1979 KX 250

[splime738]

Cheers for the info, will be checking crank case pressure when I get barrel back on, will pull it apart if any leaks on seals

[Davey Crocket]

Make sure the piston to bore clearance is not to tight [maybe to hard now with the damage]......we just had a barrel re nickasiled 3 times because oldmate gave it to much clearance and the bore was tapered.....it took a little while to make the guy understand, hence we wont be using him again. Also are you using a genuine Kawi piston [NOS] or a modern one [Wiseco/Wossner].

[oldyzman]

I usually like wossner, but in my A4 i had more luck with wiseco...

[holeshot buddy]

piston to bore too tight and piston expansion will seize on exhaust port !

[splime738]

I have been using Wossner pistons for a while now in all my other bikes with no problem. I have used this A5 for a while now, it never seems to pull right and then seizes within a couple of meetings, I bought all new old stock electrics in case it was the ignition gone a bit. I have even had an electrex world ignition (pvl copy) on it for a while. When the bike runs it feels flat, no bottom or midrange and almost like the clutch is slipping, which it isn't that's been changed anyway. Several people told me I need to go up on the jetting due to the newer fuel being poorer quality hence why I was asking about what jetting people are using. I have a new VM 38 on it set up with original spec jetting, until last week when I tried to richen it up still the same. Poor power then locks up and on inspection, front exhaust port damage. I have another barrel, but same results when that was on. I use a company called Langcourts for re plating and there work is good.

[JohnnyO]

Are you using a 190 round main jet?
What is the piston to cylinder clearance, has it been replated with not enough tolerance?
Has the head been machined for more compression?

[splime738]

Yes a 190 round style main jet, piston clearance should be ok, the same company have done my 75 cylinders no problem, same bore size. No signs of machining on the head.

[JohnnyO]

There's obviously something not right, no use assuming everything is ok. You need to measure piston to bore clearance, compression ratio or compare cylinder heads and cylinders with other std ones in case head or top of cylinder has been machined, maybe try another ignition also.
Check for air leaks, replace the LH crank seal, do some plug chops to check jetting. Plenty of things to check..

[Lozza]

Problem will be the Wossner. QC has gone to shit recently

[oldyzman]

Funny you should say that Lozza, i have a old wossner in my cr race bike for 2 seasons now, but in my kx had no luck with recent wossners...

[Lozza]

Whats even more odd is Wiseco seem to have got better.  Had every type of fail with them as well, mysterious sinking domes, ring  pegs pushed in to far, 'soft' alloy etc etc. Rings are very good on the other hand