rogergcam

I have a '72 454, Auto, Air, PS, PB and the car has a miss we can't find.
I had it in the shop-second time, and they did: new battery, ignition wires, points, condenser, rotor, distributor cap, as well as rebuilding the distributor. Found several vacuum leaks, and replaced a lot of vacuum hoses. Set timing, and dwell. I put a new rebuild carburetor on several months ago.

The car seems to idle OK. But when you get up to 40 MPH plus you can feel it miss. The tach also fluctuates 75-100 rpm. If you put it in neutral and rev up the engine you can see the tack bounce, and hear it miss.

Several questions:

How can I check for vacuum leaks? The car is a real nightmare of vacuum hoses. How can I check for leaks without doing each hose one at a time? How much vacuum should I have at idle?

Could this miss be coming from someplace other than the ignition? Could a slipping transmission cause the same feeling, and change in RPMs? If yes, how can I isolate it?

How about the jetting in the carb? Any thoughts on jet size? We are just a little above sea level.

I'm at the end of my rope with this! Any ideas or thoughts will help!

THANK YOU ALL!

carriljc

Try swapping in a generic coil. I had an intermittent miss that took me forever to find.....Just "happened" to find it late one evening while spacing-out as I pondered what to do as the car was idling and I was continuously flashing flashing the timing light on the wall...... when I suddenly noticed a "skip"...hmmm.... then as I paid attention I noticed that it was "skipping" in a non-routine manner. I had an old Datsun back then and I just swapped in it's coil..... immediately fixed that.
PS-It also eliminated a problem I had with the car suddenly dying in really hot weather.....which, as soon as I opened the hood, would cool down the coil, and it would just restart.....that was a pain.

You can pick up a cheap coil at autozone for under 20 bucks:

https://www.autozone.com/ignition-tu...9_576402_17367

Mooser

Double check your intake bolts for tightness

Maybe treat the car like any other chevy and disconnect the "extra" vacuum systems so you only have the power brake and transmission lines and even the pcv (need one of those push in valve cover breathers to replace it. Cap off the other fittings and "T's"

Then try that and see if the miss is still there. If nothing else it might separate the "everything else" from the equation

A fuel line leak, especially where it doesn't drip but will suck air when under load is usually more dramatic and causes the car to fall pretty flat on it's face once you hit the point where it starts to suck more air than fuel and then goes back to OK once the rpm drop back down to where it can keep up

Rebuilt carb (just ask Lars) might not be correct or the choke is set incorrect

Just some WAGs
M

Rescue Rogers

pull one plug wire at a time and see if all 8 change at idle. If its just one plug it could be a bad lobe on your cam or a bent valve or something along those lines. If all 8 are the same then its a random thing and your looking at the coil, a shorting plug wire, bad points, crack in the cap, cracked plug, bad gap....... If its random, when was the last major tuneup done?

540 vette

Condenser in the distributor. The one hooked up to the points.

7T1vette

Two ideas:

1. What plug gap did you use? If you have a points ignition, you should gap at .035", REGARDLESS of the coil you have installed.

2. If you did some painting on the intake manifold, you need to be certain that the metal-to-metal contact areas between the distributor housing, metal hold-down clamp, and the engine block are FREE of paint. You only need small contact areas which are paint-free. That pathway is the ground connection for the distributor.

rogergcam

Thanks for everyone's idea. Heres what I did:
Manifold bolts are tight
The car has had a complete tune up twice in last 6 months..New dist, points, condenser, rotor, dist cap, one new set of plugs and wires.
Measure Vacume-about 20 inches at idle.
Removed fuel filter...still runs rough
put in new fuel filter-no change
Put on another coil-no change
Removed fuel tank cap-no change-thinking maybe not venting?

I put my timing light on the high voltage wire from the coil to the distributor. You can "see" the miss as the timing light flashes. It seems totally random.
I took a piece of wire and went directly from the battery to the plus side of the coil-hopefully eliminating any possible loose connections. No change.

AGAIN THANKS for the thoughts and ideas...Keep them coming...


George

7T1vette

But you didn't run a jumper ground wire from some metal on the distributor housing to metal on the engine block. You also didn't mention what plug gap you used.

If all you do is check the things that you think are the problem, you'll likely not find the cause.

Cooter Tech

How do your plugs look, any fouled , darker than the others?

orourke

"Try swapping in a generic coil. I had an intermittent miss that took me forever to find.....Just "happened" to find it late one evening while spacing-out as I pondered what to do as the car was idling and I was continuously flashing flashing the timing light on the wall...... when I suddenly noticed a "skip"...hmmm.... then as I paid attention I noticed that it was "skipping" in a non-routine manner. I had an old Datsun back then and I just swapped in it's coil..... immediately fixed that.
PS-It also eliminated a problem I had with the car suddenly dying in really hot weather.....which, as soon as I opened the hood, would cool down the coil, and it would just restart.....that was a pain."


That's exactly what's happening to my 1969 Camaro now. You just confirmed what I suspected.

Thanks.

carriljc

Let us know if the coil swap works for you..... that would be cool. Sure hope so.

carriljc

From what I recall placing the timing light on the coil wire makes it flash very rapidly....
Try placing it on an individual plug lead and see if you can still observe the skipping (it should be a bit slower).
You may also have a bad plug, or 2..... I would check each plug lead if the first one I check doesn't show a skip.
Also ensure the good adequate ground as mentioned earlier.

rogergcam

I put a new coil in--no change. I am going to try a grounding wire ...i forgot about that suggestion.

THANKS

Gold Dragon

I think you nailed it in your first post. Vacuum is a killer in the Corvette. Just experienced something similar with my 69.
Locate the vacuum block on the intake. find your main vacuum line for the cars headlights and wiper door. Disconnect and cap. if you lose the miss, your leak is in the system. My 454 did not like the low vacuum signal and was running as you indicated. Now its running great, I just have to find the pin hole

forman

may i ask how many miles are on the car ... you could have a worn timing chain or gear and theres a way to check it

Cooter Tech

Could your distributor be worn out, sloppy shaft or excessive play?

rogergcam

about 59K...

rogergcam

I just had the distributor completely rebuilt. New bushings, everything. Very expensive...

Kim72

When looking for ignition misses I like to run the car in a very dark, blacked out garage. If there's a short somewhere you'll usually see a lovely blue light show. Doesn't take long to see. If you try it, make sure your exhaust is piped to the outside.

Cooter Tech

Does the shop have old test equipment like the Sun Engine Analyzer Scope and someone that can use it?

http://performancetechnician.com/pdf...ion-System.pdf