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.
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.
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)
Don't assume anything. Check the entire ignition system, part by part. Make
sure the distributor is installed with proper gear timing, or by dry timing.
Check plug wires not crossed. Could be cracks or carbon in distr cap.
Flush the entire fuel system and make sure gas is fresh.
Sounds like the timing chain is stretched way out severely retarding the
valve timing which screws up the ignition timing cause the distributor is
run off the cam.
Remove the dist.cap and observe the rotor button while rotating the
crankshaft back and forth....this will indicate how much slack is in the
Any slack is too much.........!
Thanks for all the help. I have been able to get the timing adjusted. I
found several things wrong after going back a double checking everything.
The #5 spark plug was broken where the porcelain mates to the metal. So
needless to say no fire to #5. I also found several vacuum leaks, mainly
ones going to the emissions canister.
I've replaced the manual tranny, installed a TH 350, cleaned the carb,
installed a rebuild kit, and upgraded the manual brakes to power. I should
have the old truck on the road real soon. Thanks again for everyone's
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