69 Chevelle SS 396 falls on face

[Brightwing 1]

Have a friend that has a basic 396 Chevelle with automatic. Just recently when in 3rd gear, with low throttle, just easing up to speed, around 3,000 rpm it will all of sudden stop pulling and start stuttering. If you put it in first gear manually with about 3/4 throttle, I had it up to about 4,300 rpm and no problem till I shifted to 2nd and instantly did the same thing, flat on its face and stuttering. The engine was rebuilt about 1,000 miles ago, has a replacement HEI ignition, rebuilt Q-Jet from Summit and a 350 HP Cam instead of the 325 HP cam. The fuel pump is new and in no way does it act like a trans problem. I have 50 years of muscle car, carb and trans experience and have never seen these exact symptoms. The weird thing is that it was fine after the rebuild, but this just cropped up. Thoughts and suggestions are appreciated before we start replacing parts.

[Blue Bowtie]

Welcome Aboard!

I'm guessing that is starts and idles just fine. When the secondaries are opening (lower gear, 3/4 throttle, 4,300 RPM) is there any hint of slight popping or backfire in the intake? That could indicate improper valve timing.

I'm not sure what a "350HP" cam is, but can you I.D. the cam or post profile specs? That may be a hint/guide for setting the secondary AV windup spring and metering rod/hanger selection.

[Brightwing 1]

Originally Posted by Blue Bowtie

Welcome Aboard!

I'm guessing that is starts and idles just fine. When the secondaries are opening (lower gear, 3/4 throttle, 4,300 RPM) is there any hint of slight popping or backfire in the intake? That could indicate improper valve timing.

I'm not sure what a "350HP" cam is, but can you I.D. the cam or post profile specs? That may be a hint/guide for setting the secondary AV windup spring and metering rod/hanger selection.

Thanks for replying Bowtie...Yes starts and idles OK, no popping or backfires thru the intake. The 350HP cam was just the 350 horsepower cam that was used on the L34 396 engine of that year, as opposed to the L35 that was the 325 HP engine. Probably a little more lift and duration, but don;t have the specs right off hand. The real hint to its issue is that it is stuttering when it does it and not just a smooth drop off. You can keep a steady foot on it and just buck and stutters.....no backfires, plus the fact that it all of a sudden started doing this. I am kind of swaying towards ignition because it is getting under a higher torque load in both situations.

[Blue Bowtie]

Could be ignition, but even the factory HEI does a pretty respectable job up to nearly 6,000 RPM under reasonable load (unless the plug gaps are insanely high). It certainly wouldn't hurt to verify the condition of the cap/rotor/wires.

If there appears to be arcing in the cap at either the trailing or leading edges of the contacts, that could indicate a slightly out-of-phase distributor installation. That can occur if the firing position is not reasonably aligned with a cap contact. You'd be able to set the static timing, and it wil run under light load, but may get scatter/cross-firing under load and at higher advance angles.

I have an '86 (last year of the large cap HEI) and experienced a similar condition after programming the ECM for a max advance of 52° in highway mode. I'm guessing your total mechanical/vac advance my not be that aggressive, but verifying the position is pretty easy.

[Brightwing 1]

Thanks Bowtie,

It is a good thought and we will check it out. Have a good day!

[Blue Bowtie]

BW - You picked a rotton day to check it out. My backyard is 65 miles west of yours:

[Brightwing 1]

Yuck!