Carburetor Tuning & Reader Questions - Performance Q&A

Kevin McClelland Jan 1, 2010 0 Comment(s)

Bucking Nova
Q: I am having a problem with my '68 Nova. For the past couple of years I have been doing improvements and have run a best of 11.69, but the stars must have been in alignment that night because the car usually runs high 11.80s. This past Christmas, I treated myself to a Coan TH350 with a transbrake and Coan 4,500-stall converter. The car leaves hard off the line, lifting the front wheels about 3 inches, but then nosedives for just a millisecond and recovers, running hard all the way to the finish. The only changes I made were the transmission and converter. Even with this problem, the car will run 11.90s and the 60-foot times are about 0.20 slower.

I have since installed jet extensions on the secondary side. I launch this car by placing the gas on the floor with the transbrake button depressed. The rpm of the car runs up to about 4,400. It seems like the car needs an accelerator pump shot, although I am past the accelerator circuit. I am not using a rev limiter; I've been told this would help with my problem, but I don't understand why, since I would still have the gas on the floor when leaving the line. Do I need to jet up the carb more? Thanks for any help you can send my way.
Tom Pogue
via email

A: Jet extensions should have taken care of any lean situation you may be encountering. You're right about the fact that you're well past the accelerator pump circuit of the carburetor. When you go wide open on the transmission brake, the carburetor is fully open and running on the main circuit of the carburetor. We'd be looking for a rich condition, and here's why. When the car leaves off the transbrake, there is an instant of very hard acceleration. When this happens, the fuel in the primary bowl can slosh out and through the bowl vent, dumping fuel into the airstream. This will create a rich bog for a split second. Most performance Holley carburetors are equipped with an anti-slosh vent extension to help prevent this. But if you have a crack in the plastic extension, or it's not there at all, fuel can easily come out the vent tube on the primary side.

A quick test to see if this is your problem: Install a piece of neoprene fuel line between the two bowl vent tubes of the carburetor. Cut an opening in the top of the hose at the peak between the two vent tubes. This will allow the primary and secondary bowls to breathe and prevent fuel from sloshing into the airstream.

Finally, a two-step may be a good addition to your race car. It won't help with your off-the-line stumble, but it will give you a tuning aid for when the track gets hot and slick. Rarely can a track take full power all the time unless your car is completely overtired. Also, as weather conditions change, the launch rpm of your engine will vary. The better the air the higher the stall speed, and inversely when the air gets hot and muggy. If you keep the chip rpm 400-500 below full stall, this will prevent repeatability problems related to weather changes.

Hope this helps you find your problem, and maybe win a couple of more rounds with your Nova.


New School Teaches Old Dog New Tricks
Q: Having been a gearhead for almost 40 years now, I can say without a doubt that this tech column is the best I have ever seen! Last year, I built a 383ci small-block Chevy in a high-performance engine-building class offered by Sinclair Community College in Dayton, Ohio. In this class you build any high-performance engine; only your imagination and budget will hold you back. The best part is not only do they teach you the different machining procedures, but you actually do the work yourself. The big payoff is at the end of the year when you put your engine on the dyno and see the results of nine months of work. Here is my list of the major components: a '78 four-bolt main bored 0.030 over, a Scat 4340 forged crank, Carrillo 6-inch forged rods, Probe SRS pistons -12.8cc dish, AFR 195 Eliminator heads, a Comp Cams hydraulic roller 230/236 duration with 0.510/0.520-inch lift, Comp Cams Magnum 1.6 roller rockers, an Edelbrock RPM intake manifold, and a Holley 750-cfm carb.

Using 15/8-inch headers, this combo made 479 lb-ft at 4,400 and 468 hp at 5,800 with 36 degrees of timing. What effect can I expect to see using a set of 17/8-inch primaries into a 4-inch collector, such as a Hooker Corvette-specific side pipe/headers system? What intakes can be used with the stock L82 hood? I have a Weiand single-plane low-rise Stealth model

Thanks for any info and thanks for being here for guys like me.
Bud DeWitt
Dayton, OH

A: Well thank you very much for the kind words. Last year my son Daniel went through a semester of an engine design and troubleshooting class at the Santa Rosa Junior College in Santa Rosa, California, that sounds very similar to the course that you took. They teach you about the engine combo, how to machine it, and, finally, assemble it. Taking your first or hundredth engine from cradle to engine bay is very rewarding. Many community colleges around the country offer very similar courses and are looking for men and women just like you to fill these classes. Keep them full to continue the education of our young people-and a couple of old dogs at the same time. Thanks for reminding me of a very good find.

Nice little small-block you got to learn all about. The 17/8-inch primary header with 4-inch collectors is a bit too large for your 383. You will lose some of the slow-speed torque (10 or so lb-ft), and may gain very little top-end horsepower. As for the manifold, the Corvette doesn't lend itself well to inlet upgrades. The low-rise Weiand dual-plane will be very small and really limit the top-end potential of your engine package. It may be down on power to the tune of 30 hp! Check out the Edelbrock Torker II PN 5001 single-plane. This is the only other manifold we know of that will fit under the L82 hood without mods.

In the mid-'80s, Vic Edelbrock had a '79 Corvette with a Donovan aluminum small-block and a Doug Nash five-speed, and it was one nasty piece. We used this car for the development of the Torker II engine packages, which consisted of the single-plane manifold and a camshaft package. These components installed in an old-school 350 produced 400 hp at the time. That was a nice-running street small-block back then.

Congrats on your engine build and the knowledge that you will have for the rest of your life. Enjoy your engine that you designed, machined, assembled, and dyno'd in your Vette. Keep us old guys rocking!



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