Hey everyone, new to the klx so bear with me. I recently purchased a bike with a klx 250 engine but it had a 331 bb kit and a Yoshimura TMR-MJN 34 carburetor. The head is shot (camshaft journals got so hot it turned dark red on the metals and valves got slammed by the piston) so I need to either buy a complete engine or complete head and cylinder assembly. Now the questions
1. Can I swap a klx 300 head onto a 250 bottom end with no modifications?
2. Will I need to lap valves and replace valve springs?
3. How do the valve shims work? How can I check the tolerances?
4. Should I continue to stay with a 331 kit or go up to 351? If so do I need to reject the carburetor?
5. Does anyone know how to or where I can go to get this yoshimura TMR-MJN 34 carburetor rebuilt/rejetted?
6. If the top end was toast is there a high chance the bottom end is also shot?
7. Will I need to upgrade anything else like drivetrain or cooling system while using the 331-351 bb kit?
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2. Valves got slammed by the piston and you are asking about lapping them, that's a joke, right?
4. You didn't mention any damage to the cylinder.
5. I did a search on that carb and all responses were in Japanese. I doubt that you'll find anyone familiar with jetting it for the KLX.

Rid on
Brewster

 

Lots of questions with some variables.

#1--Yes
#2--Not necessarily...depends on condition of the 300 head
#3--I'd suggest an internet search of "under bucket shims" and some youtube vids on the topic of design and adjustment
#4--If I already had a good 331 kit, I wouldn't move to a 351. Rejet?...hard to say. Probably have to run the engine in real use first to diagnose.
#5--Contact Yoshimura???
#6--Impossible to diagnose from your info here. Define the cause of the bottom end "toast"
#7--If the drivetrain and cooling system are working as design and within wear parameters they should not require "upgrade"...though you might prefer a different gearing setup and maybe fresh coolant.

One other thing...get a shop manual.

 

2. Sorry for not being clear, the question what meant for the used head assembly I would be purchasing. Would it be wise to lap the valves from a used engine.
4. Cylinder walls still had factory cross hatch but a little bit of vertical scoring. Piston is no good since it was slamming valves, was planning on just using the cylinder as a core exchange for buying a new bn kit.

 

If the cam journals got red hot, that whole head is toast, IMO.
Get a new or ebay one complete, or get a machine shop involved.
You will have to put the cam in the heads, rotate them to what would be TDC, check the clearance, order new shims, remove cam and install shims. You should be able to install on the cylinder then.

You could just get a new piston & rings, hone the cylinder and go.

 

4. Is the power and torque much of a difference from a 331 to a 351?
5. Contacted yoshimura USA many times still waiting for their responses.
6. There are 2 long black Hard plastic pieces that were attached to the bottom end that came up through the head assembly, I think it has to dp with the timing chain and/or timing chain tension. Those pieces are badly broken. I was able to spin the crank by hand somewhat freely. It's hard to tell since there wasn't much oil in the case.
7. The klx 250 engine won't have cooling issues with the stock radiator and a big bore kit?

trying to buy a shop manual soon.

THANKS FOR ALL YOUR HELP EVERYONE.

 

#4--I took from your first question that you already have a good, serviceable 331 kit. If so, it wouldn't be worth what is likely an imperceptible difference between a good 331 kit and a 351 kit. If you don't actually already possess a 331 kit, well, I'd probably go 351.

#5--I think I'd pass on using that carb unless you get some really clear direction from Yosh on how to jet it for the KLX...and the jets to do the job.

#6--Yes, sounds like the cam chain guides. That's not a common failure in such a catastrophic manner. You're going to have to break that engine down to splitting the cases IMO to insure what you're dealing with. The crank may need some professional inspection.

#7--The KLX drivetrain and cooling system are more than adequate to handle any big bore kit we've dealt with here. If abused, the clutch might be weakest link but not to any concerning degree.

Clarification: I'm taking it that the 300 head you have is complete and in good order, and the head that had valve/piston contact is the original 250 head???

 

I have a complete, running engine from an '09 bike that was crashed. Good compression, title in hand to demonstrate that it isn't stolen, crated for shipping. Not sure what year your bike is, but there are some differences between 09-present bikes and earlier models. I don't believe the mechanics changed, but electrical components may have. Others here could clarify. PM me if you are interested.

 

yea I'm trying to find a cmplete engine or a complete head from a 250 or 300. I'll just get a new 331 bb kit just for added insurance.

 

yea I'm trying to find a cmplete engine or a complete head from a 250 or 300. I'll just get a new 331 bb kit just for added insurance.