Thumper cam5/9/2023 The rear shocks had five spring preload adjustments, but were not the high-tech units available at the time on other Hondas, leading some period testers to suggest that the rear shocks were not as good as they could have been. Dual exhaust ports allowed for twin pipes to clear the central downtube and provided a nice vintage touch, twin-port exhaust being the rage in the 1930s. The single overhead cam 4-valve engine was slightly under-square, with a bore and stroke of 89mm x 90mm. Even with all these niceties, the Ascot still weighed about the same as the SR500, coming in at 374 pounds with a half tank of fuel. Named after the now gone but not forgotten Ascot flat track near Los Angeles, California, the Ascot, besides having a vibration-quelling counter balancer and a reliable electric start, featured cast aluminum wheels, dual-piston disc brakes front and back, a 35mm constant-velocity carburetor with an accelerator pump, and air-assisted front forks. At the end of 1981, Yamaha decided not to continue importing SR500s into the U.S., while at the same time Honda announced the FT500 Ascot. However, by 1982, most American riders expected an electric start, and the commuters who were the 500’s natural market just wanted to get to work in the morning without having to learn a complex starting ritual. The Yamaha design team was able to dampen the SR500’s vibration and used a sight glass for lining up top-dead-center combined with a decompression lever to ease the kickstart-only ignition. The downside of thumpers was always hard starting and vibration, which on some bikes was so bad it would rattle the fillings in your teeth. ![]() ![]() Honda also noticed that the SR500, although a best-seller in many countries, was not moving quickly off dealership floors in the U.S., due to the lack of two components: a counter balancer and an electric starter. A popular bike, the XL500 was used by many riders as a sort of urban assault vehicle, and Honda noticed. Possibly inspired by the success of the Yamaha single, Honda introduced the XR500 as an off-roader in 1979, and then used a similar engine to power the dual sport XL500. Available only in Japan for many years, it recently returned to Yamaha’s American lineup. A hit with dirt enthusiasts and desert racers, its 500cc single-cylinder engine was repurposed to power the SR500 roadster, which is still being manufactured today in 400cc form. Yamaha decided to try its hand at designing a thumper, introducing the XT500 in 1975. Some thumpers were slow and basic, but others - the Norton Manx, BSA Gold Star and Velocette Thruxton, for example - were some of the fastest bikes ever made.Īs time went on, street bikes were increasingly powered by smoother twin, triple and 4-cylinder engines, but the virtues of the single were still evident offroad. A thumper was a 4-stroke single, generally displacing between 350cc to 500cc, and it was known for its great sound, stump-pulling torque and simple maintenance. Once upon a time, there was a breed of bike known as a Thumper. Oh yea, if you use them, the 360 heads you have should easily support the power goal you have set with just a quick clean up and a good valve job.Suspension: Air-adjustable telescopic forks front, dual shocks w/adjustable preload rearīrakes: Single 11.6in (295mm) disc front, single 10.75in (273mm) disc rearįrame/wheelbase: Single downtube w/engine as stressed member/56.5in (1,435mm)įuel capacity/MPG: 3.4gal (13ltr)/45-55mpg (avg.) There's lots of opinions on builds like this, maybe some others will chime in also. 273 laid out a pretty good plan there and if you went Magnum later you could reuse some of the parts required on the new build. i would probably use the 650 on the iron manifold and the Eddy on the square bore manifold. ![]() Now do you cut and use the iron manifold and make less power or do you buy a Eddy RPM, have to cut that also, and make more power but render it useless on another set of heads because of the milling? As far as carb. If you mill the 360 heads that much, which is pushin' it imo, your going to have to mill the intake a good amount to fit. But, you need to get the compression up to increase the power and drivability with that cam. If you are gonna try and move to a Magnum engine at some part, your probably will not want to put too much into that shortblock. There's alot of smaller details here that make this difficult.
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