Re: rough running 250 straight six

I have a 79 GMC 1/2 ton pickup with a 250 straight six that I

> purchased a while back for my son as a project. I was told by the > previous owner that the truck wasn't driven for at least a year and > was only started once or twice during that time. The engine runs > rough, but doesn't smoke or knock. I checked the timing and noticed > that it was at least 30 to 40 degrees below top dead center (BTDC). > When I adjusted it to around 8 to 10 degrees BTDC it ran even worse > and would backfire some and would hardly crank. It has new plugs, > plug wires, distributor cap, rotor, etc. What could cause the timing > to be so far out and run smoother than when it is timed properly? > Could the distributor have been previously put in and be off a tooth? > I'm basically trying to figure out whether the engine is worth > messing with. I had planned on putting a 350 in later, but would like > to leave the 250 in for the short term until my teenage son proves he > can stay out of trouble. I only paid $225 for the truck, so a new > engine wouldn't be a major setback. Any help would be greatly > appreciated. Thanks. > > Chris

How are you timing it? The worst way to time an old engine is with a light, as the vibration damper on the crank often moves on the rubber over time.

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Commentator
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tiger69k

Vacuum guage. Essentially, adjust to high vacuum. If you get a ping, back off in small increments until it stops.

Alternatively, you can either change the vibration damper, or pull a plug, turn the engine to TDC, and make a new mark on the damper to allow you to use the light.

I would just use the vacuum guage.

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Commentator

Vacuum gauge can get it close. short of replacing the balancer there is no way I know of to get it dead on the money. there is no timing chain, its gear to gear so that rules that out. I would imagine the carb is in pretty nasty shape internally, and minimum drain gas tank and replace fuel filter. There are lots of parts out there for that 250 performance wise if you know where to look. And even in stock form it will go just as fast as a 350, just takes a bit longer to get it wound up. Something else came to mind, the grease under the ignition module may have dried out, and your not getting a good ground. make sure the vacuum line to the advance is hooked to the correct port on the carb, and disconnected and plugged when setting timing (not trying to be insulting here, just not making any assumptions) Whitelightning

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Whitelightning

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