Help!! Engine Miss

[buzzinhalfdozen]

I've been reading this post and there has been alot of good suggestions mentioned here. Two things that grab my attention are the 3000 ish RPM limit and new switches being installed. The RPM the problem occurs almost seems to be a clue, I've encountered these issues due to several different things, low battery voltage, improper triggering of the ign. controller along with obvious things like loose or ommited grounds. I noticed Pat Collins had asked you to check the wire colors from the Dist. to the MSD, very good idea as it takes a short amount of time to actually look and be absolutely sure they are correct, I've caused myself many head aches by assuming things. My self I believe the first thing I would do is put my trusty MSD tester on the car and spin her up to make certain the box and coil are up to the task. If it sparks all the way up we can say the control box and coil are OK. During this test may be a good time to put the DVOM on the Batt. Pos. as well as the switched pos. ( small red wire) to make sure we have sufficient voltage. Is it possible your new switches have an issue such as a large voltage drop thru them, testing in this way will tell you. As with any problem we need to break it down into smaller units and test the units that we can, by eliminating one unit we can concentrate on the next one and so on. No testing the carb. thats another story, as others have mentioned it may be easier to swap one out and see if you get the same result. Belive me I understand your frustration trying to isolate this concern, I deal with these kind of issues every day, unfortunately no o9ne can give you the magic fix via the net however I'm sure by properly testing the things you can test you'll resolve this problem. Good luck Joe

[Ben Kallies]

Adger,

I've tried 3 different distributors,each with their own new cap and rotor, so I believe I've eliminated that possibility. I've also replaced the coil, plug wires, plugs, MSD box, battery, and rerouted and insulated the MSD connectors, double-checking the polarity of each. I appreciate the encouragement and continued support.

Randy,

Thank you for your input.

FED 387,

Thanks for your input. Pulling the engine is the last resort, but I've been considering it.

buzzinhalfdozen,

I believe that I've eliminated the "insufficient voltage to the ignition system" by installing the HEI with a single wire direct to the battery. However, this does still leave the door open to a possible ground issue elsewhere in the vehicle. I've also tried a second carb, eliminating that possibility. The new switches are definitely on my list of things to eliminate. Perhaps I'll disconnect them, and hardwire the components one by one. Thank you for your input.

-Ben

C

[buzzinhalfdozen]

Ben, is the battery for this vehicle in the trunk or under hood ? If in the trunk how is it grounded ? If it's in the trunk and grounded somewhere in the trunk area do you have a dedicated ground wire to the front of vehicle? Also you stated that you "hard wired" an HEI unit direct to the battery with the same results which would nearly eliminate the switch as being the issue as you have bypassed it alltogether, we still have the question of sufficent voltage. From rereading your posts I'm starting to believe you have a mechanical problem assuming your voltage is sufficent to operate the vehicle. Have you performed any mechanical diagnostics, compression test, leak down, checked spring pressures, checked valve action while rotating engine with valve cover off? just some thoughts. Joe

[mtkawboy]

Ive had a broken valve spring cause a similar problem

[Ben Kallies]

Joe,

The battery is in the factory location behind the driver's seat ('71 Corvette), and the negative cable goes through the floor and grounds directly to the frame. I've got an additional ground wire from the cylinder head to the front of the frame.

I haven't performed any mechanical tests yet. I've had the valve covers off, and reset the valves, but it's worth a second look.

-Ben

[P Bailey]

Originally Posted by Ben Kallies

Joe,

The battery is in the factory location behind the driver's seat ('71 Corvette), and the negative cable goes through the floor and grounds directly to the frame. I've got an additional ground wire from the cylinder head to the front of the frame.

I haven't performed any mechanical tests yet. I've had the valve covers off, and reset the valves, but it's worth a second look.

-Ben

Ben,You might want to check the ground at the frame. We have had problems with the connector on the end of the cable inside the rubber cover.Good Luck with your problem. Phil

[Tony Curcio]

It sounds like you've isolated everything external to the engine- after 67 posts it's hard to take it all in.

As you said, the valve train is your next suspect. You've had the valve covers off- did you run the engine while they were off? This is a quick way to see if a lobe is flattened, a pushrod bent, a stud pulled out of its threads, or a broken valve spring. You'd probably be able to spot these things even if you just cranked it with the covers off. Any of these will make a popping sound in a consistent rhythm.

An overtight valve just sounds like crap, hard to describe, but a valve is not closing, so it's just weak and shakey.

One more thing I remember chasing long ago- a head gasket that is blown between two cylinders. Spits out the exhaust and intake of the two affected cylinders.

All of the above have a distinctive sound. If it was possible to post an audio file, many here would probably recognize it.

Hope you solve it soon, and hope you'll post it when you do.

[junior barns]

Originally Posted by mtkawboy

Ive had a broken valve spring cause a similar problem

This is what I would check