The Stovebolt Site Forums The Shop Area Engine Shop Truck dies/bucks at Highway Speed

I can't seem to find any information, so help me please. Here is what I have. 1949 with a '57 235. New fuel tank, pump, carb kit, converted to 12V with a HEI ignition. It drives around town nicely (30mph), when i floor it, it takes off, but when i hit 40-50 mph and try to hold that speed it hesitates, bucks and dies. If i pull the choke about 1/2 way out it stays running and I can keep driving. I had the carb rebuilt by a friend of mine, I want to think it is a carb adjustment issue, but am looking for some guidance on this. Thanks.

I drove mine with no choke at all with no problem, does it back fire when it is acting like a bucking bronc.. did the problem start when you installed the rebuilt carb, why was the carb rebuilt.

No problem with backfire, carb was rebuilt because it had been sitting for about 30 years. It doesn't happen driving around the block, just on the highway.

if just on the highway, it is starving for fuel. That is usually either a weak fuel pump, or a restriction somewhere. You have a new tank, did you replace the fuel lines? (especially the rubber ones). I would re-check fuel lines, filters, and as a test, run with the gas cap loose to make sure it is not creating a vacuum in the tank.

Check your exhaust heat damper. If it's stuck open you may experience carb icing.

for sure you need to check as 69cuda mention and those are some simple things to check on a 235....in a worst case it might be a bad cam but I doubt it...

SWEET and I worked through a persistent issue with my truck that had the same symptoms you described. I had trouble with it for almost a year (as I was rebuilding things on the truck). I would have SWORN it was fuel pump/delivery/carb issue.

For me, the bucking only occurred under power, but going up even the slightest of inclines. If that's you too, you may want to look at your timing. SWEET dialed mine in and it stopped the bucking problem.

If fuel-related, which I also experienced, I fixed mine by ditching the Rochester that leaked gas all over my engine (even after two rebuilds) and went with a rebuilt Carter YF from Carb-x. Another bolt-on fix. Still running that great carb.

Best of luck!

Jim

Try a new gas Cap

Try a new vented gas cap.

Make sure that your carb gasket is correct for your carburetor. There is a hole in the gasket which needs to match the holes in the carb base and the intake.
Carl

I had a similar problem with my truck a few years ago. The solution in my case was a new fuel line from the fuel pump to the carb. It was releasing some small rust particles that would keep the fuel flow to the carb restricted, so that when driving with a load on the engine it would starve and start bucking. It was sometimes an intermittent problem, as the small particles would eventually pass through the needle valve in the carb and allow it to act normally. It seemed to act up at the worst times, like when attempting to merge with traffic.

OP -- Indeed, as Sweet says, I had the *exact* same issue. I replaced or rebuilt *everything,* even the wiring going to the distributor from the ignition. Nothing solved the issue of that bucking under load in 4th gear. Turned out to be carb icing. Check your heat riser flapper.

Another easy thing to try -- and this sounds stupid, but it works -- wrap your carburetor in aluminum foil. The idea is trap heat from the manifold to keep the carb warm -- even in warm weather. It that cures the issue, which it did for me, then you can confirm carb icing.

If it's stupid but it works ... it isn't stupid.

Thanks for all the ideas. Once I get time away from work I will try these things and let everyone know.

John, at what position is your heat riser flapper stuck?
Carl

Its stuck in the closed position (no heat coming up)

John,
With it stuck in the closed position (no heat coming up), are you only having driveability issues prior to the engine reaching normal operating temperature?
The reason that I ask this is because having it stuck in the closed position is far superior to having one stuck in the open position (heat coming up).
I am not questioning your highly prized, extravagant solution (I had the privilege of viewing it in person). I am trying to wrap my head around how a stuck-in-the-closed-position flapper would cause icing once normal operating temperature was reached. If that were true, then all 216s would ice up once operating temperature was reached.
I think that I read somewhere that a lean carburetor will increase the chance of icing. Since climatic conditions need to be just right for icing to occur, one may go a long time without experiencing it even if the carb was set up too lean.
Out of wind now. Feel free to tell me to just shut up, just don't call me Fatso.
Carl

Sounds like the fuel filter needs to be changed.


Pete

I had a similar problem on my 58 burb with a 250. Turned out the two nuts holding the carb to the intake worked lose. I had rebuilt the carb and reinstalled it without lock washers. Ran fine until the engine heated up and things loosened up a bit. Then the truck would die in the most inconvenient place. While your checking everything else, put a wrench on the carb nuts and make sure they are good and tight.