ith all the talk lately about fuel-injection this and fuel-injection that and pump pressures and ECMs and re-mapping and O2 sensors you’d think that the good old days of the carbureted Harley-Davidson were long gone. You’d think wrong. Carbureted V-twins are still very much part of the scene. A big part. Virtually every custom bike built has a carburetor on it, there are a bazillion pre-FI Harleys still out on the road, thank you, and how about this–both TP Engineering and S&S chose to use carburetors, not injectors, for their latest and greatest EPA-compliant engines. The carbureted motorcycle is alive and well, and the guys at Speed’s Performance Plus see plenty of them as they travel the country with their mobile tuning center and repair shop.
     More than just seeing all those bikes with their jets and emulsion tubes and float bowls still in place, though, Wayne Hanson and his crew have perfected the art of tuning those carbureted motorcycles to perfection. Wayne, Jamie, Jason and the rest of the Speed’s team are certainly versed in the intricacies of getting the most from modern fuel-injection, no doubt about that, but they haven’t forgotten a thing about what started it all and what’s still a big part of the Harley world, and that’s the carburetor. What follows, then, is a sort of primer on performance carburetion with Wayne Hanson leading the class through the ins and outs of first choosing the right carburetor for the job and then getting into some of the subtle tweaking and tuning to get the most from that fuel-mixer. And right along with that we’ll let Wayne explain how he goes about upgrading and adjusting that carbureted bike’s ignition system, too. One most definitely goes with the other when the goal is a crisp, spunky running motorcycle that’ll snap your head back at each gear change and then settle into a glass-smooth ride out on the highway. And yeah, you most definitely can get there with a carburetor and a good ignition.
     “The vast majority of bikes coming up to our truck and tuning center,” Jamie Hanson says, “are pure stockers or customs with bigger-displacement but still fairly mild engines. They’re brought in by riders wanting just a little more out of their machines. Most don’t want all the fuss and added maintenance of an all-out performance set up, though. What they’re looking for is crisp throttle response and the extra push that a boost in low- and mid-range torque will deliver.” The first area addressed to get there is simple enough: It’s the air filter. An OE filter in its restricted airbox or a too-small one on a custom is a major impediment to airflow.      Fortunately the FatBook is filled with good alternatives to improve not only the airflow, but also the look. But either way, whether its just performance that’s required or performance with some style, “For a stock-displacement engine,” Jamie says, “nothing smaller than a 2 3/4-inch deep filter should be fit. That’s the minimum it takes to move enough air.” Bigger is better, of course, and there are plenty of oversize, high-flow filters in the FatBook from K&N and Drag Specialties and others and Speed’s carries them all, along with their own “2 Step” forward-facing filter kit and the oversized “Speedy Flow” which neatly tucks behind a stock Harley cover. With the filter situation handled one way or another the guys then look at the exhaust currently mounted on the bike in question. Reduced to the basics a V-twin engine is nothing more than a big air pump; getting the air in and then getting it out fast is key to performance.      “And in our experience,” everyone at Speed’s agrees, “a 2-into-1 system with a collector seems to be the most efficient pipe design you can install. It really scavenges the exhaust.” But a performance exhaust doesn’t have to be a 2-into-1. If you can’t live with that look, or the unique sound of a 2-into-1, there are lots of good alternatives. A set of equal-length staggered duals, preferably with some sort of crossover pipe or connecting chamber to balance the exhaust from cylinder-to-cylinder is a perfectly acceptable pick from a performance standpoint. So are many of the stepped-header systems available. Rinehart, Hooker, Samson and others all offer them and they look good, perform well and preserve that V-twin rumble. With those basics addressed, the air filter and the exhaust, it’s time to start looking at the carburetor itself.
     Not too many years back a 95-inch V-twin was a big engine. Today that’s just about average. 103-inchers and 110-inchers are offered as dealer options right at the showroom, and in the aftermarket the 107s, 114s, 117s, 124s and 131s are everywhere. And for one of these increasingly popular big motors, don’t consider a carburetor any smaller than an S&S “G” (at 2 1/16-in. / 52.3mm), a 45mm Mikuni or if it’s a CV model something as large as 51mm. These are all ballpark numbers, though, because each individual engine with its own compression ratio and cam timing will respond differently to different carburetor combinations and this is where the real need for custom dyno-tuning begins to show itself. Speed’s, as an example, often ends up custom building a carburetor to the application by over-boring one of those S&S mixers as much as 0.100 of an inch then custom jetting and venting it. And on a big-inch engine with a big carburetor the air-filter requirement jumps up, too, to at least 3-inches thick. One of those forward-mount filter kits with their big, tapered high-flow elements begins to make sense here, and for more than just appearance.
     Back to those carburetors and their setup. Whether it’s a stock CV-model on an 88-incher or even a 95, which can be made to work just fine given proper attention and parts like the Dynojet Recalibration Kit and ThunderSlide or the recalibration kits from Vance & Hines and Zipper’s, or it’s one of those aftermarket replacements from S&S, Mikuni or Edelbrock, they’re all going to need some fine-tuning to perform their best. Rare indeed is the carburetor that comes optimally setup right out of the box. Now, properly tuning and setting up a carburetor can be tricky business, a trial-and-error process of switching jets back-and-forth and fiddling with needle heights. To do it right really requires access to a dynamometer and an air/fuel sensor. Consider the possibilities. Some carbs are designed with two fuel circuits, others have three. Some use adjustable needles to regulate fuel delivery through replaceable jets, some don’t. Some are the constant velocity, CV design, others aren’t. Things can get complicated and an inexperienced fiddler can get lost fast and actually end up worse off than when he started. And there are some things that inexperienced tuners might not even think to look at, like float-bowl venting. This is one of the most overlooked areas in carburetor tuning and it’s one of the most important, taking into account all the variables introduced with the different types of high-flow air filters that might be fit and the different exhaust systems and varying durations and overlaps of camshaft sets that might also be installed. The vacuum signal across the carburetor, which actually pulls the raw fuel through the jets, can widely fluctuate making float-bowl venting crucial. When the venting’s not right you’ll see things going from lean to rich to lean to rich again and happen all in one power pull from idle to redline.      It’s something definitely felt out on the road as surging, and a change in jet size or needle height alone won’t do a thing to correct matters. From approximately 2,000 RPM right up to redline all the fuel is coming through the main jet, yet with an improperly vented carburetor the air/fuel mixture can be all over the map. How do you make one jet change to correct that?
     Speed’s does it by first carefully addressing that venting issue. Simply put, optimizing the venting evens out the fuel delivery through the main jet, which can then be properly sized. With the float bowl properly vented the engine will either be rich all the way through its power pull or it will be lean all the way. No up and down variations. From there it becomes a fairly straightforward matter of changing the main jet accordingly, going bigger or smaller as needed. Carburetor venting becomes a tuning issue more often than you’d think, too. It’s commonly required, the techs at Speed’s say, after bolting on some of those forward-facing air cleaners. Those big filters move a lot of air but they also increase the length of the intake track and speed up that airflow, which can overcome standard float-bowl venting. This float-bowl venting situation most often occurs, they say, with CV carburetors and most S&S models, carbs with a relatively small venting system to start with. On a CV carburetor that’s lean under part-throttle conditions they’ll even work on the venting first, before making any changes to the needle height. And sometimes, especially with stock- or near-stock engines, correcting that venting situation cures the problem all by itself. If you feel a definite surge under a full-power pull float-bowl venting is certainly something to suspect and a dynamometer with a fuel/air sensor can confirm the situation. And speaking of that dyno, and all that’s been mentioned so far, proper carburetor tuning really requires one. Without a dyno and an air/fuel sensor it’s going to be pretty tough to figure out if the problem is jetting alone or if it’s a combination of float-bowl venting and jetting and/or needle height and you can spend untold hours trying out every possibility in the book. Specialized carb tuning like this is an art and Speed’s Performance Plus has made a career of it.
     For real carburetor performance it’s crucial to match that optimized fuel delivery with an optimized spark delivery. Performance tuning the ignition system can bring a lazy engine to life and tame a highly-modified one for the street. Of course most Harleys seen today have totally electronic systems, factory sealed and tamper-proof. These systems rarely deliver the spark at the precise time and with the intensity a specific bike requires, though. The aftermarket has cured that, and once again all the options are in the FatBook. There are dual-fire ignitions and single-fire systems (which Speed’s has found to be somewhat smoother running) and those add-on ignition modules from Drag Specialties and Crane and Daytona Twin-Tec, Dyna and a host of others all allow a level of adjustability that’s almost beyond belief. But just as with those carburetors, setting that ignition curve just right takes some experience.
     Say you’ve got the air/fuel ratio just about perfect, but there’s still an annoying ping in one area, maybe around 2,700 RPM. With one of these totally adjustable ignition modules you can move the timing in that one area only. Or maybe the bike’s a little sluggish on the bottom end. You can adjust the timing for that, too. The beauty of all this is that you don’t have to throw out a complete ignition curve that’s performing nicely everywhere else and settle for a less aggressive curve overall to cure a problem in one area. All of the systems mentioned supply a number of pre-written performance ignition curves as a starting point, curves tailored to the broadest applications of displacement, compression ratio, carburetor size and camshaft. They’re great to begin with. Speed’s generally limits the ignition curve to a 6,000 RPM redline, however, the idea being to make the power quicker and keep it lower in the RPM range where the engine will be most used. Keeping the power down in that 5,000- to 6,000 RPM range as an upper limit will greatly increase the engine’s longevity, too. And much like carburetor tuning every bike will require a slightly different ignition curve to perform its best, all depending upon the other modifications made to it, its rider, the way the bike is used and where it’s used. That said, the overall advance can end up being anywhere from 30-degres all in to a whopping 50-degrees total, and how it gets there–the steps of advance and where they occur–can be equally varied. But with those aftermarket tuning modules, the pre-written performance curves as starting points and a laptop computer you can easily manipulate that ignition timing yourself and get pretty darn close to optimum. Couple that with some high-output coils, and there are plenty, some quality wires and spark plugs, lots of good choices there, too, and you’re all but guaranteed to be correctly lighting the fuel supplied by that carburetor.
     Carburetors are alive and well, and will be for quite some time. The FatBook is packed full with them, and with all the jet-kits, needles and filters and accessories to fine-tune these timeless fuel-mixers to perfection. Couple all that with a hot new ignition upgrade and, well, you might be doing it old school but it’s old school with some serious punch…

Speed’s Performance Plus is a Hanson family affair. Wayne and his sons Jamie and Jason travel the country leaving sweet-running Harleys in their wake. Their shop on wheels carries plenty of stock, too.

Speed’s isn’t hard to spot wherever they set up. If the bright colors and all the banners don’t grab you the sound of that dyno will. Wayne’s tuning those carbs morning ‘till night.

Single-fire, dual-fire, hot coils, low-resistance wires, total adjustability–it’s all part of an upgraded ignition system that can wake up a lazy bike and tame a wild one. Crane’s Fireball is always a favorite, and for good reason.

Don’t expect a carburetor to come out of the box ready to go, perfectly tuned and matched to the application. The FatBook’s loaded with everything to make things right, though, like these jet- and slide kits by Dynojet.