Posts
16
Joined
1/8/2021
Location
GB
Edited Date/Time
2/24/2021 5:26am
hi i want to build another race bike grasstrack same as flat track i use to run a 74 KX250 with great sucsess but want a change and fancy a cr250 or yz 250 both 1974 or what is the mx250 like all im after is the fastest one HP wise i will be modifying it to the max but want the best to start off with
many thanks for any veiws or other engine idears
many thanks for any veiws or other engine idears
The Shop
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Another bike I thought of was Eyevind Boyesen's 250 Husqvarna project in1971 IIRC, which pioneered the reed valve + bypass passages to the transfers that he patented. Supposedly a mid-top engine w/ 35hp at the rear wheel. He insisted on using a button flywheel ignition on it and it sounds like it was not always fun to ride; Husqvarna passed on it and to me that's a big what-if of 1970s MX.
Some degenerate mucking up the first one . . .
The start long stroke watercooled motor will need to be changed to a short stroke air cooled one a lot of machining
This is the frame its going into
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Pit Row
Head Time
Thank you for taking the time to post this.
@Shredder, IIRC a magazine added just a Skyway pipe to a stock '74 or '75 Can-Am 250 and it made at least 35, maybe nearly 37 rwhp on a dyno where other stock 250s were still trying to reach 30, without turning peaky, although it was a lot to handle. Skyway was making some good pipes then, like for the Elsinore and YZ 125s, but that was like factory-level power with just a pipe and without it they still made more than 30. There's a magazine test of the 1975 that raves on the power but says they asked Can-Am why it didn't have the 200mm travel straight-leg Betor fork, and the answer was that all production of those was allotted to Spanish marques. I guess their production chassis didn't catch up to the engine until 1977, and by then other brands were closing the power gap significantly.
I don't know if the white-tank 250 engines were the absolute best raw material those years, but clearly they belong on the short list if the rules emphasize period mods. Speaking of which, did Husky screw Boyesen over by borrowing the non-patented aspects of his prototype for the early reed-valve Husky models?
The generation prior had CZ and Husqvarna showing the way with again, advanced cylinder ports, this was why the 250's of both brands were really the only logical choice in their time.
FWIW, in the post 1975 period it was again Husqvarna showing the way with it's Boyesen designed reed valve models challenging the KTM and Rotax, Puch was also producing the twin inlet 250 that proved to be the engine to measure others by HP wise but never were available in quantity to ordinary people so they really don't count.
One last point, while these engines mentioned above were no doubt really quick in their time we must not overlook "usable power" , those user friendly traits were important too. While the big hp is impressive there needs to be an element of "usable" power to the ground, while Yamaha, Suzuki and Maico were far from the highest HP they did in fact produce the right kind of power to make them competitive, keep in mind all the power in the world is useless unless it can be put to the ground.,The HP race sort of sat in limbo for a while until the suspension made advancements, once the big players had worthy rolling units the HP wars took off for real with the advent of water cooling and variable ports.
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