Funny someone should mention this, as i'm getting ready to start building one, well not really a hybrid but a project dual shock EVO husky 4 stroke MXer.
See i have a fairly complete 1986 Husky 510 (air cooled 4 stroke), that i bought as a junker, but it's single shocker & a 1981 250CR rolling chassis that had a toasted motor with dual shocks, both had for a $100
Here's what i found out/ know
1) Yes i know Husky made a twin shock 4 stroke in 83-85, but they
had problems with overheating & so-so magnetos. From what i've
read these were fixed in the 86 model, but by that time Husky had
already pretty much lost out on the MX war & worked mostly on
staying out front in the woods/enduro front , so the
chassis/suspension wasn't up to MXing.
2) The 83-90's? Husky 4 strokes used what i think is sort of a core of
the 2 stroke bottom end, that's factory modified, for the oil reed
valve & cam chain. Even the cylinder is a sort of cut down version
of a big bore 2 stroke, with a slot for the cam chain & of course
modified to except a 4 stroke head.
3) I put my spare 82 430CR bottom end, into the 86 510 frame & all
the motor mounts bolted up perfectly,the only difference is the 86
uses a larger diameter rear motor mount bolt , because it's a single
shocker?
SO i know the 86 motor will fit a 1981 thru 85 twin shocker frame,
with no mod's.
4) Also own a 91 350WXE (4 stroke) & the motors bottom end looks
identical to the 510, so a 87-90's water cooled motor should bolt in
as well, but you'd have to make radiator mounts at the very least.
What i'm planning
I'm sort of looking to build a Husky version of the Yamaha HL 500, which by the way used a Husky frame when they built the prototype.
Already got a spare 78 250CR tank, the 81 rolling chassis, but with Ohlins 16" reservoir shocks instead of the piggy back ones (for looks) & of course the 86 4 stroke motor. I haven't worked out the exhaust mounting & air box yet, but i'll come up with something. I picked the 81-82 chassis as it has by far the best suspension of all the Husky made, long shocks & long travel 40mm forks.
My biggest thing is not to destroy anything so i can return it to stock in the future

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Here sort of semi photo shop version i made of what i'm planning, but with the 81-83 style tank instead of the 78 type.
I'll take plenty of pictures as i go, and i'll post them here

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Later
John