Not sure if i've covered this before [can't find it] but here it is again. Total cockup motor i had in had roadrace power & revs, hopeless with wide spaced box. So due to an odd conrod & shortened cylinder i thought leave it & build another motor as had a crank with a 214 rod in [my] usable condition lol. One way of getting rid of excess parts too--make a motor. Anyhoo, had a genuine RT2MX cylinder & modified std head so went for that--oh & had a 2X5 82mm piston in. Found a new piston kit in my bits so is "less loose". Cylinder must've been dropped as piston is tight at the bottom but piston stops just short so left well alone. Trans is spaced hard [think i've written this somewhere before] trans i have running against circlips double washered on the other side & all that sort of thing. Many MANY boxes of junk parts i sorted through. Only needed to use the plunger & spring for the gear cam, & clutch long pushrod of the other motor. So, it started first kick--oh & the mag off the other which looks like it shouldn't run but doesn't falter. Started first kick & sounded WILD. So off to the farm but nowhere to let it loose [pumpkins again]. Clutch slips [no its not all wheel spin]. But funny thing is, same problem has shown up but not as bad. To much top end & hollow in the middle. So the pipe is too short & too fat RM250 pipe from years ago. So i'll try adding 100mm into it somewhere & might try building another pipe to specs i once had & did build to those specs with good results 30 years ago. Same bike [half of it], sold it then got frame minus PE arm back then this cylinder 20 years later. So this time its on short legs with DT250 forks & shocks with short travel despite looking long. Arc welded short gas Girlings into PE bodies. Don't think i'd be brave enough to try that again with 200psi of nitrogen & oil immersion waiting to 'get me'. Terrible short video coming. Should've turned the phone camera around to make it wide [learning]. Some day i'll do a better one when i've sorted the pipe & clutch. Was half pie doing this to sell it but damn it the thing still gets me pumping. Frame once had a triumph unit motor in, so been around. Wonder if my tenant would let me rip between the pumpkins. Be many rows of standing 1/4 drags in 20 acres lol.

writing doubled up, think i've sorted it. first pic showed the change arm roller that was hiding in plain site on the floor & made another before i saw it there haha thinking i didn't have one. Just used $2 shop instant gasket in a tube stuff. Used it on the clutch cover too, no gasket, same on inlet reed & manifold.
Also a pic of how it used to be, which you've seen before & was a bit overdone with RM 125 arm longer than the PE one i had in years ago when i sold it then eventually got back minus that arm. Cross bracket is still across under the seat as i didn't have heavy enough damped long rear shocks so i add two short gas girling's minus spring vertically under the seat & onto the S/arm cross piece--alloy PE arm & worked a treat.
Also there's one pic of the jet setter from something, maybe my DT1 & tried an extra Oring [was long ago] which worked good as an easy option when i couldn't find a small one, Sat in the taper lead in under tension just right. Kickstart gear has half the width of the teeth chewed off so had to set it over as hard as pos plus run the idler gear with washers behind & against the circlip to get the half with teeth covered. Damage for about half the circle if teeth but they revolve flatout constantly so a bit worried.
 

 

How many engines in yer collection?
Sneezles61

Might have enough bits for one more. But each time the parts get get worse with worn selectors & rounded off dogs plus damaged high point of the selector drum tracks mainly 3rd & 5th gear. If i ever try again i'll have to somehow block of 5th so it stops as a 4 speed. Nearly got caught out many years ago using a "68 selector drum which has the wrap around selector which pin engages the track from over the back. So was using 'that drum' with later selector on its own shaft & on my practice assembly checking engagement i found it went 1-2-3-5-4--bugger. As "Mr Know It All" would have said--RT325 you really should have known lol. Tricks to catch us out.

The mx cylinder porting is a bit overdone. You could make better power with a ported RT2 cylinder. I would raise the RT2 port about halfway to the MX spec and widen it to 68% of the bore diameter. It will make more low-mid and bring the power on sooner.

You can always raise that port up higher but once you go too far it's done.