78 gmc w/ small block dieseling problem
 

I have a 78 with a 350 small block. I just changed all the plugs because of a dieseling problem, noticing that one plug was exceptionally fowled with what looked like oil debris buildup (an assumption made after consulting my chiltons manual). Went for a quick test drive and it's still dieseling. Where do i go from here? do i need to get into the motor?

im sorry but im not sure i follow on this word dieseling
and dont start tearing into the motor just yet
check the fuel pressure first also check the distributer and coil and distributer rotor and the coil in it also
i guess i would need to knwo what dieseling means first
also it is carburated maybe the jets need cleand
also if its fowling out the plugs check the gap setting on the plugs it should be .035 if im correct go to a local parts store and get a gap setter there only a buck or 2
and as for the oil build up you may be burning oil casued by a bad oil ring on the piston
but again would you care to elaberate waht dieseling means
and i noticed you were a new member have you gone to the new member forum and introduced yourself
and welcome to the forum and hope you stick around a awhile as otehrs come and go after they get there answer
keep us posted

Way to soon to think about tearing into the motor. How many miles are on this motor? Any rebuilds or mods done? Today's octane may not be high enough for this engine, does running premium gas help?

dieseling means it keeps running after i turn the key off.

when i bought the truck the guy who sold it said he had rebuilt the motor. If he was truthful, and i think he was, it probably has around six thousand miles on the engine now. I have driven it pretty hard a few times out on the mud trails. No mods done to it, just rebuilt. could it be the fuel/air mix is too rich?

that sounds like its and issue with the ignition tumbler wireing also if im correct your truck has a carb and a manual choke
i do know if your choke sticks on or open it will keep dumping fuel in it and keep it running
so maybe the adjestment screw in the choke is loose or to tight
also if the guy was truthfull about it being rebuilt engine i would look into it because people will say that and then it wont be did he produce any reciepts of what he put on it

keep us posted and take ssoem pics of your truck
and i go muddin sometimes but not as much as i used to due to the wife and kid
but also be ready to replace your hubs on your truck they wear pretty fast when goin mudding ive replace quit a few of them on my buddys trucks
and my own
and also keep an eye out on your altenator as it will eventually go bad from all the jerking and bumping from goin on the trails i learned that the hard way after a day of mudding and haveing to pull people out then i drove home and the battery gauge started goin down then i heard a horible noise from the engine turns out the beaerings inside the altanator were broken so it wouldnt spin

If he kept the same compression ratio as stock then the octane raing of the fuel might be to low. Could also be the timing.

yes i guess it could also be the octane rating in the fuel or timming