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Carburetor Tuning & Reader Questions - Performance Q&A

Kevin McClelland Jan 1, 2010

Tuning In The 21st Century
Carburetor tuning has always been up to the driver and his/her senses, to determine how a car behaves and performs. We can use vacuum gauges to get the idle side of the carburetor correct, but the main circuit of the carb can be quite tricky. What does a lean bog versus a rich bog feel like? Can you easily determine which direction the jetting needs to go? Since fuel changed back in the '70s to unleaded mix, we can no longer read the spark plugs for mixture; most unleaded plugs look completely white when the fueling is correct. The best way to set up the main jet calibration has always been at the dragstrip. Tuning the jetting for maximum mph in the quarter-mile is what the engine wants, but with ever-changing weather conditions between the density altitude and wind conditions, some jetting changes will only yield small changes in mph, which can then be skewed by a shift in the weather.

chp_01_z Carburetor_tuning_and_reader_questions Performance_parts 2/2

For years we've been using wideband oxygen sensors on engine and chassis dynos. They have given us a window into the combustion chamber and the air/fuel ratio (AFR) of the burnt gases. These dyno-grade sensors have been expensive and require data-logging capability. This past weekend I had the pleasure of setting AFR with a very inexpensive wideband oxygen sensor for permanent installation into your vehicle. Advanced Engine Management sells a complete Universal Exhaust Gas Oxygen (UEGO) Gauge Controller featuring a Bosch UEGO "laboratory grade" sensor. This sensor has an accuracy and repeatability to 0.1 AFR. The gauge is a standard 21/16-inch with a sweeping LED readout, and a digital readout in the center of the gauge. We had one on our Stock Eliminator wagon tuning the highly modified Quadrajet, and another on an EFI-equipped Stocker. Within two runs on the wagon and two metering rod changes, we had the AFR honed right into 12.8:1. This is where the engine made peak power on the engine dyno. On the EFI car we had been chasing our tail working out other calibration issues. The EFI was commanding 12.5 AFR but was actually at 15:1 going down the track! We quickly adjusted the fuel tables in the EFI and the AFR came right in line at 12.9:1, and we picked up 0.4 in the quarter-mile. I sure wish that the simple rod change would have given me that kind of performance increase in the wagon.

The AEM Gauge-Type UEGO controller kit, PN AVM-30-4100, is available from Summit for $218.95. For the price of this sensor you can have your car dialed in quickly and accurately. After doing this for years, and using a wideband at the track for the first time, I will never tune another car without one. This is just another easy tuning tool you should have in your toolbox and install when changes have been made that require new jetting. I'm tired of working so hard!

Thumpr On
Q: Can you offer a recommendation for my project '55 Bel Air? I purchased a 400hp LS2 from an '06 Corvette with less than 10,000 miles. I want to deviate from stock a little, to give the engine a better sound and a lope, as well as gain some power, and need some help with a cam choice. I have read a little about the Thumpr from Competition Cams and wanted your opinion on which version will best suit my needs. Can I use the stock lifters and pushrods since they are still somewhat fresh? Will I have to have the computer reprogrammed to compensate for the different bumpstick? Will the injectors need to be upgraded to make the whole package work? Is there a better choice or combination to give the engine a lope with a gain in horsepower and torque? I love your articles and layout of the magazine. Thanks for the help, and keep them coming!
Mike Yore
Indian, AK

A: Having a cool, choppy idle in a '55 Chevy is just about mandatory. Doing this without killing all the slow-speed torque to pull your very nice street rod around is another thing. The Comp Cams Thumpr camshaft line does give you somewhat the best of both worlds. With the advanced inlet lobe and tight separation angles, it helps boost slow-speed torque while maintaining idle quality. We'd recommend going with the smallest of the Thumpr line for your project. The 275THR9 camshaft specs out with advertised numbers of 275/295 degrees duration, 219/233 degrees at 0.050-inch tappet lift, 0.553/0.536-inch max lift, ground on 109 centers. Anything more than this will hurt the slow-speed driveability of your very refined LS2 small-block.

The original injectors will feed this combination just fine. You will probably need to do some calibration work to clean up any driveability glitches. With the swap, you'll need to create an air ducting system to feed the air valve. Incorporating the mass air sensor correctly into this tube will make the most difference in the calibration. Whatever the factory ducting size is at the actual mass air, must be maintained in the fabricated air inlet system. This will keep your fuel trim numbers as close as you can in your calibration. This is where you will want to do any final cleanup with your engine swap and camshaft installation.

The rest of your factory LS2 components will work well with the Thumpr camshaft. Enjoy your engine swap and many years of happy cruising!

Need A Little Content
Q: Are there any magazines or catalogs out there with performance parts for Chevy Cobalts? I see little things here and there in the import magazines, and pretty much nothing from the Chevy magazines.
Jeff Anderson
via email

A: The Cobalt SSs are little rocket ships, first with their supercharged Ecotec engines, and then in '08-and-up with the turbocharged upgrade. Sorry for the lag in coverage here in CHP. However, our sister publication GM High-Tech Performance has run a few stories over the years on these models. Just lately they featured an '09 turbo direct injected tuning story. They came away with some pretty impressive horsepower gains.

As for component parts, you'll find most of the performance parts in the sport compact arena. After some digging we've come away with several one-stop shops out on the internet. Check out Cobalt Addiction and Modern Performance. Both feature a wide range of performance upgrades for Cobalts of all years. Modern Performance supports most sport compacts, and Cobalt Addiction only caters to the Cobalt enthusiast. Hopefully, these three avenues will feed your need for content.


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!


Whoa, Baby
Q: I am the luckiest man in the world. My wife is just as big a car nut as I am. (You've got to love a woman who's actually happy to get a carburetor for her birthday.) We have a few old Camaros, a couple of '68s, a '69, and a '72. My wife got me into drag racing a few years ago; she drives and I get to make the car go faster with her '81 Camaro. We run 6.50s in the 1/8-mile. Although the car still isn't fast enough for her, she can hardly get it stopped at the big end of the track. No vacuum means no power brakes! Does anyone make a kit that I can convert this power-brake car into a manual-brake car? Or what would be the easiest and least expensive way to make this thing easier to stop?
Steve Burch
Harvest, AL

A: Getting our wives' support for our habit is a blessing. My mom warned my wife before we got married that I wasn't going to change-and I haven't. Enjoy racing with your best friend. Let's see if we can keep her safe.

A couple of things jump out with your question. First, you're right that the vacuum level is quite low because of the camshaft you are running. This should only affect the brakes' operation when idling around, or from a fresh startup. When you're lifting off the throttle at the top end, the engine vacuum goes very high (20 plus inches of mercury) and the power brakes should function normally. If they are not, look to the booster's condition and the check valve in the booster line from the carburetor. The check valve is to hold vacuum in the booster when the vacuum falls, to give you at least one good brake application.

Manual brakes were standard on your '81 Camaro, but very rare. It used a smaller master cylinder diameter and a specific pedal pushrod to give you the proper pedal ratio and brake pressure. You could try to round up these components and convert the car over with factory parts. The master is discontinued, as is the pedal pushrod. You could also go with a vacuum booster pump to increase the vacuum supply to the booster. There are some very nice pumps on the market and may be the cheapest option. Check with Master Power Brakes for all your brake needs. MPB offers a very nice Heavy Duty Vacuum Pump kit, PN AC2724K, which will produce a constant 18-20 inches of vacuum. This kit comes complete with pump and vacuum switch to turn the pump on only when needed. Also, if you still wish to convert the brake system over to manual, MPB can hook you up with the components to make this happen.

Get these brakes working soon-you don't want one of your wife's legs to be bigger than yours! Good luck, and be safe racing.


Q: Do you know if the '67-69 Camaro Z/28 302 motor was a bored-out 283 or a destroked 327 (283 crank used in the 327)? Thanks for your time!
Daryl Lange
Oroville, WA

A: The last year of the 283s comes in right at '67, which is the first year of the 302 small-journal engines. The 327s had been around since the '62 model year in Vettes and Impalas. Sliding a 3-inch stroke crankshaft into the 327 is the simple answer. Most of the 283s won't accommodate a 4-inch overbore without the cylinders becoming dangerously thin. Only some of the early blocks could handle a 4-inch overbore, and it certainly wasn't into the late '60s. The tops for most of the 283s were 0.060 inch over, bringing the engine to 292 inches. We know this isn't a definite answer, but it makes sense logically.

Sensor Madness
Q: Can you help me install a '95 Z28 engine in my '96 Impala? The '96 has a crank sensor behind the harmonic balancer, and the '95 does not. Using the '96 harness and computer, can I trick the '96 sensor plug and computer by grounding off one of the wires coming from the sensor? Thanks for your help.
Cliff Blacke
Piqua, OH

A: The sensor you're referring to references engine misfire for the OBD II computer system mandated in 1996. There is a small reluctor wheel behind the front cover that gives the sensor a 4X signal to the computer. With the differences in the water pump and damper between the Camaro dress and the B-car Impala, it would be easiest to just swap over the front cover, sensor, and reluctor wheel into the '95 engine. This would make the computer happiest and complete the swap properly.

When converting over the '95 engine, make sure you use the damper hub from the '96 engine because it's 0.100 inch shorter to accommodate the reluctor wheel spacing. Also, you'll need to swap out the Woodruff key that indexes the crank sprocket. The Woodruff key is slightly longer (0.100 inch) to engage the reluctor wheel.

Fooling the computer is really not an option without tuning software. You can disable the misfire detection with calibration software, but a little more labor will make this a painless swap.

Stovebolt Racer
Q: I've been trying to find drawings showing the hole locations and sizes, center hole size, for various bellhousings and transmissions. I need to machine a trans adapter. We just finished building a 261 (0.060 over) inline-six for my '50 Chevy station wagon. I'm putting a T5 from an S-10, and an S-10 4x4 rearend with 4.10:1 gears in it. Isky ground a custom cam for more low-end torque for this heavier-than-average car.

Yeah, I'm a geezer, but I also help with tuning an NHRA TAFC and race go-karts during our Minnesota winters, so I'm not done yet. I realize this question probably doesn't fit the column, but I thought you can probably steer me to the info I need. Thanks for any help.
Darryl Bergerson
Prior Lake, MN

A: From most of the digging around we couldn't find an adapter plate available for your combination. Surprisingly, Advanced Adapters no longer lists this swap. Surely it had this adapter available in the past, but it's been 45 years since the last time the old Stovebolt six-cylinders were produced. But we've found something else.

The factory 261 manual transmission bellhousing has the same bolt pattern as the mid-'80s T-5 transmission! The input shaft is slightly longer than the three- and four-speed transmissions of the day, and it is a standard 27-spline shaft. Even the pilot bushing stub is the same size. You may need to shorten the pilot bushing stub of the input shaft slightly with a cut-off wheel and dress it with a sanding disk. You will need to check for clearance to prevent the input from bottoming out in the crankshaft. Next, drill out the transmission ears to accommodate the 1/2-inch bolt from the 13mm fasteners used in the later-model applications. The rest is just simple bolt-ups and driveshaft shortening. The original clutch, throwout fork, throwout bearing, and pilot bushing will work. You will need to pick up a clutch disk from a '71-and-later manual application with the 27-inch spline count. Just match the pressure plate size that you have.

Good luck with your very cool '50 wagon. My dad is going to be jealous. He's about half done with his '50 sedan delivery but has too many projects going on to finish it.

Serpentine Solutions
Q: A while back in "Performance Q&A," you talked about a company that puts together GM serpentine kits for crate engines. Could you please give me the name and how to get a hold of them? My son gets your rag and since it's my mailbox, I get to read the Performance Q&A first. Thank you very much.
Bob Bussworth
Avoca, NE

A: We're glad to see your son has good taste-and that you've got quick hands. He should learn that age and treachery will win out over youth and enthusiasm every time.

Our buddy Ken Casey at Burt Chevy is the hookup for factory GM Serpentine kits. He has sold a ton of GM crate engines and the components to make any swap easy and free of surprises. If he hasn't seen it all, we're just a phone call away. You can reach Kenny at 800.345.5744 and he will be glad to help you out.


Technical questions for Kevin McClelland can be sent to him at



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