The Stovebolt Site Forums The Shop Area Engine Shop Backfire at 55mph engine quits won't run occaisional backfire only

Last week I was happily cruising down route 3 in MA in my '55 GMC 3/4 ton w 270 (not stock engine for that ear, but its what is there.) at 55 mph. Suddenly, there are a couple spaced out huge backfires, the engine quits, and i coast to the side. She won"t start, spins fast and spectacularly back fires thru the carb. We get towed home, and i am figuring that 1985 Nitrophyl float has had it. Or maybe the main fuel jet fell out again into the fuel bowl.

Took carb apart, a Holley 1904/1960 single barrel, everything looked normal except the carbon in the horn. Got an NOS brass float and needle/seat assembly from ebay, put it all together, and still same gig. no run, backfire.

i am wondering what could fail suddenly like that...valve timing engine timing..will check that next. Thoughts?

First thing I would look at is the distributor, if that checks out then the valve train.

sounds like a timing gear

Continue w/the easy checks. Pull each plug and examine, they can usually tell you if theres an internal problem.
Make sure the ign system is right, check points rotot, dist cap, wires and check/reset static timing.

If all checks out, pull the valve cover and inspect. Make sure they all open/close when spinning the engine. Theres several likely causes up here.

Good luck
Dave

When you said she spins fast,take the valve cover off I'll bet it's the timming gears.

Pete

Timing gear would get my vote.

Jon.

Since we are voting, I too vote for timing gear/chain.

Well, do I have a story. I took out all the plugs. They looked very good. I then took off distributor cap, turned engine by hand, rotor moved. Took off valve cover, turned engine, valves moved, proper sequence, went thru several cycles, head scratching starting.....then I touched the rotor. Now a key piece of data is I had been driving in a field earlier that day, and a relatively non gearhead friend had suggested that might be the problem. She was right. The rotor spun freely. I took it off, and noted scratch marks on the locating nub inside the rotor where it locks with the shaft. I got out my old old distributor cap, (never throw old parts away) and noted that the central electrical contact extended almost a quarter of an inch farther into the cap than on the new one...and the rotor had been contacting the cap contacts at their lowermost edges...the rotor had popped up enough to stop turning with the distributor shaft, was arcing on one contact, and when ever gas went into that cylinder, BOOM, regardless of valve position. No timing. None. I cleaned up the rotor, put it back, locked it into the distributor shaft, put the old old cap back on, fired her up and she roared to life. Warmed up, purred like a kitten. Idling better actually with that new float.
I got a new cap and rotor from NAPA, and noted that the new rotor has the locating nub, but also instead of a round hole, the shaft casting has a triangular shape, so the rotor is locked by the nub and slot as well as the shaft....this has happened before. The center contact spring is also stiffer. The new cap has the center contact similarly recessed the way my malfunctioning one was, but i felt the stiffer spring and triangular hole would make up for it. For now i am running the old rotor and old old cap. And I am a happy camper. Everyone sensing a complete lack of timing was correct. This experience was a lesson in carefully proceeding to investigate problems in an unhurried and observant way.

Thanks for the responses, they gave me structure to check things out in a way that found an unbelievably simple problem that likely started bouncing in the field that morning.

Dave