Ok, so I've got a 1978 F150 4x4 with a 351M and a C6. It has been running fine and has been my daily driver for the last couple of months. However, it has recently started dying on me while driving. The first time it happened it didn't actually die, but almost did. I was driving down the freeway at about 60mph and it started sputtering so i slowed down and switched lanes and started pressing the gas again and it acted normal again. And then about 4 days ago while driving at about 40mph it started sputtering again and then just died. I pulled over to the side of the road and got it started again and drove it home just fine (only about another half mile). Then yesterday i was on my way home for lunch and it started sputtering again at about 35-40mph, so I started slowing down and pulled off to the side of the road, where it was running rough and then died. I started it again and it idled fine, so i started to drive again and once i hit about 20 mph it again started sputtering and died, bringing me to a stop again. So i once again got it started, put it in first gear and started driving, i managed to drive it through first gear at about 15mph, then into second gear up to about 25 where it started sputtering again, at which point i hit a school zone and slowed down to 20mph and limped it home. I checked the fuel filters (one by the carb and one connected to electric fuel pump) and they were in pretty bad shape so I put new filters on it and called it a night. I drove it to work this morning just fine, but when leaving for lunch again today it started doing the same thing, but this time as soon as I try to drive it. Now here is the really strange part...it will drive fine in reverse, and will idle fine, but as soon as I put it in any forward gear and try to drive forward it starts sputtering and wants to die and will only let me get up to about 15mph. I have no idea where to start or what to look for, can anyone help? I'm sorry this was so long winded!

 

My guess is there is dirt in the carb or that the pump is not pumping enough to keep up with the demand for fuel.. That is why you can't got fast.. Your sucking the fuel dry..

 

Thanks for the reply! That makes sense, the only thing is that it does it to me now even at 5-15mph, but i can floor it in reverse and go fast in reverse, and rev the crap out of it in neutral or park, just can't drive forward. May be a stupid question...but what is the transmision modulator and what does it do? Could this be tranny related at all?

 

The tranny module is responsible for shifting the tranny.. I have had them go and the tranny won't shift up so the engine will whine..

It doesn't sound like a tranny issue to me, but I guess anything is possible

 

OK, thanks, just figured I'd ask. I guess I can just sum up my problem for anyone reading by saying that my truck drove fine until recently. Now it will sputter and want to die if I try to drive it in any forward gear, and won't let me go above about 10mph, but it will run/rev/drive just fine in park, neutral, and reverse...it's also my daily driver so i need to fix it quick, any info helps!

 

I believe you need to clean carb probaly a 2bl they act funny that way sometimes.

 

Sounds like the float height is too low, or the fuel pump isn't putting out enough volume. Either way it sounds like fuel starvation.

 

I just had the same problem, the arm on the fuel pump is not being lifted enough to let the right amount of gas in for high speeds, they opened up the front of the motor tightened up the wheel inside that turns and lifts the fuel pump arm to let gas in, it is fine now, I hope this helps.

 

Thank you all! I guess I should have noted that it's got an aftermarket (Mr. Gasket) fuel pump, and it has a rebuilt Holley 2bbl, that I actually just tore apart and it was all clean. Would a clogged or malfunctioning fuel pump make it act this way? An additional note...it ran fine all the way home after work today. I just can't figure out why it runs fine in reverse, but not going forward, and intermittently. Thank you all again for your help, I really appreciate it!

On a side note...i have access to a rebuilt (supposedly, there is no paperwork or any way to prove it) 351M with an RV cam, edelbrock carb, and edelbrock intake for $375...is that a good buy? The guy says that he would refund me my money if it didn't run good as long as I install it within a few days of buying it.

 

i had a dura spark module go bad intermidently and it would feel as if the ignition just cut off and came on. i blamed the carb but the carb ran good on another car. also dont buy a autozone module they are crap get a mortocraft or something good.