Avispa

The red thing has developed a sickness at about 5k at full throttle under load. It stops pulling hard and sputters a bit. Sitting in the driveway I can rev the thing through 6500 without so much as a whimper. The misfiring happens only under load and this is about a year old problem. Before this the motor would pull cleanly and strongly (no hesitation, sputter or misfiring) right through 6k.

Timing is spot on at 36 degrees centrifugal (all in at 2500 RPM) and about 10 degrees static. Starts up perfectly. Up to the sputter point it runs perfectly and will burn the tires just mashing the throttle.

It does sit for long periods of time (couple of months) and has had old gasoline in it (no Sta-Bil or other additive).

Any ideas? I'm guessing the secondary metering block has corrosion in it from the damn corn syrup in the gasoline, but I'm not sure.

Carb is a Holley 3310 with conversion to secondary metering block to fit with factory L72 fuel lines. Running 68s up front and 74s in the back. 6.5 power valve.

Thanks for any suggestions.

MikeM

I would suggest #72 primary jets and #76 secondary jets. That's not what's causing your problem though.

Clean/gap the plugs? Clean/gap the points?

Avispa

Mike: did the plugs. Points are gone 20 yeas ago to a Unilite module (can't think of why it would start doing something like this, but....). Definitely not the jets causing the prob. I've had the 68/74s in the carb for 15 years.

Powershift

Agree with Mike M that problem is probably ignition related. But since you have no points anymore, I would suspect the Unilite module or the coil.

Not certain how to confirm this short of a chassis dyno or simply replacing parts.

Larry

EDIT: Although you said you cleaned and gapped the plugs, I would simply install a new set of plugs of the proper heat range (45 heat range). This is cheap to do. Then go from there.

MikeM

If you have an electronic tach in that car, if the ignition breaker starts to malfunction, it think your tach needle might show it by being very erratic?

ghostrider20

Faulty ignition coil. Had this happen twice. Coil would bench check fine, but fall on its face at high revs and load.

In my case I initially thought fuel flow, so went down that road. IF your car ran fine before, changing jets or carb tuning is not the answer.

MikeM

I only throught that because he brought it up. I used jets like that BEFORE E 10 and E 10 is a little lean so it wouldn't hurt to change the jets at his leisure.

tbarb

Before you start spending $$$, get a digital ohm meter and check the spark plug wire resistance

tach drive 61

My guess is distributor bushings

toddalin

I'm thinking the fuel pump. Mine used to do the same thing at the big end of the quarter and a gauge showed I only had about 3# of fuel pressure under load.

A new Melling high volume pump fixed it right up.

nassau66427

It wouldn't take much to temporarily rig up a fuel pressure gauge so that you could see it from the driver's seat.

Run the car through the gears and see what happens.

Avispa

Thank you guys for all the suggestions. Probably time for new plugs anyway. They're cheap enough. Not going 45 heat range, though. I need 43s here in the outdoor sauna, maybe even 42s.

Yes, the car ran fine before, but it had sat for at least a couple of months without being run. That was why I thought some gum or corrosion got into the secondaries. They don't get used much (no, really...lol).

Plug wires are fine. Resistance checks out perfect on all 9 wires. I love my Fluke 77 DVM. Found a bad wire in the set right when I bought them of all things; had to chop off the bad part.

Will definitely check fuel pressure and the coil, of all things. I had a 20+ year old Accel Super Stock coil (came with the car when I bought it in '90) that I changed out with a new one just before this stuff started happening. Never bought a bad coil before, but I'm sure it happens.

Distributor bushings are perfect. I had it out to clean it up inside before all this started.

I still have the factory mechanical tach, and I've checked it against more than one electronic gauge so it's probably accurately telling where the motor is starting to sputter.

I have a whole tray full of Holley jets. After I get this issue sorted out, I'll take it somewhere I can put an AF gauge on it and see whether the jets need to be changed. I've run 68/74 on a model 3310 for more than 30 years and it seems to be just about perfect for a stock L72.

66since71

With Todd. I had the same problem. The pump solved it.. By the way, it's probably easier (and safer) to just change the pump, than to run a fuel pressure gage to a place where you can see it... (temporary fuel lines scare me!)

Harry

Powershift

A 43 heat range plug is going to be too cold unless you and you car are constantly running at the track. If you drive your car anywhere beside down the 1/4 mile or around the oval, then you should at least use a 44 heat range. If you drive mostly on the street and aren't street racing for $$, then the 45 may be best.

Larry

Nowhere Man

Since when does a BB have any more power above 4k

nassau66427

This is Corvette Forum.

Team Chevelle is over thataway.

Nowhere Man

Jeez thanks but really I thought the peek power band on BB were around 2-3K RPM.

nassau66427

For '66 the 390 was @5200rpm w/460 ft/lbs of torque @ 3600 rpm.

The L-72 was 425 hp @ 5600 and 460 ft/lbs @ 4000.

(From the AMA specs.)

tbarb

If you are sure the spark plug wires are good I suspect something in the secondary side of the ignition system. If you installed one of those reproduction coils start there and swap a NAPA coil in and try that.

What do the spark plugs look like. If they are sooted that can cause misfires so clean them or get a new set. Are there any plugs more sooted than others?

Selecting the best main jet for a Holley should provide a nice lean cruise without surge that way the spark plugs stay clean. At that point, enrichment is provided through the power valve channel restriction at the selected power valve opening vacuum point.

If you want to tune the Holley it's best to start with the idle circuit first then the main circuit but I don't think that's the problem if the car ran fine before.

leaky4150

Early one Sat. morning on interstate near where I live no traffic L72 3:70, 125 mph @ 5200 rpm, engine not working hard more to go.These cars/motors are beasts (tach/speedo 48 yrs old)