Dying KX !

[sherrif]

Hi guys, I desperately need some help ! I've just bought a '90 KX 250, and I'm having lots of trouble with it. The main fault is that it takes ages to start, and then it won't idle at all unless I screw the idle adjuster all the way in. If I rev it then, it takes ages for the revs to drop back down, and usually cuts out again. Also, it's chucking out loads of smoke, and it seems to be misfiring at low revs. Above around 1/4 throttle, it seems fine.
I've cleaned the carb and filter, fitted a new plug, but things are no better. It's on a standard needle, I've tried 160 and 165 mains, 45,50 and 60 pilots, and the air screw everywhere, all to no improvement.
I was just cleaning the bike, and noticed a crack in the bottom of the crankcase around one of the bolt lugs. I can't tell how deep it is, but would an air leak here give the symptoms described ? If so, any ideas on an easy fix ?
Sorry for the long post, but I'm pretty new to the two stroke MX thing, just come across from years on the road.

 

[YZ Joust]

Good lord man, don't scare me like that.... The bikes on life support but she'll pull through... From your description there are a number of things that could be wrong or need to be checked.... Check the crack for one thing!! How long have you had the bike? Do you know anything about how the bike was maintained? Was it running when you bought it? I'd say you may have to replace the case... Sounds like alot of probs but I would start w/ the damaged case.... :eek:

 

[sherrif]

I've only had her about two weeks, and don't know anything about her past, but I've just noticed that the gearbox drain plug has been drilled for lockwire.......Gulp ! I'll check out the crack today, any other obvious things to check for ? I suppose if I didn't have bad luck, I wouldn't have any luck at all....

 

[Heilbronner]

Start by doing a compression test, if its low (like below 125 lbs.)pull the top end off and replace the piston, rings etc. Have a good shop check the rod for endplay and see if its within specs. If its got some hours on it replace the rod also.
Remove the carb, and inspect all jets well, sometimes the pilot will be clogged. Look at the reeds for chipping or cracking, that could also give you starting problems. :| We get this scenario at the shop constantly and most times its just minor. If nothing works for you take the m?c in to a good shop and have them do a estimate for you. Hope this helps.

 

[Heilbronner]

Sorry, another thing before I fall asleep. You might have an air leak on the mag side. Again have a good shop check it out. Could be bad crank seals.

 

[kdxtaz]

"Could be bad crank seals"- Heilbronner
That's what I would check for, especially if his other (easier) suggestions fail. I believe those are only replacable by splitting the cases on that bike, but you need the case repaired anyway. Are you losing trans oil when you ride? (not due to leaking) A bad rt. side seal will suck it into the engine causing excessive smoke. (not to mention poor primary compression)

 

[dave186]

I've just noticed that the gearbox drain plug has been drilled for lockwire.......Gulp !

I do believe that it came what way from the factory.

 

[wolfmangk]

Air leaking would result in some of those symptoms. For the rest, check the timing. Braze shut the crack.

 

[sherrif]

Just pulled the engine out. The cylinder plating looks to have worn through in two places,and there is a wear ridge at the top.There is also lift in both the big end and main bearings. To make things even more fun, I managed to break two clutch spring lugs off when I removed the clutch nut ! Can you braze ally, I was thinking of trying one of these new fangled epoxies, 'cos the crack meets the crankcase joint, and I'm a bit concerned about heat distortion. Anyone had any luck with "plastic metal" or similar ?

 

[mxer1973]

Hmmm, sounds like you got yourself a project bike. I did the same thing myself, just not quite as bad.I wouldnt trust the epoxies or jb weld type products as this is a high revving, high vibration type engine and it will tend to fail when you least expect it or want it. Just have the crack heliarced(also know as TIG)< get your cyclinder bored and replated(or replace with a new one), change you bearings(make sure and check the rod and crank also), or just look for another good engine. Well good luck. By the way, the water pump failed about 5 mins after i got mine home and the top end siezed(had to bore, replace piston,ring,bearings,seals,waterpump seals, and couldnt ride my "new bike" 3 weeks, lol

 

[Nevada Sixx]

man, it sounds like your about to go thru what i went thru when i got a my old yz. check my posting..
one thing you want to do is check your water and make sure it's green.
and check your reeds also for chipping. clean out the choke on your carb, and repack the silencer.. when i got my 89 yz for 700 bux, i thought it just needs a few jets make it run well,, i ended up restoring the bike for 2000 more. I'd think about selling that bike and buying a newer one from someone you know who's actually riding the bike and it runs and cranks well.

 

[wolfmangk]

No. Brazing is good because it does not have nearly as much heat as welding, this is one of the principle factors people use to braze. It is not as strong, but it is 1,0000 times as strong as epoxy. You can braze copper to aluminum, or aluminum to steel, or steel to clay. Brazing is very versatile, so yes, you can braze alloys. Since you have a lack of experience in it, I'd take it to someone who knows how to braze.

 

[sherrif]

Just took the engine to my local shop, turns out it needs a replate/liner, piston,con-rod, big end bearing, main bearings and seals, oh, and the case has to be welded... So much for buying a "cheap" bike ! Gonna cost me £460 to repair...It was meant to be so much fun.......