fresh motor blew top end

[gargamel777]

I have a 1996 kx 250. i blew out the connecting rod bearing on it so i replaced the crank, bearings, connecting rod and piston and rings. while breaking in the motor, took it down the road shifting first through 5th, taking it really eazy on the motor and blew the top end. the rings welded themselves to th piston right at the intake port. It,s a hot rods crank and wiseco piston. any ideas?

 

[MXer666]

did you let it go through heat cycles? for the first 5 hours your not saposed to go more than 1/3rd throttle i think

 

[magneto]

Did you check the end gap on the new rings?

 

[gargamel777]

yeah i checked the ring gap and did not go hard on the throttle- was just making sure it ran through all the gears and warmin it to change the bottom end oil again. Ive heard kx have a really thin nicasil coating and i did hone my cyclinder- maybe i need to get it resleeved?

 

[hot125mod]

isnt there a lot more load on a motor in the higher gears. Stressing it like that right away even though you went slow could have been the problem.

 

[dirt bike dave]

Did you warm it up thoroughly before jumping on the road?

Problem on the intake side are also known as a 'cold seizure'. If the motor is cool, Not as much fuel is vaporizing as it enters the engine. The liquid gas can wash the lubricating oil off the intake side of the piston and cylinder. Might be more of a problem on a new motor with tight clearances.

 

[76GMC1500]

did you let it go through heat cycles? for the first 5 hours your not saposed to go more than 1/3rd throttle i think

Seriously, where did you get that number? Honda says you're supposed to replace the piston at 7.5 hours so I highly doubt a break-in takes 5 hours. I give it 1-2 15 minute cycles and you're good to go.

An intake side seizure sounds kind of like a lack of lubrication.

 

[keefbikes]

Realy, 7.5 hours. That seems like alot of work for a short time of riding. You would spend more time working on the engine than you would riding. Is that the case with all bikes?

 

[rmc_olderthandirt]

There are a limited number of possible causes, so let's see which ones can be eliminated.

The rings will sieze like that because they got hot. So why did it get hot?

1) No cooling. Is this air cooled or water cooled? If it is water cooled, check to see if something was preventing water from circulating. Perhaps a gasket was put on wrong which blocked a water passage or the water pump wasn't working at all.

2) Excessive friction: You said you checked the ring end gap. Did you check the clearance of the piston to the cylinder? Was the piston a tight fit into the cylinder?

3) Lean mixture: If you had an air leak in the intake or crankcase it would result in a lean mixture, which would cause overheating.

4) If the sieze had been on the exhaust side instead of the intake side I would suspect that the piston didn't have a necessary oil hole for the bridge. I am not familiar with your bike at all, does the intake have a bridge (a vertical bar dividing the port)?


Just had another thought: The piston has a small "tab" in the ring groove where the end gap of the ring goes. If you are not careful you can have the ring positioned wrong and then that tab holds the ring out farther than it is supposed to go. If you managed to compress the ring and force it into the cylinder then the ring would be way too tight. I believe that the "tab" is on the intake side, or at least away from the exhaust port.


Rod

 

[MXer666]

Seriously, where did you get that number? Honda says you're supposed to replace the piston at 7.5 hours so I highly doubt a break-in takes 5 hours. I give it 1-2 15 minute cycles and you're good to go.

ok, who here actually replaces a piston every 7.5 hours like Honda 'recommends'?? That is if your doing hard racing its acceptable. Here is my clymer manual word for word, it says: "For the first 5-10 hours of operation no more than 1-3rd throttle should be used and speed should be varied as much as possible....." I dont know what kind of break in procedures you do but a few heat cycles is not enough.

 

[crkid33]

5-10 hours 1/3 throttle?!?! i usually go easy threw a tank of gas then after that i ride how i ride.5-10 hours seems a bit way to long

 

[MXer666]

hey im just quoting my clymer manual.

 

[crkid33]

oo yea im not saying i dont believe you, it was just wayy different from what i do.

 

[Rich Rohrich]

" I dont know what kind of break in procedures you do but a few heat cycles is not enough.

On late model plateau honed, nickel plated cylinders a break in using a few short heat cycles is more than enough. The seating of the rings on a plateau hone finish is pretty much immediate.

There is a reason smart riders buy the factory service manual and use Clymer books for kindling to start camp fires. ;)

 

[gargamel777]

could it possibly be a jetting problem? my bike was raced on an ice oval this winter so the main jet has been changed. I put a 167 main jet in it but am not if this is correct. Also running a pro-circuit pipe. Any idea where i could find the actual jet size i should be using? or maybe slide the needle up or down?

 

[magneto]

On late model plateau honed, nickel plated cylinders a break in using a few short heat cycles is more than enough. The seating of the rings on a plateau hone finish is pretty much immediate.

Do I still have a plateau honed cylinder surface when I run a ball hone (240 grit?) through a modern plated cylinder?

 

[griffbones]

Sorry post error.