Help Needed - '68 c20 w/ 396

[Rippin-J]

I've had my '68 now for over a year, and the guy I purchased it from "had the motor rebuilt" but then it sat parked in his shop for 5 years before I purchased it. It's never run great, but for the past 6 months or so is been SUPER rough, too sketchy to even rip around the block.

It'll turn over okay (after a jump..... alternator maybe?) and even idles OK for a few minutes. Once it warms up it begins to spit and sputter a bit, sometimes sounding like it's on the verge of dying. The left side (driver's side) seems to be the worse as it blows black smoke (not TONS) out of the pipe. Also, when I dump the throttle and throw open the carb, it is VERY late to respond and near dies.

First, the thought was carbs because the Quadrajunk that was on there wasn't even opening up with the throttle. Replaced it with a brand new Holley Street Avenger. That didn't fix it, but the motor gets more gas now.

The thought now is that the issue is something internal: compression, valves, gasket, piston (rings), etc etc. My major dilemma is that my network of 'gearhead' buddies are all in Colorado and I now live in Charlotte where no one in my network fiddles with or knows much about old cars. Oh, and I know very little about the diagnosis and repair of internal components.

So here's what I'm asking the forum for help on:

1) What novice level tests can I do to further diagnose the issues I am having.
2) Does anyone know anybody or is anyone in the great Charlotte area willing to help me learn the ropes.

It's a very clean truck and motor, so I'd hate to just scrap it and replace it with a rebuilt 350.... but am willing to if that seems like the best case scenario.

Anyways, you all have been very helpful in the past so I have no doubt that someone, somewhere will have some ideas.

Much appreciated,

Jordan

[kwmech]

I'd go after a valve adjustment first, followed by a compression test.

[stllookn]

I agree with KW but would do the compression check first since it will be much easier with your level of experience. All you have to do is purchase a compression gauge from your local auto parts store and follow the instructions. Some of your problems could be timing as well as compression due to valves or poor break-in on the cam.

[BigDaddyDave]

Have you tuned it up? I can tell when mine starts running rough that it is time. I spent a lot of money on my junk,then replaced the distributor and it made a world of difference.

[fun in dirt]

?bad gas? Drain lines, tank & all. Just a thought if it's been sitting with same gas for over a month or so, especially as crappy as gas is now. I spent $100+ on mine one time, when it was acting like that, & it was just the gas.

[Rippin-J]

You know, I hate to admit, but I've heard the term 'tune up' used my whole life and I can't say that I know exactly what that entails.

When you say tune up, what all does that include?
timing, distributor, wires, oil, ?

Any chance you feel like making a trip down I-85 to help out? I'm just off the interstate!

I think the tune up sounds like a good first step, followed up by compression test and then valve adjustments. My assumption is that tune would eliminate a bunch of other possibilities, then the compression would confirm others, and finally a valve adjustment would be the last case scenario.

[Rippin-J]

Gas is another good point to check... the gas that's in there is at least 3 months old now.

[Longhorn321]

Have you tried a new set of plugs? I would start there if you haven't tried that already and then look at the timing, compression and cylinder leakage, and then the valve adjustment as stated above. The timing and valve adjustment would be more suspect to me due to the fact that you said the motor had been rebuilt and had not run great since you have had it...who knows if the PO knew what he was doing.

[Rippin-J]

yep, new plugs for sure. replaced those with the carb in April.

[kwmech]

Originally Posted by stllookn

..................................................................................... Some of your problems could be timing as well as compression due to valves or poor break-in on the cam.

Hence the reason for the valve adj first----if the valves are too tight then the compression is gonna be a false reading

[Longhorn321]

Originally Posted by kwmech

Hence the reason for the valve adj first----if the valves are too tight then the compression is gonna be a false reading

Tru dat! I actually had that same condition...missing on 3 cylinders and compression was horrible. Adjusted the valves and she ran like a top.