1979F150

I've got a 1979 Ford F-150 4x4 with the M-Block 400 Motor and a C6 Trans. Just purchased it and it had been running fine. R2 the NSS on the Trans and then took it out back to climb some hills. Afterwards decided to go into town and the transmission wouldn't up shift on it's on into 2nd (Manually I could still select it on the column). Finally after experimenting into town it up shifted on it's own but my RPM's had to be through the roof. Same problem going from 2nd to 3rd in drive. Think I messed up a valve cause now I'm getting a pretty consistent back fire through the carb. Anyone experienced this on the Transmission before? Thanks for any help.

Junkyardjeff

Bad modulator valve or a leak in the vacumn line.

1979F150

Thank you very much Jeff...Tracked the vacuum line out of the Modulator Valve up to the back on the intake manifold and found that I had knocked it off while running the wiring for the NSS. She's shifting fine now. Just need to get that Back Fire sorted >.<

13Harleyfan

Pull your plugs, they will tell you the story of your misfire, one should look different than the rest, probably caused by the vacuum leak.

rusty78

Hey guys I gotta 78 f100 and it does what you were sayin about it back firing through the carb and it shifts fine its got a lil lag in it and I also just replaced the distributor, wires, and plugs and it still does it on occasion especially if I let my gas get close to E any suggestions?

1979F150

Some how the back fire seems to have worked itself out...weird but welcomed...lol

1979F150

So the back fire came back...pulled the plugs to look at them...didn't really see anything...then while running the engine pulled each individual plug on at a time to see which cylinder it may be...found out which one...part of the plug wire innards stayed on the plug itself an the back fire stopped...R2 plug wires and all cylinders going like a champ with no back fire Hope that may help 78...probably the cheapest fix to start with...