Speed’s Spotlight | Carburetor Cures - Hot Bike Magazine
Last time we spoke about the benefits of a dyno tune on a fuel-injected Harley and how in the hands of an experienced tech, the results can be nothing short of phenomenal. All true. On a carbureted H-D, though, the importance of that dyno and a savvy tech behind it becomes even more important.
Historically, choosing jets and setting needle adjustments has been something done right at home—the results checked by seat-of-the-pants road testing. Talk about a time-consuming, trial-and-error process. Without a dynamometer and its exhaust-gas analyzer, setting a carburetor for optimum performance becomes akin to searching for a needle in a haystack and never knowing when you’ve found it—or stumbled onto that carburetor’s optimal air/fuel ratio (AFR) for the low-speed, cruising, and power ranges. Check things as you go with a dyno, however, and you’ll get darn close to the calibration and efficiency of that properly tuned fuel-injected bike, that sweet-running, cool-running, power-producing 13.7- to 13.9:1 AFR range. Yeah, it’ll cost a few bucks, but so would all the gas burned up with those back-and-forth test rides, over and over again. With an exhaust-gas analyzer in the pipe, the correct low-end calibration can be set, the partial acceleration and mid-ranges dialed in, and then the upper ranges of the throttle, the areas controlled exclusively by the main jet, can be analyzed and adjusted. This holds true no matter what carburetor is on the bike, a stock CV, an S&S, or a Mikuni. And don’t be so quick to dismiss the original-equipment CV. Properly set up, it just might be all the carburetor that’s needed.
“The first thing we’ll ask any rider of a carbureted bike is if there’s any spitting or popping out of the air cleaner, and does he have to pull the choke out to make that bike run better?” says Jason Hanson, ace tuner at Speed’s Performance Plus. If either of those conditions occurs it’s a safe bet the bike is running way too lean. And it might not be just the jetting. “A lot of times, we see leaking intake gaskets, especially on bikes with a few miles on them,” Jason adds. By first replacing the intake gaskets and sealing those air leaks, something often overlooked, you’ll give yourself a much better chance of hitting a homerun tuning the carburetor.
Performance-wise, a carburetor that’s too big is almost worse than one that’s too small.
Something else often overlooked, says Jamie Hanson, Jason’s brother, is the carb’s venting. Without getting overly technical, a carburetor works by pressure differentials. Fuel is drawn from the float bowl and through the various jets and passages by the differences in air pressure in the venturi and on the fuel in the float bowl. The vacuum signal across the carburetor, which pulls that fuel through the jets, can vary widely and when things are not right you’ll see the mixture go from lean to rich to lean to rich again all in one power pull. The AFR will be all over the map and no jet change will correct that. However, increasing the venting evens out the fuel delivery with no up-and-down variations, and from there it becomes a fairly straightforward matter of changing the main jet accordingly. This venting situation is especially critical with CV carburetors and on most S&S models, carbs with a relatively small venting system to start with. When the guys at Speed’s are confronted with a CV carburetor that’s lean under part-throttle conditions, they’ll work on the venting first and quite often that cures the problem all by itself.
Speaking of that Harley CV carb, along with that venting fix, the next best move is a Dynojet Thunderslide package. It comes with virtually everything needed to upgrade and recalibrate that carburetor, but beware cheap offshore imitations. Some use brass needles, unlike the fully adjustable titanium Dynojet part, and they’re not worth the money you pay for them. They wear out quickly. Additionally, that Dynojet kit has a better, lightweight slide, a significantly better spring for the diaphragm, and a new, complete Main-jet system with an assortment of replaceable jets. On a stock bike, or even a moderately modified one, that OE carburetor setup with a Thunderslide and dyno-tuned to near perfection is going to be a pretty sweet deal. And it’s affordable.
When is it time to switch to a bigger carburetor? “When you start putting some lungs into that bike,” Jason says. “Go bigger on the displacement, install better heads with bigger ports, a larger intake manifold, things like that. At that point a bigger carburetor pays off.” Having said that, the switch to a slightly bigger carburetor, like a 42mm Mikuni or the S&S Super E, will result in more torque and horsepower on that relatively stock motorcycle, too. Just don’t go overboard, Jason warns. Performance-wise, a carburetor that’s too big is almost worse than one that’s too small.
Now, having said all this, bear in mind that a carburetor, unlike an FI system, is going to be a lot more sensitive to everything else on the bike, the pipes, for instance. As an example, a carbureted bike with a set of ultra-short, large-diameter unbaffled drag pipes is going to be pretty tough to jet correctly, no matter how hard you try. In situations like that, the tuning quite often becomes a compromise, settling for something less than the optimum AFR. If you must have that show bike look, it’ll come at a cost, and it’ll be more than just the price of those pipes.
For more information on all of this, talk with the Speed’s Performance Plus team yourself or take a look at their website—much has been written about the benefits of dyno-tuning a Harley’s fuel-injection system. Putting a carbureted bike through those same dyno paces is just as important, maybe even more so. HB
Speed’s Performance Plus
(605) 695-2272 | (386) 405-7898