Mikuni on Betsy

THE MIKUNI HRS FULL RACE CARBURETOR

Posted on 11. Dec, 2014 by in Gypsy Bikers

by Scooter Tramp Scotty Kerekes

I’ve been riding evolution engines for 24 years. Although the Evo offers great reliability, it’s low compression, relatively small 80 inch displacement, does not offer the power of today’s big motors. For smaller motorcycles like Softails and Dynas it’s plenty really, but the big touring bikes are another story. For their weight and wind resistance, even when fitted with a large modern engine, these bikes are dramatically slower than their smaller counterparts. My 1988, 524,000 mile full dress, Evo powered, Electra Glide gets around okay. It’ll do the job. But give it a steep mountain grade, heavy head wind, passenger, or combination of these, and it struggles noticeably. In fact you may have to drop a gear on occasion for uncommonly steep grades. In truth, the bike becomes pretty much of a slug.

1 Bored with this lack of ponies I installed an Andrews EV27 cam years ago and this made a dramatic difference. Still, it wasn’t quite enough. I began to toy with the idea of head-work and higher compression pistons.

Living on the road full time, by summer I travel the country and often work as a mechanic for the many venders that 2 permeate the large motorcycle rallies across this big nation. It was at one such monolith event that I walked to the vendor next door (a big outfit called RacePro that specializes in Mikuni carburetors and a handful of other performance parts that they sell and install) to ask about borrowing a broom. Rob, the owner, gave me his broom and, over the course of this event, we became friends. I later told him of my ideas for doing head-work and such. His opinion was that the simple installation of a Mikuni HSR series flat slide carburetor would produce the results I was looking for. “Stay on the outside of the engine,” he said “Leave the inside alone.” Rob seemed adamant, but I’d heard sales pitches before. Every vendor has one. Besides, I’d been using stock CV carburetors for years, had learned to tune them very well, and was pretty happy with their performance. Then, both of us being in the vendor biz, Rob offered to sell me a Mikuni at just over his cost—a substantial discount. It was obvious that this guy really believed in his product. I turned his offer down but thought about it over the course of the next year. By the following season I’d decided to buy one. So I did.

3 Although the Mikuni HRS comes from the factory already set up for evolution engine applications, Rob tweaked the new carburetor just a little to accommodate my high-flow air filter, Cycle Shack exhaust, and Andrews cam. The Mikuni kit also includes the proper jets for twin cam engines as well; although these must be installed before mounting the carb. Rob is also only a phone call away from any customer who needs proper tuning information and advice.

Half believing I might feel a little improvement from this thing I installed it and took the first ride. The difference was 4 shocking. I’d not expected that at all. Mikuni carbs are known for quick throttle response—a dramatic difference really—and it was necessary to retrain my clutch hand to properly meet the fast revving engine. The overall power increase was damn near the same.
I’d installed this thing while working at the 2014 Sturgis rally and began riding back and forth to work. It was almost like a different bike. I’d had no idea.

5 While at work my ignition switch broke and a coworker loaned me his bike (identical to mine except slightly newer) to get home that night and back to work in the morning. What a fucking slug! Later he rode mine and immediately wanted the same cam and carburetor. Even offered to buy the Mikuni from me.

In the interest of fine tuning I decided to approach Chris at Cycle Solutions (I’d worked for him in the past) to see if I Mikuni vs stock could weasel a free dyno tuning job. He agreed to do it. The exhaust sniffer on Chris’s highly computerized dyno machine measures exactly the air/fuel mixture that the carburetor is feeding into the engine. It showed a perfect 13.5 to 1 mixture ratio across the RPM range. The setup was fine so there was no need to go inside the carburetor. The only adjustments Chris made was to turn the idle mixture screw from the factory two turns out, down to 1¼ therefore enriching the idle mixture a little. Next he tightened one of the adjustable accelerator pump screws just a tad. That was it. When finished the bike ran perfectly.

8 The dyno then showed that the old evo now produced more power than a stock 96 inch twin cam, and was even creeping up on a stock 103. What a trip.

For the following months I traveled this big country on the fully loaded Electra Glide and was easily flying up the left lane when crossing steep grades of the Smoky Mountains or their equivalent. Pretty cool. 9

Of course all carburetors loose power in higher altitudes and the Mikuni is no exception. Since its application I’ve been as high as 6,000 feet and could certainly feel the drain. The main difference being that, since I now have more power to begin with, there’s still more than before even at these altitudes.

Now let’s talk gas mileage:

10 When new, this full race carburetor is set up to produce maximum power, but the tuning manual included in the kit tells of adjustments that can be made to increase gas mileage as well. Because I travel almost constantly on a full dress bagger that’s always heavily loaded down, I installed the next size smaller accelerator pump nozzle (there are three) and also dropped the needle setting one notch. Although these changes increased mileage, neither really seemed to affect the power.

Since performance carburetors make more power by giving the engine all the gas it asks for (the stock CV carb is 11 stingy) instantly and on demand, gas mileage is where they suffer most. Whereas both my stock CV and the new Mikuni like to stay in the low 30s MPG in town, the CV immediately jumps to 40 or 42 anywhere else while the Mikuni (a full race carburetor) likes to stay around 35. Sometimes it gives me 37, but the best I’ve gotten is 39, and that was only once. So this is where the trade off comes in: more power equals more gas.

But if it’s power you want then this carburetor is an absolute must. This is my personal, road tested, no bullshit, experience.

I got this performance carb from Rob over at RacePro. He’s been in the Mikuni biz for many years and is an expert with these things. Here’s his info:

Rob Hassay
RacePro Motorsports
1-888-RACEPRO(722-3776)
www.888racepro.com

Cycle Solutions dyno tuning:
cyclesolutions.com
Scooter Tramp Scotty

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