Stovebolt.com Forums The Shop Area The Engine Shop Chevy 250 inline 6 runs rough after warm.

Installed a Chevy 250 inline 6 in 65 C10 with 350 turbo, think it came out of a 76 nova. Runs good at idle even when put in drive/reverse (cold), timing set at about 8 deg BTDC. Played with timing about ever which way you can. Runs strong and smooth down the road get her warmed up then rough idle when going slow and when stopped while in drive/reverse, the whole truck vibrates. EFI distributor (appears to be stock) new plugs. Was told it had been sitting 3-4 years. Oil is clean , no smoke, timing gear little noisy. Any suggestions is greatly appreciated. Thanks

I would first check out that noisy timing gear. That noise could be a disaster waiting to happen.

Come on bolters, this gentleman need a few suggestions.

“It’s never the carburetor”

The timing gear if came out of a nova would most likely be made out of fibre ,I had the same setup 72 nova 250 in a 69 c10 no noise but on a nice sunny day at -40 c about 5 hours away from home all stop .The good thing is if the gear goes it won’t damage anything else at least it did not on mine.I would put it on your list of things to do ,I would start off with checking the easy cheep stuff,plugs wires then move on to the carb to see if the choke is off when warmed up .

Does it have the two separate manifolds, or is the exhaust cast into the head? Could be a vacuum leak from a worn gasket or cranked manifold opening up when it heats up.

I like the vac leak theory as first place to check. Get it warmed up to the point it starts acting up….and spray something like carb cleaner at the carb base, manifold connections, etc. If you hear the idle change up or down, you’ve found your leak.

Foxx Take your propane torch not burning ,send propane in around your manifold area,when it smooths up you have hit the leak,or acetylene torch has better nozzle,not burning . Better fix that timing gear noise before it hurts something.

You guys all beat me to the vacuum leak problem. Check your vacuum modulator on the transmission if it has one. Check the vacuum lines going to the HVAC doors, etc.

Thanks all, great suggestions on things to check.

Joe H, has separate manifolds.

Adjusting timing helped with the noise behind cover, however cruising down the road there’s like a roaring noise from some where.

I check all vacuum suggestion.

Besides the timing gear concern it has to be something simple (I hope) as it runs great when cold.

Remove the distributor cap and to see if you have side to side movement in the shaft. I had that problem on 2 250's and both were the distributor bushing worn out. Are you running a clutch fan and it's seized up?