Posts: 0
Likes: 24
Misfire Problem
I referenced the carb because of the rich gas smell.
Another area is the Vacuum advance might be seized,try testing it with a Vacuum pump while you're watching to see if it moves.
And yes, distributors have a way of turning in an orbital pattern rather than perfect circle when they are getting worn out.
Dennis
78 F-150 429CJ C6 ,Silver w/Explorer Pkge
641/2 Mustang,Pre-World's Fair Car #8092
64 Fairlane S/C waiting for a 390-4spd.
68-Mustang.Sunlit Gold 80,000 miles
Another area is the Vacuum advance might be seized,try testing it with a Vacuum pump while you're watching to see if it moves.
And yes, distributors have a way of turning in an orbital pattern rather than perfect circle when they are getting worn out.
Dennis
78 F-150 429CJ C6 ,Silver w/Explorer Pkge
641/2 Mustang,Pre-World's Fair Car #8092
64 Fairlane S/C waiting for a 390-4spd.
68-Mustang.Sunlit Gold 80,000 miles
Elder User
Joined: Jun 2001
Posts: 997
Likes: 0
From: Corydon US of A
Misfire Problem
Smurf, this would be a simple solution but make sure u check because i've seen plenty of people throw money down the drain because of this. check you coil wires make sure they are in good, Ford's stock coils arent very good, and i've had to soder up the hot and ground wire on my coil, not saying thats what it is but if you had a bad connection it would backfire. i've done it before, might help
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Senior User
Joined: Nov 2001
Posts: 144
Likes: 1
From: Savannah US
Misfire Problem
It sounds like an ignition problem. But it could be the power valve in the carb if its a holley. One good backfire through the carb can wipe it out. They are fairly simple to change, and very cheap $3-4. I'd say test the plug/coil wires, coil, timing and the distributers condition as well. I had a points distributer do about the same thing in my 2wd. If it spits up through the carb, it may be to large of a carb. If thats the case you can jet the carb down, or install a lesser CFM carb.
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