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Jeff Smith Technical Questions and Answers - Ask Anything

Hesitation Solutions

Patrick Duf; Freeport, NY: I have been reading your magazine for years (thanks, Dad!), and I am finally writing to receive some advice on my '79 Malibu beater. My Malibu has a 305 Chevy and come next summer will have a 350 in it. My problem is going from idle to wide-open throttle. I recently threw in a Comp Cams XE268 cam (224/230 degrees duration at 0.050, 0.477/0.480-inch lift), a 2,200-2,400-rpm converter, L98 aluminum heads, an Edelbrock Performer RPM Air-Gap manifold, and topped it off with an Edelbrock 1405 carb per Edelbrock's recommendation. The main problem is when I put my foot down, the engine stalls out and does not restart. I checked the No. 1 plug to see if it got wet when the throttle was opened all the way, but it was bone dry. If you have any suggestions on where to start looking it would be greatly appreciated. This is my first engine build.

Jeff Smith: Thanks for taking the time to ask a question, Patrick. We all have to start somewhere, and it appears that yours is a relatively easy fix. The issue is one of combinations. When creating a high-performance parts mix that includes a carburetor, an intake manifold, a camshaft, and a cylinder package, you have to expect to do a little tuning to make the system work properly. It sounds like something as simple as merely increasing the accelerator pump shot, but before we get into that, it's important to ensure that the fuel delivery system is in good shape. Often, what appears to be a carburetor-related problem can be traced to low fuel pressure and/or low fuel volume. We will assume for a moment that you are using a standard, engine-driven fuel pump, since that's the simplest and most common fuel delivery system. Even a stock fuel pump should be able to supply sufficient fuel for a mild performance engine like yours, so check to make sure the fuel line is not obstructed or somehow kinked in a way that would restrict fuel volume. Next, you will need to borrow or buy a good fuel-pressure gauge. Those tiny, inline fuel-pressure gauges that are about 1 inch in diameter are notoriously inaccurate, so locate a larger, more accurate gauge. Ideally, set it up so you can drive the car and have a buddy watch the gauge. We've done a quick test with a T-fitting in the fuel line and taped the gauge to the bottom of the windshield. Make sure you don't have any leaks, then drive the car easy and eventually hit wide-open throttle in Second gear, somewhere safe (and legal—getting a ticket while tuning puts a damper on the process!). You should have a minimum of 4 psi of fuel pressure with a preferred 5 to 6 psi. We can assume from this that the pump is supplying sufficient volume—that's why we test under load instead of just at idle. If the pump cannot keep up with the demand, the pressure will drop as rpm increases. If the pressure drops, it may not necessarily be the pump's fault. In older cars in particular, odd things, such as big dents or twists in the fuel-supply line, can restrict volume. You might also check to ensure the tank is properly vented. The simple way to check is to do the same test with a cap you know is properly vented.

Assuming there is sufficient fuel pressure, the next thing to try is changing the accelerator pump linkage. There is a small arm that connects the throttle linkage to a small vertical pin that is actually the top of the accelerator pump. The linkage will have three adjustment holes, and the hole closest to the carburetor body will produce greater leverage to deliver a little more fuel. That's the no-cost trick. If that helps, but the bog is still there, you will probably have to add a larger accelerator pump nozzle. The stock nozzle diameter for your carb is 0.028 inch. Edelbrock sells a nozzle kit (PN EDL-1465; $12.95 Summit Racing) that includes three squirters at 0.024, 0.033, and 0.043 inch. Try the largest one first, and if the hesitation disappears, try the 0.033-inch nozzle.

Another tuning item that might help is to make sure both the initial and total ignition timing are properly adjusted. I'd suggest setting the initial timing at 12 to 14 degrees. This is what you'll read on the harmonic balancer at idle, using a timing light with the vacuum advance line unhooked. If you add a timing tape to the balancer, you can use a standard timing light to read the total timing at around 2,600 rpm. Again, with the vacuum advance line removed and plugged, you should read around 34 degrees of total timing. Don't advance the timing much more than 36 degrees, because the 305 engines tend to be detonation sensitive, and too much timing can make them rattle—and that's a quick way to both kill power and possibly break a piston. If you find you have other questions related to tuning, you can also call the Edelbrock Tech Line at 800/ 416-8628.

More Info

Edelbrock
Torrance, CA
310/781-2222
www.edelbrock.com


CC Quickies

Here's a picture of this month's Horsepower! star, Greg Monroe's '01 Bullitt in his shop, Racer's Edge Tuning (RET). Want a turbo kit installed on your car? Call RET at 562/ 622-2508.


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