After reading serveral very similar post on timing and carb and black plugs I am still dealing with this same issue. I have a bone stock 455 just rebuilt the engine about 90 days ago car has not be on the road since rebuild. I let it run 3 or 4 times a week in the shop at home. First I want to say I have no experience with timing and carburators. But when at idle the engine shakes like it is missing and the plugs have a small amount of black sleet on them I changed the plugs and still no improvement I ran some Seafoam and the black smoke did stop coming out of the tailpipe, but the motor still shacks just a little when at idle and the plugs are still black. I think the timing is ok because for one I trust my engine builder (30 years of experience) and the car starts perfect. So again what do I do. Oh yeah my engine buider said the car needs to be driven but I don't know. Oh yeah and it has good throttle response as well Nor does it misfire when on the throttle or will driving the car a good amount of time solve this I have heard that any gm rebuit motor will have some smoke and shaking until broke in

 

What is your idle RPM?

 

A very small amount may be normal.
Maybe it's missing - have you checked all the spark plug wires and the distributor cap and rotor? Do you know anyone with a SUN machine, which you could use to check this?

If all the engine does is idle, the plugs will be black.

Neither of those things guarantees that the timing is right.
Have YOU checked the timing?
Trust is a wonderful, warm, fluffy thing, until you realize you can't trust any or the people you thought you could.

So let me get this straight - You paid a big pile of money to a guy to rebuild your engine, so that you could have a car that was fun to drive, and you go out to the garage a few times a week and start it, but you won't DRIVE it?!?
Please explain.

You can't tell how it's throttle response is if you're not driving it [see above].
All you know is that it revs up well in neutral.
Why not check the dwell and timing, and double check the idle speed and mixture settings? Maybe you'll be surprised.

- Eric

 

If the choke never opens, the plugs will get black!
If the carb is on the rich side, the plugs will get black!
If the timing is off, the plugs will get black!
If the engine never gets up to operating temp, how did you break-in the cam??
Did you use a zinc additive?

 

I think you should get out there and drive that thing. My Chevelle did the same thing after a rebuild and breaking it in and driving it often set it straight. I'm not saying thats what needs to be done with yours but it definitely helped me out.

Good luck

 

antweave: You sound like me in about two weeks from now. My engine is being rebuilt by a man I have known and trusted for 30 years. I have had bad luck with my previous un-rebuilt engine and I just want the thing to work right!

I need lots of torque and no spark knock!

I've spent the big bucks to do everything to make my rebuild (better than) new again. I just hope that it runs right! I want to burn rubber like there is no tomorrow... If things work out ok, I'll then have my posi-traction axle installed!

We will see in a couple of weeks.

Drive'er MacGyver!

Jaybird

 

AFTER SOME DEAP THOUGHT iT HIT ME LIKE A TON OF BRICKS i DID NOT GA THE PLUGS AT ALL i WENT OUT TO MY HOME SHOP AT GAP THE A/C delco plug to .40 the wiped as much of the black **** of and put them back in. Will this help the plugs are new to the build again car has not been drove since rebuild on in the back yard I feel like a complete dumb ***. Oh yeah the car is a project with limited funds so it has no license plates so it just sits in it's spot in the shop until funds are available.