1970 Coupe Deville engine miss? Please Help!!

[PsychoBillyCadillac]

Hello,
Maybe you guys can help me with my frustrating engine problem on my caddy. It's a 1970 Coupe with a 472 ci. V-8 and has about 115,000 miles on it. The engine has been freshened up quite a bit since I got it including a top end re-build, full tune up, new exhaust system, etc. Recently I noticed that the engine has begun stuttering. It still starts perfectly, but seems to have this skip and shakes throughout the throttle range like it has dropped a cylinder. It does it in neutral and while under a load, it never goes away. I did a compression test, it has ample in all cylinders. I put in a new set of lifters thinking maybe it had a collapsed lifter, but that didn't change the problem at all. I checked the cap and rotor, and wires (all fairly new) and it has ample fire throughout. I replaced the plugs, which were black with carbon, but it had no effect on the problem at all. What am I missing here? When I changed the lifters none of them looked overly cupped or deformed on the bottoms so I really don't believe it's the cam. Could it possibly be a rocker or pushrod? The tips of the rockers where they contact the valve stems did look well worn. The pushrods looked straight from what I could see. I looked all over and didn't find any sort of vacuum leak or anything around the crb or intake. I'm at a loss. Any suggestions would be really, really appreciated at this point. THANKS!!

[Hooligan63]

Check your timing first,then go from there.Did you do the firing order correctly after the top end rebuild?

[PsychoBillyCadillac]

I will check the firing order but I am fairly certain it is correct. The top end re-build was several thousand miles ago. The car was running perfectly until it just started this shuddering business right out of the blue the other day. It's really got me scratching my head.

[Hooligan63]

See,if it's a bad lifter,a bad pushrod or a bad valve you'll hear a loud tick,and possibly a knock while it's running.Might want to check for vapor locking in the fuel lines too.Now,I'm guessing when you say shudder it's like when you drive a manual transmission and try to take off on a higher gear at too low of an rpm?Only way I can describe what you're trying to describe.Also,might want to get rid of your points and drop in a pertronix unit.Points are a bitch man.With a Pertronix,no more having to change points and condenser,or gapping points.But with the way you described your plugs,sound like you're burning a lot of fuel and running a hell of a spark,so might want to definitely look at the timing,and possibly air/fuel mixture at the carb.Also,run some Sea Foam through the fuel and oil,which will get rid of a lot on nasty crap in your system.

[BRUCE ROE]

Any car that old needs the timing chain changed, did you? Regardless of miles, the sprocket has plastic teeth that can wear or crack & fall off at any time.

You can check the chain by removing the distributor cap and rocking the crank by hand. If you move the crank 6 degrees before the rotor starts to move, the chain is shot. More is impending disaster. A roller chain replacement is best. Bruce Roe

[cadillac_al]

It's always the ignition points.

[steelybill]

I'm with Al. If the rubbing block on the points is worn a bit, the point gap may be too small, working OK on some cylinders but not all. Just a touch of run-out in the distributor shaft with points too close can make it run strange. Check the gap.

[PsychoBillyCadillac]

Wow,
Thanks for all the suggestions guys, some of these I would not have thought of. To describe the "shudder," when the engine is at idle it looks just like someone has it by the manifolds and is rocking it from side to side very quickly, you can feel it vibrate throughout the car. When you rev the engine, as the RPMs drop back to idle it gets worse for a moment and then goes back to a steady shaking. Under the hood it is quiet, there is no ticking or knocking at all, but if you hold the throttle part way you can hear a faint ticking through the exhaust which mirrors the shaking. I didn't fool with the points/condenser when I did the engine re-build as the car was running well prior. I have never changed a set before so can someone tell me what to look for if they are bad and how to go about replacing them if they are? I know some engines do strange things if the points are out of whack so I think I will try that approach next and see what happens. THANKS!!

[steelybill]

You might consider installing a Pertronics electronic ignition kit in your point type distributor. It will have great spark, and you will likely never have to fool with it again. They are available in many auto stores and online, at places like Summit Racing, Jegs etc. The spark plugs will last longer also, due to the high frequency type spark they put out.

[cadillac_al]

Dude, when I ran points cars I had to change them about every year. They would start running bad and I would think the engine is getting worn out. Put in new points and it would feel like a new car again. That's why I refuse to use them anymore; they just don't last long. I convert everything to electronic ignition now. If your points haven't been touched then it will run like new with a new set (for a little while).

[steelybill]

You are right Al. I seldom got more that 10 or 12 K miles on a set of points in the old Chevy small blocks.

[PsychoBillyCadillac]

Well here is the disheartening update for today. I opened up the distributor and looked at the points, which I actually had put in when I first got the car, just forgot about it. The block that rides on the distributor was obviously worn and had decreased the gap. I re-gapped them, started the car, no effect on the shaking. I then took them out, cleaned the point surfaces themselves, which were a little bit burnt, tried it, no effect. Then I went to the store, got a brand new set, tried them, no effect. I also played around with the gap a little just to see, which didn't do anything either. I checked the timing and firing order, all are spot on. The only thing that has changed is that the exhaust smokes ever so slightly, which it didn't do before, and you can hear the ticking and sputtering in the exhaust more prominently now it seems. Am I wrong for wanting to stick a rag in this thing's fuel tank and light it? Any more ideas gentlemen?

[BRUCE ROE]

What about checking the timing chain? Bruce Roe

[Hooligan63]

Man,better hope your crank isn't screwed.Vibration and noise makes me suspect the crank,but take it to a mechanic instead of asking all of us.

[steelybill]

Bruce is likely right about the timing chain, with the plastic teeth being on the edge.
Maybe you should take a look at the rocker arms. Even though you did a top overhaul a short time ago, the stock rockers are a weak point in these engines.
I had one that the bolt that holds a pair of those came loose. Shouldn't be too difficult to take a look. We are still trying to get some ideas here
Did you replace the coil? Heat related problems could affect the coil on some ignitions with points. The old style coils were oil filled, and prone to heat problems as I remember. The spark is strongest at idle, because of longer coil saturation, and any carbon tracks on the coil tower or anywhere in the cap will affect the way it runs.
You could also start it up after dark, and see if any stray arcing is evident in the ignition system.
If you suspect an intake manifold leak anywhere, like at the cylinders that are affected, dribbling a few drops of gasoline along the manifold gasket edge will bring RPM up if there is a vacuum leak.