The engine is in my Vette, but it is out of a 69 Chevelle L-78 396/375, so forgive me.

Ignition is the stock HEI that came on the 75 350 Vette(How about that, Tom Mobley! I've been a HEI believer for a long time too!). It has been re-curved for the L-78.

After it runs for about 15-20 minutes, it developes a "dead miss." It runs great until 15-20 minutes or so. Then, it seems it slowly gets worse. Sometimes the engine would die if I didn't keep my foot on the gas. Sometimes it is like pulling one plug wire off, it will run but there is an obvious miss.

Two things to check in my mind:ignition and/or carb. The plug wires are fairly new, but I'm going to change them today. I'll let you know if they are the problem...You never can tell with the wires. Plugs are new.

I had a buddy who had the same symptoms....the hot wire to the distributor was almost burnt in two. He "spliced" it and that took care of it. I am going to look for this as well......

Any ideas? Coil, ign module going bad?

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-Mark
TC# 717
ACES #1641
1967 Chevelle SS396/375 (http://www.chevelles.com/showroom/1BadRat2)
1967 RailVelle (http://www.chevelles.com/showroom/1badrat3.jpg)
1964 Chevelle 283/195
1975 Corvette 396/425 (http://www.chevelles.com/showroom/1BadRat1.jpg) "Wow, that's a mighty strong 350!"
Rat Garage (http://www.chevelles.com/showroom/RatGarage.jpg)

Don't know if this will help you or not but I suspect it explains your buddies burnt ignition lead at least... The wire going to the positive coil terminal for point type distributors requires a resistor, GM did this by using a wire with built in resistance. HEI's on the other hand like full voltage and in fact perform a little better with a full 14.5 volts.

Back to your problem, did you replace the power lead to your coil when you installed the HEI?


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Rod

67 396
SS clone

Daddy's Red Car (http://www.geocities.com/rthlc/index)

Rod,

This is in my 75 Vette, which has always had HEI. It originally came with a 350 and HEI. I am using the same distributor and factory wiring on the 396. My buddy, on the other hand, had a 77 Vette w/HEI and the wire burnt from being too close to his headers.

I replaced the plug wires and it didn't help. On to something else......

if you let the motor cool down, can you go out and start the motor and have it run smoooth(no missing)? will it run smooth before it gets hot?

Mark, Does it still have the solid lifter cam? If it now has hyd lifters, I can offer something to try.

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von '69 300 Dlx SS TC #15 ACES #1575 HSCA #5
My '69 SS (http://mywebpage.netscape.com/jerryacheson/vons69.html)

Mike, after it cools down, it runs fine, and will run smooth until it warms up again.

Von, it's still solid lifters, stock cam. It's an all original 69 L-78 that I've been tuning for 20 years! This one has me stumped. BTW, I'll be in Indy at International Truck and Engine, either the first or second week of Jan. Might give you a holler! Are you close by?

Not much time to work on it during the week, but I'll keep everyone posted.

I think you hit it on the head, I think your module is going bad. another thing you want to do is make sure you grease the module up real well with the electric grease(don't remember the name) that comes in the package. Heat kills these things, make sure you use it. Thats what makes me believe this is the problem you are having with your car. you can pick these up rather cheap at your local parts store ($15.00).

The grease is called Di-Electric Silicone. You can get it in most all parts houses, even Wal-mart sells it in their automotive department, but you might have to look real hard. It also comes in the package with most new electrical components. ie the module you must need.
Billy