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1991 ktm 250mx rebuild down on power & revs

9K views 4 replies 3 participants last post by  racermon 
#1 ·
I was given an older 1991 250mx 2 stroke basket case with a bad cylinder/powervalve/etc. I got a new powervalve and had millenium redo the cylinder with a wiseco piston, etc and new powervalve flapper. I ordered the special KTM gauge for setting the x dimension and set it up very (too?) conservatively with the slightest bit of daylight possible visible between the piston crown and setting gauge- I used a 1mm cylinder base gasket vs the .50 that came with the wiseco kit, which when set up the piston lifted the x dimension setting gauge off of the cylinder (I got all the thicknesses of gaskets available- even a .75mm stack was having the piston lifting the gauge a bit). Squish measurement with rosin core solder on the 1mm cyl base gasket is .97mm.
Not being too familiar with the power valve setup for this, I set the gear tooth engagement as shown in the photo, which may be wrong. Note the thin scribed line closest to the tiny dimple on the flapper rack gear is not a factory mark, but the fatter ones appear to be. When that non factory mark is set up as shown the barrel valve is open fully at the limit of flapper travel. I think that the resting position of the flapper is correct, because I was told that the hole drilled in the flapper rack is to align with the hole in the bracket below it, but I may have the engagement with the mating barrel valve screwed up- but when I assembled the thing the barrel valve ports were open all the way when the flapper was at the limit of its travel- is that correct, or is the barrel valve port supposed to be open at a point that is not at the limit of flapper travel? As is it set up now in the photo the bike starts super easy and putts around great but is not making near the power it should (will not wheelie) but runs cleanly. Also seems to rev about half as high as I would expect it to. Keep in mind that my last dirt bike was a new 1983 RM250 that I last rode over 20yrs ago- I figured I'd do this project to see if I still enjoy riding & then get a newer bike later on. As it stands this makes nowhere near the power of the old RM...
The bike also has an aftermarket pipe. The idle/choke plunger on the Keihin PJ carb was broken when I got it, I replaced it with a new plunger that when turned does not seen to have much effect on the idle speed. I wanted to set it up conservatively- the old barrel had evidence of seizure and I'll need to go thru the jetting but it seems like either I have the x dimension way wrong or the powervalve gear engagement screwed up for the bike to be this weak.
Never having set one of these engines up I'd appreciate any advice.
thanks
tom
 

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#2 ·
anybody home?

I repositioned the power valve drum valves to where the marks look like they should be, now I have better low rpm torque but unchanged power at higher RPM, still runs cleanly but not making any power at higher RPM.

Any ideas?
 
#3 ·
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