Help!! Engine Miss

[Adger Smith]

Is it really running rich or is the ignition system so weak it isn't lighting off the mixture correctly? How is the carbon tip in the dist cap?

[Ben Kallies]

Alan,

I was moving toward a possible mechanical issue because I thought that I had eliminated any electrical possibilities, but it sounds like there could still be a gremlin lurking somewhere affecting the ignition. As bad as the miss is, particularly under load, I would expect that if it were a mechanical issue, it would show itself more readily. Maybe I'm wrong.

Regarding whether or not the miss can be isolated to one or two cylinders: I've pulled the plugs a number of times and I don't see anything that screams out as being an offending cylinder.

I like the idea of hooking up a voltmeter to the 12 V sources. I'll add that to my list. Thanks again for your input.

Ed,

I'll be honest, I can't quite envision what you're suggesting. I don't have an undampened vacuum gauge, but I'd consider getting one if it could help me isolate the issue. Is there somewhere I can go to see this setup in action? Thanks for the tip.

Adger,

I was suggesting exactly what you stated, that it's running rich due to an unburned mixture. Maybe I'm wrong, but I assume that if my problem was purely a vacuum leak, then I'd actually be running a bit lean as opposed to rich? The carbon tip on the distributor cap looks brand new. Thanks.


-Ben

[Adger Smith]

Ben,
Please don't take this the wrong way, but "Looking good and being good" are two entirely different things. There may be a problem there. I would "Check it" with a Volt /OH Meter to see if it has high resistance. Some of them are spring loaded and can burn the spring out. I've also seen the path from inside the cap to the brass contact broken by being burned out , yet it "looks good" on both sides, resulting in Open circuit or high resistance. I think you need to go over the eletrical system with a fine tooth comb. get friendly with your V/O meter and check the system, loaded, not static. "Load it" by running the engine. It could be a switch that "looks good" but has high resistance when a load is put on it. I would even put another battery in it. I've seen batterys in some dirt cars look fine and load check fine, but when you load them with "G" forces the plates bounce around and arc internally and kill the ignition. Start taking loads off the battery and you might want to put a dedicated one for just the ignition. Have you tried moving the plug gap down a little to see if the miss leaves? I've had that happen and it could indicate several things. Not "enough" coil output to jump the spark at the plug is usually the fix for that. The acting like it is running rich and your carb guy says the carb is healthy makes me think the electrical system is suspect. As the "Engine Guy" I usually end up figuring out what the problem my customer has because it is always "My Engine" at fault. I've got a long list of "silly things" it always turns out to be. Don't get tooooo frustrated. Problems like this can make you ... well, you understand.. :~) Good luck.

[randy wilson]

Look at the ground, it will be the problem. You may have to ground it in several places, and also, put a timing light on the motor in the car, rev it up to peak, and see if the timing moves. If it does, it is a ground.

[FED 387]

timing on a engine with a chain drive will tend to retard as the engine increases in rpm and advance when the rpm decreases if you time it by reving it up its the chain stretching ever so slightly or taking up the slack that causes this movement---try timing it at a steady rpm of say 5000 rpm see what it does if its moving around or not---I'm gonna bet tht you have a bad wire somewhere either broken or bad connector maybe even a bad sparkplug try swapping out plugs one at a time see if it helps I've seen bad ones from the factory not often but it does happen---works and then kinda works depending on the load and other things---- -- maybe go so far as to take the engine back out of the car and run it on the dyno again see if the problem persists or goes away---if it persists its the engine if it goes away its some thing in the car---if its in the car then you'll hafta start looking at the entire electricla/ignition system----Comp

[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