79 351M Backfires
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From: Midwest
79 351M Backfires
I have a 351m in my 79 F150, ran decent until i recently replaced cap, rotor, spark plugs, plug wires. now it back fires or hesitates when it is under a load under driving conditions, you can rev in it park to the moon and she sounds great, im stumped. Dan
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Joined: Nov 2001
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From: Renton USA
79 351M Backfires
>I have a 351m in my 79 F150, ran decent until i recently
>replaced cap, rotor, spark plugs, plug wires. now it back
>fires or hesitates when it is under a load under driving
>conditions, you can rev in it park to the moon and she
>sounds great, im stumped. Dan
Did you route the plug wires on the left bank (driver side) right? On these engines, they are picky with how they are routed. According to the '79 Ford shop manual, the routing should be from distributer starting from the front - #5, #7, #8,#6. In other words this is how they lay togather in the clips until you are past the valve cover, then they go to there respected spark plug. If they are routed in numerical order-#5, #6, #7, #8, you get a possiblity of a spark crossing over to the wrong cylinder, which may have a intake valve still partially open. This is what I remember a mechanic told me after I did some mods on my 400 and it ran like yours. All he did was route the wires right, then it ran like a dream for 265,000 miles;-) . Something easy and cheap to try first anyways. Take care, Mike B.
>replaced cap, rotor, spark plugs, plug wires. now it back
>fires or hesitates when it is under a load under driving
>conditions, you can rev in it park to the moon and she
>sounds great, im stumped. Dan
Did you route the plug wires on the left bank (driver side) right? On these engines, they are picky with how they are routed. According to the '79 Ford shop manual, the routing should be from distributer starting from the front - #5, #7, #8,#6. In other words this is how they lay togather in the clips until you are past the valve cover, then they go to there respected spark plug. If they are routed in numerical order-#5, #6, #7, #8, you get a possiblity of a spark crossing over to the wrong cylinder, which may have a intake valve still partially open. This is what I remember a mechanic told me after I did some mods on my 400 and it ran like yours. All he did was route the wires right, then it ran like a dream for 265,000 miles;-) . Something easy and cheap to try first anyways. Take care, Mike B.
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Joined: Oct 2002
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From: NiagaraRiverArea, Riversi
79 351M Backfires
Does the engine have orginal cam shaft?, some replacement cams require to use 351w sparkplug firing order!
take one wire off at a time to find out what one is firing out of order only two at a time....
take one wire off at a time to find out what one is firing out of order only two at a time....
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Joined: Aug 2000
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From: (near) Denver USA
79 351M Backfires
The 351W firing order is the same as all 335-series engines (1-3-7-2-6-5-4-8), and of course, you can't use a 351W cam in a 335-series engine anyway.
This firing order issue is peculiar to replacing the cam in an early 5.0/302/289/260/221 engine with the 1-5-4-2-6-3-7-8 firing order.
This firing order issue is peculiar to replacing the cam in an early 5.0/302/289/260/221 engine with the 1-5-4-2-6-3-7-8 firing order.
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From: NiagaraRiverArea, Riversi
79 351M Backfires
I did buy a TRW cam for my C351 and the instructions were to use Windsor timing order, it was an aftermaket mild racecam.
The only other idea is that if the engine sat , say allsummer, the accelorator pump in front of carb is dry and has a pinhole in it and needs replace'n. You can buy that for a few dollars snd replace it without taking carb off engine. jim
The only other idea is that if the engine sat , say allsummer, the accelorator pump in front of carb is dry and has a pinhole in it and needs replace'n. You can buy that for a few dollars snd replace it without taking carb off engine. jim
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Posting Guru
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79 351M Backfires
All 351 and 400 engines use the same firing order. (I can't say for sure about a reverse rotation marine engine). TRW should know this.
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From: the soo
79 351M Backfires
Have you checked the spark plugs wires for continuity.
I did a tune up on my 351C with all new stuff even bought the new,expensive fandangled siliconed filled spark plug wires.
After installing everything had very bad backfire when the engine was under load. Checked everything timing, gap of plugs you name it.
finally put old wires on and the backfire was gone.
Found that one of the spark plug wires was faulty.
Get out your multimeter it could be that simple.
I did a tune up on my 351C with all new stuff even bought the new,expensive fandangled siliconed filled spark plug wires.
After installing everything had very bad backfire when the engine was under load. Checked everything timing, gap of plugs you name it.
finally put old wires on and the backfire was gone.
Found that one of the spark plug wires was faulty.
Get out your multimeter it could be that simple.
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Joined: Dec 2002
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From: chicago burbs
79 351M Backfires
like telesqrt is saying, route #5 and #6 away from each other.let 5 go directly to the plug by the front of the engine and take 6 the far way around the cap and on the far side side of 7 and 8 wires. change wires for length if necessary. the concept is called 5-6 induction misfire. no guarantee that's your problem, but it might help
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