Q&A

Mailbag: “What is Causing My Car to Hesitate?”

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We’ve got the answers—the Summit Racing tech department tackles your automotive-related conundrums. This week, we find possible causes and cures for engine hesitation.

R.H. Grand Rapids, MI

Q: I have a 1979 Trans Am powered by a 406 small block Chevy. The engine has 9.67:1 compression, GM cast iron cylinder heads with Crane roller rockers, an Edelbrock Performer cam (214 degrees duration at .050, .442 inches of lift), a Holley 600 cfm carburetor on an Edelbrock Performer RPM intake, and Hooker Competition headers. The ignition is a GM HEI with Mr. Gasket advance springs and weights installed.

The engine runs strong (even with 2.73 gears), but has a slight hesitation at all rpms. The carburetor was completely cleaned and has a Holley Trick Kit. I changed the squirters and accelerator pump cams, but that did not help. What is causing my car to hesitate? Is the problem with the distributor advance or perhaps fuel pressure? I’m thinking about replacing the distributor with a Summit blueprinted HEI distributor, and replacing the stock fuel pump with a Holley mechanical pump. Would these changes help cure the hesitation?

A: Your hesitation is likely related to the mechanical advance setting in the distributor and a lean condition due to the restriction posed by the relatively small cfm carburetor. The Summit distributor is a good idea, and you should also consider upgrading the carburetor to a Holley 750 cfm vacuum secondary.

We’d recommend that you stick with the stock fuel pump because it is compatible with the stock fuel tank venting system. If the pump is bad, use an ACDelco replacement pump and add a fuel pressure regulator to monitor pressure to the carburetor. Finally, we’d suggest changing the rear axle gear ratio to a 3.42 or 3.73 to help with torque multiplication.

 

 

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10 Comments

  1. Most likely the cause is that Chevy engine.
    Hah!

    Just kidding.

  2. I wouldn’t switch to a carb with a vacuum secondary. I had a Holley with vacuum secondaries and it sucked. car stumbled all over the place. Switch to a quick-fuel with mechanical secondaries. much better carb. not a lick of trouble in 3 years and picked up 50HP on the dyno. A long shot but I’ve seen this happen too: Check the RVP of your gas. You may be running crap gas whose boil point is low and you start getting vapor into your fuel line. I live in GRR too, and our winter blend gases from October to April are horrible.

  3. I think it would do better with a 750 CFM Edelbrock or even a Quadrajet. The QJ might be a hassle with the Performer RPM manifold, though, if it doesn’t have a spreadbore mount.

    Frankly, I think the 2.73 gear needs to be thrown out immediately with that heavy of a car. Also, the timing might be a contributing factor because I would think that it has to be very conservative / late to keep from detonating with that tiny cam and 9.67:1 c/r, even with 93-octane. I think it needs a Performer RPM cam (or bigger), a tricked out Q-jet, and gearing more like 3.73.

  4. A 406 can handle a 650-750 carb. The cam you have with mild lift and low duration paired with decent 9.6 / 9.7 compression should make for a tire burner at low – mid rpm. My guess is your timing curve needs customized for your combo. If you change gears make sure to do it before having the distributor re-curved. Most aftermarket companies only use one set of weights and one general curve for SBC engines. A good shop will use a distributor machine to tune distributor and adjust for any slew rate.

  5. Install new spark plugs, 36 degrees timing by 3000 rpm, at least 12 degrees at idle, adjust your bowl levels to the bottom of the sight glass, open your idle feed screws in 1/8″ increments and test,you can also increase squirter size until stumble is gone but it’s probably not necessary with your mild cam. Buy a temp gun and check your header tube Temps at idle also.

  6. Check you power valve to in that holley. I had a 289 ford and when you pressed the accelerator pedal it would pause a bit before the power valve would operate. I needed a lighter one.

    • 650 double pumper was the carb I had. Torquer intake. 512 lift, dont remember the duration. Cam Dynamics. 180 150 valves. It wasn’t a stumble, just a delay in enrichment.

  7. Holly not performing right
    Adjust the pump on primairy
    Do the same 2 secondary’s

  8. Check your vacuum at idle with a diagnostic vacuum gauge. Whatever you are making for inches of vacuum, halve that number and get that power valve. For example if you are making 12 inches of vacuum you want a 6 power valve, 8 inches of vacuum you want a 4 power valve.

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