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Best xr 75 motor set up?

18K views 26 replies 6 participants last post by  vintagemx 
#1 ·
What's the best option for the xr75 motor? Was thinking of just doing a powroll stroker bore and cam. Can't really find a whole lot of other option? Looking to build a vintage mx racer for my dad and it needs to come apart to weld the cases.
 
#8 ·
vintagemx said:
What year engine? There's not a lot of performance parts available for the '73 - '76 4-speed engines. The '77 - '78 5-speed 75 and '79 to current XR/CRF 80 motors have a few performance options. Plenty of performance stuff for the '81 to current XR100 engine. All will fit in the XR75 frame.
He would like to use what he has. A stroke and bore will be more than enough for what we are doing with it
 
#10 ·
dr-z said:
He would like to use what he has. A stroke and bore will be more than enough for what we are doing with it
The engine out of my '74 had weld repairs to the case where the foot peg bracket had broken off. I bought the bike like that and after a while an oil leak developed from the weld repair. Case halves, complete cases and even complete bottom ends for the '77 & up XR75/80 are cheap and plentiful on e-bay.

Powroll has the most experience with these engines, but there are a few other builders out there. There's a couple different options for a cam along with Kibblewhite valves and springs. Porting the head, larger carb, pod filter and reverse cone exhaust improve performance as well.
 
#11 ·
im not looking for a rocket by any means, i think doing the crank and piston, exhaust and a pod filter will put us where we want to be. i will keep an eye out for the cases in the mean time though. does anyone know what the comp ratio will be with the crank? i know the piston is 9.5:1 but the stroker is +9.5mm if i recall
 
#16 · (Edited)
57mm slug 53.1mm stroke oversized valves from the factory 24/20.5 to 28.5/23 and some port work possibly cam regrind and needle bearing it, maybe best option?

how intake came from factory
not finished but its pretty close
how exhaust came from factory
not finished and not sure what to do w/ it cause short turn looks like hell in a handbasket
side view of finsihed combustion chamber
another view of combustion
comparison to xr 80 head
comparison to another xr 75 head
 
#20 ·
vintagemx said:
I've been told the entire CR80 front end up to '93 should bolt up. '86 to '93 gives you a disc brake. Use tapered steering head bearings to get rid of the 42 "Jesus" balls.

The problem is the shock length. Max length you can use is 14.25" before the swingarm hits the frame.
Hmm would this be clamps and all? I would like the disk brake and 17" wheel. I don't have a issue with pull the forks up in the clamps a little or shortenting the forks if need be. What shocks work good with a stock swinger?
 
#22 · (Edited)
82-86 CR80R front end will slide in, 87 and up will mean replacing the steering stem.

It's gonna be choppered hard, so you're going to want to stretch out the rear big time, XL 125/185 swinger and shocks will get it done.

An 85 and up XR100R engine can be made decent for less than $200 with a TB bore kit and a bit of jetting. If you want 10HP plus, it's quickly one of the most expensive mini bike engine families. Powroll still builds cranks for any of the uprights (75/80/100), though they tend to get pretty rough on kicker gears beyond 132cc (just ask Chad...). Yoshimura has been flogging alot of their older stuff and you may find a deal on their website (12.5:1 comp ratio on their bbk, vs. BBR's 10.25:1).
 
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