Q&A / Tech

Mailbag: The Cure for Stumbling Problems

Got questions?

We’ve got the answers—the Summit Racing tech department tackles your automotive-related conundrums. This week, we’re helping a Camaro overcome its stumbling problems.

                                

D.P. • Bridgewater, PA
Q: My 1979 Camaro is powered by a 350-horsepower Jasper Performance engine fitted with a Weiand Stealth intake, a Holley 650 double-pumper carb, Dart iron heads, HEI ignition, Dynomax 1 5/8-inch headers, and a 7 1/2-psi 80 GPH mechanical fuel pump. The cam is a 230-degree duration, .480-inch lift model; the Turbo 350 transmission has a B&M shift kit; and there’s a TCI 10-inch Street Fighter torque converter. Out back, there are 4.10 gears in the 10-bolt.

My problem is that every time I nail the throttle from idle, the engine stumbles and misses very badly until 3,200 rpm. After that, it’s smooth and responsive. I’ve tried 50cc accelerator pumps in the primaries and secondaries; I’ve tried .028-.037 shooters and both pump cams; I’ve changed the initial timing from eight degrees to 16 degrees; and I’ve sent the distributor back to have it checked. I’ve even tried four different carbs, and vacuum and mechanical secondaries.

A: That combination of parts on your Camaro should perform very well, but after reviewing the symptoms and all of the modifications you’ve tried, it sounds like you have a vacuum leak. You should check for both internal and external vacuum leaks. If the heads have been milled or the block has been decked, your intake may be leaking from inside the crankcase.

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5 Comments

  1. Mike Johnson says:

    An intake leak from INSIDE the crankcase!?! I did not even know about such a thing!
    Play that music from Jaws with answers like this!

  2. Plug wires

  3. That’s a lot of carb on that engine setup IMHO, what is the exhaust doing at the time of stumble? If you are seeing any smoke of any color disable the rear pump. Sure sounds like it’s overfueling till the engine starts breathing. Never had much luck with double pumpers on auto trans setups.

  4. Holley Sniper will fix that problem.

  5. Air filter size or cold air seen many stock air filters not letting enough air in. Causes stumble need air and fuel.

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