What about the best pre75 engine by weight and HP

[EML]

As a follow on to the "best engine ever fitted to a dirt bike" can I have some votes for best engine fitted to a dirt or road bike built before the end of 1974.
It can be 2 or 4 stroke (or anything else) but must be judged by power to weight and, just to add spice, it must be readily available and easy to service/maintain.
I leave it in your hands gentlemen.

[cyclegod]

For me it HAS to be the 73-75 Yamaha RD350, torquey with plenty of power, TZ bits fitted on no probs and you could get serious power with a bit of tuning and pipe work.

[Davey Crocket]

You can get a 440 stoker crank for them.....Vaughn wouldn't be able to hang on.

[Lozza]

Kawasaki 350 Big Horn, needs a rod/piston swap, good enough to beat a TZ500.

[jimg1au]

lightest and most power
jawa 500 or jap 500 speedway motor
no question end of story

[firko]

Are talking motocross road racing or speedway?
If we're restricting it to motocross it's be hard to go past the 50+hp of the first series 501 Maico. Offering up the 350 Bighorn with a " rod/piston swap" is a tad out there. If that's the criteria I pick the Kawasaki Z900 fitted with a 1450 stroker kit and 3 stage nitrous oxide kit ........lets keep it real kids.

[Davey Crocket]

The homo wants to put it into a pre75 trolley.....Hey Mark, I wonder if anyone has put a TZ700 motor in one?....they came out in74.

[Lozza]

Are talking motocross road racing or speedway?
If we're restricting it to motocross it's be hard to go past the 50+hp of the first series 501 Maico. Offering up the 350 Bighorn with a " rod/piston swap" is a tad out there. If that's the criteria I pick the Kawasaki Z900 fitted with a 1450 stroker kit and 3 stage nitrous oxide kit ........lets keep it real kids.

Real enough? Kiwi Big Horn just dyno'd at 43hp with no real work on it.
https://www.youtube.com/watch?v=xHJD0yOXPZ4

[Husky500evo]

Are talking motocross road racing or speedway?
If we're restricting it to motocross it's be hard to go past the 50+hp of the first series 501 Maico. Offering up the 350 Bighorn with a " rod/piston swap" is a tad out there. If that's the criteria I pick the Kawasaki Z900 fitted with a 1450 stroker kit and 3 stage nitrous oxide kit ........lets keep it real kids.

Real enough? Kiwi Big Horn just dyno'd at 43hp with no real work on it.
https://www.youtube.com/watch?v=xHJD0yOXPZ4

Never had a go on a Kawasaki Bighorn. But they had want to be good in a motocross application , to be better than a rotary disc valve Rotax motor from a Can Am TNT or MX1  .

[EML]

John Elliot put an RG500 in one but it spent all of it's energy spinning it's wheels on one spot.
So lets keep 'em coming....with some real power to weight comparos.

[Tim754]

What you think I am going to suggest

Well lets us see .
 Power to weight? stock 67HP show room floor in a unit type dry sump set up,  not bad for a  4 cylinder 4 stroke released in 1968. 
Only two valves per cylinder, nut type valve adjust not poxy shims, points ignition (or oodles of aftermarket electronic conversions that can be fitted easily by a  dyslectic jellyfish)  more than a    couple     sold all round the world, so parts new or used not hard to find. Stock clutch and gearbox that rarely gave trouble.(When was the last time   if ever    you heard someone whinge about a CB754 box heh! (Yes I know I run back cut polished strengthened cogs and a Barnett full race heavy clutch but if they are sitting on your parts pile !!!) Has four easy to adjust round slide carburetors. (Remember them ) Solid kick start mechanism that does not shear off when a hundred and twelve year old gently rests a soon to be amputated foot on it in anger. Does not rip he other leg off with rocket launch kick back either! (Or maybe just press the standard electric start button to fire it up ) enough for now  Tim754

PS Which bike has won the most "Bike of the 20th century awards"       

[firko]

Never had a go on a Kawasaki Bighorn

I have and they're a ploddy old trail bike, no better or worse than an RT1 or a TS400. Lozza's right though in that they are a sleeping giant that can be made to really hoot with a bit of work. A friend of mine fitted one to a TZ350 roller and it kicked arse. BUT, just about any motor can be made to produce performance they were never designed for. I assumed this discussion was about stock motors, not "the best pre75 engine by weight and HP once it's hotted up" or " the best dog engine that can be transformed into a GP winner".  

[EML]

So far I'm liking the Jap and the Yam twin....light and fast.
The Jap will need and AMC or similar g/box though and I have no dimensions of the RD350, it should fit ok.
Any other options??

[EML]

Bump,
Com'on guys
What about Maico 501 or 440? What do they weigh and what HP?

[Slider29]

I'm with Jimbo on this. A longstroke 2v Jawa 500 or a JAP 500. Standard they are over 50 bhp.
The tricked up ones racing Longtrack on the Continent were more like 70/75 bhp