BUILDING A BETTER TWIN

Results

 

THE RESULTS:

    Part of this project included dyno testing at Jerry Branch’s facility. Rather than go to the dragstrip with the stock bike and then the Stage I and Stage II versions, we chose to put the bike on the pump. Dragstrip testing is limited in that it will not provide a very good idea of how a motorcycle will accelerate at low rpm, and since one of our main goals was to develop useful power throughout the engine’s rev-range, we felt that comparing the stock engine to the Stage I and Stage II versions should be done on a dynamometer.

    Our first trip to the dyno was with the stock bike and with the Stage I components. The stock (1979 Special) engine performed about as expected, although it made more peak horsepower than anticipated. Power in the lower half of the power band was unimpressive. As the tach climbed from 5,000 toward 7,000 rpm, the XS began to wake up and run. Above 6,000 rpm, the torque began to drop off and the horsepower followed the downward trend. By 7,000 rpm the engine was really suffering and fell on its face at 7,500. The stocker reached a peak horsepower reading of 42 at 7,000 rpm.

    Our first modification was to install the Bub Enterprise two-into-two exhaust. It was a bit scary; we were going to find out if our pipe design was really going to work. No wild claims could change the cold reality of what the dyno found. The first horsepower sampling was taken at 4,000 rpm, and gave a 37-percent increase in power over the stock engine at the same rpm! Our Bub-piped stock engine made 27.6 horsepower whereas the factory-stock engine gave only 20.1. By 6,000, the Bub two-into-two’s advantage had dropped to only six percent and we were disappointed until it occurred to us that riders hardly ever rev their XSs that high. Jerry also suggested that perhaps the stock carburetors were beginning to adversely affect performance. So we added the 36mm Mikuni kit to the Bub pipes and made another run at the dyno.

    The Branch Mikuni kit did not improve power output until the engine was past the 6000-rpm mark on the tach. Then the restriction of the stock carbs became very noticeable in comparison. The stock (with Bub pipes) carbureted engine reached peak horsepower at 6000 rpm. Adding Mikunis moved the peak power rpm up to 7500. The useful power band was raised 1500 rpm by simply bolting on a set of carbs. Peak horsepower went from 42 to 48. Just to see what the Mikunis would do for the XS by themselves, we replaced the Bub pipes with the stock items and made a final run with the Mikunis in place. There was a two-horsepower gain at 4000 through 6000 and a whopping 4.7-horsepower gain at 7500, where we stopped the run. The gain at the top of the engine’s range was expected: the fact that the carbs made more power at low rpm without the Bub pipes simply told us that the jetting was not correct for the Bub pipes.

    The combination of the Bub Enterprises two-into-two pipe and the Branch-Mikuni carb kit raised the power output 26 percent at 4000 rpm and 14 percent at 7500. This is a big improvement and feels like more than the numbers would have you expect. The real surprise was just how nice the immensely broadened power band is to use on the street. We found ourselves spending a lot of time in fifth gear instead of fourth and not bothering to downshift when passing. The Branch/Bub engine is more responsive, faster and-get this- smoother running. Maybe it was the pipes or the phases of the moon, but the project bike vibrated less after we installed the pipes and carbs.

    After building the 750 version, we went back to the Branch dyno. We knew from the way the thing responded and sounded that we were going to see some results worth bragging about. We weren’t disappointed. At 4000 rpm, the Stage II engine (with Bub pipes and Mikunis) made 29.1 horsepower, 45 percent more than stock! At 5000 there was a 27-percent advantage and at 7000 rpm the Stage II engine made 52.1 horsepower for a 24-percent increase over stock. It wasn’t through: at 8000 the Sage II engine reached 54.5 hp for a 30-percent gain over stock. All this even though the engine was running a bit rich. A full 60 horsepower is probably available, and the jetting we finalize will be furnished with the Branch Mikuni kit.

    You have been reading a lot of numbers and, we hope, been impressed by what can be done with the XS. The finished bike’s performance is much more than impressive numbers. The Stage I bike is good-handling, fast, responsive, sounds great, and doesn’t cost much to build. Stage II costs a lot more, but will blow the doors off stock 750 fours in a roll-on, and it will handle well enough to roadrace. The continued sales success of the big Yamaha twin clearly states the case for simple strong, and traditional motorcycles. Since many of the XS650’s purchasers are first-time buyers who don’t want to spend a bundle. Yamaha has cut a couple of corners. For those of us who want the XS for its character and would prefer more quality and performance, there are aftermarket parts to help. We hope that you are more than tempted to take advantage of some of the things we have learned about improving this venerable twin. You will be pleased with the results.

    There are many hop-up parts out there for the XS650; many exhausts, cams, and piston kits. Do what you thing is right, but remember; If you duplicate our Stage I or Stage II engine, you Will  get the same results, no guessing. Enjoy your Excess 650.

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