Insufficient compression + low coolant = ??

[KaTooMer]

The bike: 1999 KTM 300EXC

The symptoms: Loss of compression, coolant level low.

The story: My last ride was a race, and could tell that compression wasn't what it used to be. Ran O.K., noticed the smell of antifreeze after dropping the bike a couple times, finished my race, but near the end the engine didn't feel right (blamed in on a very dusty race and a dirty air filter). In prepping the bike for a pleasure ride yesterday, I had to add a lot of coolant. Engine wouldn't start yesterday, so I pulled off the cylinder head today. Guess that flaky stuff on the piston is coolant residue. Kind of cool - sort of peeled right off and the piston was still shiny underneath.

Cylinder head has some carbon residue on the front side where the leak must have been. However, the O-rings appeared to be fine. No melting, no nicks or breaks (Cometic o-rings). On a flat surface, cylinder head shows no signs of warping.

Would seem to be a case of a bad O-ring, but before I put it back together, does anyone have any other ideas, things to check out, take apart, etc?

Thanks.

 

[Stew312]

Sounds like a new head gasket will fix your problem.. With a blown gasket, compression can leak out into your water jacket, forcing anti-freeze out of the overflow and you'll notice a loss in compression...

 

[Rockey5000]

Was the coolant bubblely(is that even a word)?

 

[David Trustrum]

That’s weird! Usually if the pressure gets to the o-ring it will terminate it. If you have a path I would suspect that it is slightly warped. A flat surface (like your missus favourite mirror) & some very fine sand paper taped to is with a wipe of oil should dirty up the entire surface, if any areas are not touching then the centre may have pulled up. If so it may be a good idea to get the head professionally skimmed.

Also check the head can seat fully & any dowels are not restricting this or if the problem was at the rear I’d suggest a head engine mount if so equipped.

Replace o-ring as a matter of course, they can ovalise.

 

[DEANSFASTWAY]

KATOOMER : Ive also seen the late model KTMs have problems with the base gaskets slipping in the front at the water jackets . Like Mr TRUSTRUM says you should have the head skimmed but youll have to do it on a lathe as the KTM cyl head has a shoulder around the outside perimeter which sould be turned down as much as the face . Theyre like the only current bikesI have ever seen like that . You may want to check the cyl bolt threads to make sure they are not pulling out . A client of mine had this problem when the motor mount bolts loosened up and the cyl head stayed tight . Actually he just did a Top End on the bike and I guess the top mount was trhe only mount that was tight . The motor was literally moving around in the frame . The head stay was the only thing clamping it . Use new orings . How did you check the cyl head ? Has someone already turned it down? Was that on the 02 ? Maybe they changed the head design. GOOD LUCK

 

[KaTooMer]

Good info, thanks everyone. One more question on the cylinder head: what series of events would lead to the head getting out of shape? The common answer would seem to be a severe overheating situtation, the kind that causes piston meltdown and cylinder wall damage (of which I've never had with this bike).

Thanks.

 

[KaTooMer]

Here's a lesson on how multiple problems can lead you in the wrong direction. When I tore down the engine, I saw that the right-side radiator had been pushed back into the fat part of the pipe, enough that one of the metal hose connectors (bottom of radiator) was kinked. So there's where the coolant was leaking. The lack of compression was from rings that were worn just a little bit past their tolerances - gap was about .09" (and up to that point, the engine actually ran, well enough place in the top 20 of a Missouri hare scramble). The Wiseco top end only lasted about 40 hours...must have ridden it harder this year. Anyway, a new top end got me back in business.