A few issues ago we talked with the guys at Speed’s Performance Plus about performance-tuning a Harley with a carburetor. “And with the right setup,” Jamie Hanson said, “a carbureted Harley can be a strong runner. Smooth, responsive, and plenty crisp.” Running down the options for putting some pep into a carbureted Harley, a complete swap, we learned, is a tuner’s dream. Especially when it’s switching to a Mikuni. “A big advantage there,” Jamie went on, “is the ability to really fine-tune that carburetor. Mikuni jets are available in smaller increments, the needle is adjustable with lots of clip settings and the accelerator pump is adjustable. You can really dial these things in. But the biggest plus is the Mikuni’s flat-slide design. The carb is super-responsive to the throttle.”
Fair enough, but all that adjustability and crisp performance comes at a price. “Which is why we also keep the Dynojet Thunderslide kits in stock,” Jamie tells us now. “When your wallet can’t swing the switch to that Mikuni, a Thunderslide is a great second choice.” Coming with everything needed to upgrade and recalibrate a stock CV carburetor, bringing it to near-Mikuni performance, the kit includes a lightweight slide with a height-adjustable needle, a new spring for the diaphragm and a completely new main-jet system with an assortment of replaceable jets. “An OE carburetor setup with a Thunderslide is going to be as near perfect as that stock mixer will get,” Jamie says. And the price is right, less than $150 plus a little labor to put it all in.
We ran into the Speed’s Performance gang during Bike Week ’13 and got to see firsthand if Jamie Hanson was right here. Could a Thunderslide kit really make that much of a difference? In a little more than an hour’s time, we had the answer. A customer had rolled up to the SPP mobile shop with an ’03 FXD Super Glide that already had a free-flowing air filter on it along with a SuperTrapp 2-into-1. The bike’s CV carb had been re-jetted in accordance with those changes and the bike was a good runner, but not a great one. The rider complained of occasional spit-backs through the carb, and response to the throttle, he felt, could be better. “A perfect candidate for a Thunderslide,” Jamie said. “Stick around.”
Installation of everything in that Thunderslide package is a straightforward operation and this isn’t the time or place for a step-by-step explanation. Suffice it to say that with its assortment of main jets, a replacement Emulsion Tube, that lightweight slide with its new spring, the redesigned and adjustable needle along with all the clips and spacers to fit it all into that CV everything’s there, including detailed instructions and tuning recommendations. Complete removal of the carb isn’t required either, at least not as it’s done by Speed’s Performance. With the choke/enrichener cable released from its bracket, the carburetor can be wiggled out far enough to be disassembled to make the necessary changes. “Lots quicker that way,” Jamie says. Following Dynojet’s recommendations and their own experience SPP set up this particular carb in this application using the 185 Main Jet supplied in the kit, the E-clip was positioned on the fourth groove of the new needle, the No. 48 Pilot/Slow jet in the carburetor was switched back to a No. 45 and of course the stock metal slide was replaced with the new, lightweight Thunderslide and its spring. And just because, the carburetor went back onto the manifold with a new seal.
Result? With the mixture-adjustment screw set at 3-1/2 turns off lightly seated, the bike settled into a smooth, steady idle. The real change came when the throttle was twisted. Response, as promised, was immediate. That lightweight slide coupled with proper jetting was doing its thing. All traces of the previous spit-back condition were gone too. On a quick testride the bike was lots more rideable with what seemed like a bit more oomph. A trip to the SPP dyno confirmed that seat-of-the-pants impression. First of all, the air/fuel ratio stayed comfortably in the sweet zone throughout the RPM range, “Right where I like to see it,” Jamie’s brother Jason noted. And the power? Seventy-two horsepower and 83 lb-ft torque, this coming from a basically stock 88-inch engine with great throttle response. Can a carbureted bike be a sweet runner? Absolutely. Next time the Speed’s Performance crew is in your area, stop in and see for yourself. And bring that carburetor.
“The real change came when the throttle was twisted. Response, as promised, was immediate.”
Speed’s Performance Plus
605-695-1401 – MN
605-695-2272 – SD