Crank Up Your Carb

Properly tuning a carburetor...        read full caption Properly tuning a carburetor can be one of the most cost effective things a racer can do to his engine.

Clockwise from top left—air...        read full caption Clockwise from top left—air bleeds, gaskets, power valves and baseplate screws—key parts in preventive maintenance for carburetors.

Proper float adjustment is...        read full caption Proper float adjustment is critical to a carburetor’s performance.

If you want to improve that Late Model or Street Stock racer, a few basic tips for tuning the carburetor will pull better performance from under the hood and pay dividends on the racetrack.

One of the foremost things a driver or mechanic can do is simply pay attention to available information, particularly if a problem exists with the performance of a carburetor.

Braswell Carburetion provides carburetors for most Winston Cup teams and is involved with all levels of stock car racing. From hands-on experience in carburetor development to offering advice to countless drivers and mechanics, owner Dave Braswell has seen about every scenario possible with carburetors. When a racer calls Braswell with a problem, he asks a few basic questions as a diagnostic starting point. He first determines fuel pressure, and therefore advises racers to invest in a fuel-pressure gauge.

“Probably most of the tuning problems we have are either the fuel pressure is too high or the fuel pressure is too low, or the float level is incorrectly adjusted,” Braswell says. “And you can’t really adjust the float level with the sight plug on the side of the carburetor. We make our own floats now, and we’ve molded a line in them that you use as a reference point to measure.

“You flip the float bowl upside-down and you measure from that point to the roof of the bowl. A starting point for that measurement is 750/1000ths. Twenty or 25/1000ths difference in that measurement will affect the way the motor runs. There’s no way you can have fuel gushing out the hole in the side of the float bowl and jacking the needle and seat up and down and determine a level setting within 20 or 30/1000ths. You’re lucky to get it within a 16th of an inch.”

A racer should spend a few minutes adjusting float levels before mounting the carb. “That’s the number one thing to make sure before you even put the carburetor on the car that you pull the float bowls off and set the float levels,” Braswell says. “If you use the brass-style floats, you’re going to end up with a reading somewhere around 850/1000ths from the seam to the roof of the float bowl. Measure with a ruler or a pair of calipers or however you want to do it.

“Then beyond that you also have to make sure you don’t have more than 7 or 8 pounds of fuel pressure. I would rather have 6½ pounds fuel pressure than I would 8. If you don’t have enough volume at 6½ or 7 pounds fuel pressure, then you need to go to a larger needle and seat. Too much fuel pressure and you just aerate the fuel in the bowl and it makes it too hard to control the fuel level.”

Carb Knowledge

Jerry Dooley, sales manager for Barry Grant Inc., manufacturer of Demon carburetors, says having a basic understanding of the parts and functions of a carburetor will allow most racers to make minor adjustments that will work wonders with a car.

“This means having an understanding of the baseplate, the position of the butterflies in relationship to the transfer slot, how the float level affects things, how the squirters work, how the power valve works, and when, stuff like that,” Dooley says.

Dooley offers several tips on tuning a carburetor that will help direct a racer toward optimum engine performance.

“Probably the most important thing is the adjustment of the butterflies in the baseplate, the positioning in the butterfly with the relationship to the transfer slot,” Dooley says. “This doesn’t require any financial outlay. It’s more an understanding of what’s going on and how to adjust the carburetor.”

Dooley offers a good baseline figure for the adjustment.

“We normally like to say a safe place to be is that you like to see about 20/1000ths of the transfer slot exposed, or (it’s) what we call ‘squaring’ the transfer slot,” Dooley says. “In other words, whatever the width of the slot is, it would be the height, which is about 20/1000ths. You get some guys who say they can’t see 20/1000ths without their glasses on, but they can see the slot and square it. Now, we’ve had cases where we’ve got to close them almost all the way, but that is a safe place to start.

“Where you run into problems is when you get 40-50/1000ths worth of transfer slot exposed. The motor is going to suck the fuel out of that slot, so you’ve just got this great big puddle rolling in there, and it’s uncontrollable. There is no adjustment for that. That’s what makes them rich in the middle of the corner, then all of a sudden they clean themselves out and then it’ll go.”

One of the pitfalls of not understanding butterfly positioning is that it can lead a tuner astray.

“What happens is that if this is incorrectly adjusted, which it can be and the vehicle can still run, the guys end up making a lot of Band-Aid changes to try to over-compensate for the problem that this misadjusted baseplate or butterfly position is creating,” Dooley says. “They end up with a lot more squirter, they end up running higher float levels, they do all kinds of things to try to over-compensate, and they end up making the situation worse.”

Ahead Of The Game

Jeff Dorton, carburetor tuner at Automotive Specialists, says a little preventive maintenance can go a long way toward optimum carburetor performance. “The big thing is just keep the gaskets and the power valves changed and the air bleeds clean, and tighten baseplate screws and that sort of thing,” Dorton says. “If (a racer) does that, he should have no trouble whatsoever. A big problem is guys who let the gaskets sit for a while, for two or three weeks at a time. What happens is those things swell up.” Dorton recommends changing gaskets, power valves and cleaning the air bleeds every two or three weeks.

Dorton has a simple tip to determine if a carburetor is at or near peak efficiency—and one tip on what most competitors should avoid doing.

“I have most of our guys look about 12 inches into the tailpipe,” Dorton says. “That will tell them more than anything. You get guys out in the field looking at plugs, you’ve got problems. You determine a lot by the stopwatch and by the tailpipe. You get the tailpipe a nice gray-white.

“Most of the Saturday night cars, they like to run them lean to make power, because they’ve got cast-iron heads. Or they have a bigger engine where you run a bigger carburetor and it’s not going to matter whether you’re on the edge or not. The stuff where they need to be on the edge, we have them look at the pipe. It just takes somebody who has been doing that a long time to really read plugs.”

Jeff Dorton’s father, Keith, a respected engine builder who owns Automotive Specialists, helped Holley Performance Products develop the Keith Dorton Signature carburetor series, marketed as a line of race-ready carburetors.

“We’ve done a lot of things to the new carburetors that the guy can buy it and use it right out of the box now,” says Robert Writesman, manager of circle track motorsports for Holley. “Before, he had to pay to have a lot of this stuff done. The internal calibration of the carburetor is a little bit different but can obviously be made even better through someone massaging it.

“But it’s a pretty good piece right out of the box. This way we offer a carburetor for the guy who may not have the extra money to go out and spend a lot more money having the older carburetors brought up to speed.”

According to Jeff Dorton, the out-of-the box Holley carbs work fine for most applications. “They’ll just need to jet it,” Dorton says. “I wouldn’t make more than two jet size changes at a time, and I wouldn’t advise they try any funny fuel without consulting their engine builder. Plus, they should keep their fuel filters changed and that sort of thing.”

Making The Right Moves

One common misconception in troubleshooting a carburetor problem involves jetting, according to Dooley of Barry Grant. “Most of the guys, whenever they run into a problem, the first place they’re going to go is, ‘Well, I need to jet up or down,’” Dooley says. “What they need to understand is the jetting of the carburetor is based primarily on wide-open throttle performance going down the straightaway, at the end of the straightaway.

“If they’re excessively too high in jetting or excessively too low in jetting, it will have an effect on the car driving through the corner. So will the idle circuit as far as the adjustments with the idle mixture screws.

“What guys need to understand is the idle quality is important, very important. It’s almost like a stacking process that if you’re missing the first two steps you’re going to be doing some things that over-compensate.”

Braswell offers some practical advice for competitors who run low-compression engines, which are common in Saturday night racing.

“At a 9:1 compression ratio, the flame travel in racing fuel is way too slow for that compression ratio,” Braswell says. “Probably the easiest and most cost effective thing a guy can do if he’s a Saturday night racer is go down to the local gas station and get the premium unleaded and mix it with his racing fuel, like 50 percent or even two-thirds, because it will burn faster and it will lower the heat in the motor, and you don’t need the octane requirements of the fuel.”

Braswell also recommends removing all 90-degree drilled fittings from a car’s fuel system, instead using something with better flow characteristics.

“Practice due diligence on the fuel system from the fuel tank all the way to the carburetor,” Braswell says. “If you’ve got to use a 90-degree turn to get in to something, then use a sweep end, like a hose end 90. Those are tolerable. Just make sure from your pick-up to your carburetor that the fuel line is good and you keep it cool.”