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rm250 kart engine
hi all , i have an rm250 engine on my kart , and it keeps seizing , i just had it fully rebuilt and i ran it in , then it only done a warm up lap and then one fast lap , then nipped up in the bore again . it seized at ruapuna in january and tore the nickel out of the bore , i had it sent away and they replaced the sleevewith a cast one . i put a 67mm piston in it and it seized not long after . do these cast bores expand alot more (when hot) than the nickel bores ? the exhaust temp only got to 1094 degrees and water temp was only 46 degrees . its not running too lean , as it seizes right round the piston . any help on amount of clearances required would be a great help . thanks jamie
Replies to This Discussion
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Permalink Reply by Ross Tait on -
Did you run it in both times ?? Also oil fuel / ratio is differant between Nickel and cast bores. Also some oils cause issues with some fuels (do not stay mixed and do not mix properly !) Was the clearance measured correctly and with the right gear ??
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Permalink Reply by jamie ruddle on -
hi , yes ran in both times , i sent the new piston away both time with the barrel , the sleeve was fitted by boss engineering up north first and then cleaned up by nigel sheppard the second time . i was running shell 95 octane and elf fully synthetic oil , fresh each time and a 25.1 ratio . should i be running avgas ? and a different brand of oil ? i have a 205 jet in the carby , so shouldnt be running lean .
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Permalink Reply by Ross Tait on
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Fuel / oil ratio was that for a Nickel bore ? Cast needs more oil than Nickel ! . But piston clearance could be a question and also the type of piston as some expand more than others .From the full seize Av would not help I would think say !!
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Permalink Reply by jamie ruddle on
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yes that was the ratio i used with the nickel bore before it seized and the i also ran that fuel oil ratio in the cast bore . it wasnt a real bad seize this time , so the ring will free upo and piston and bore would clean up to get it running again , do you think i should put more oil in the fuel and try again ? im not sure what sort of piston it had when it seized the first time with the nickel bore , but the last two were wiseco pistons in the cast bore ,
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Permalink Reply by Ross Tait on
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yeah I would use more oil for the cast bore ! yamaha kt cast bore is around 16/18 ratio and that is a smaller piston. So not sure on the bigger size ! Also some bores are not perfectly round and are tapered and that can cause issues. I would try more oil !
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Permalink Reply by James Jackson on -
Hi
is it seizeing at low throttle % you could be leaning out on the low. And 95 ocy on a 250, is it stock?
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Permalink Reply by jamie ruddle on
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hi , no it seized the first time on full throttle at ruapuna , but thats when it had a split boot between the carb and barrel , then i got it fixed ( and replaced the boot) then it seized at about half throttle , then i got it fixed again , then it seized at full throttle . and its been ported and has a custom pipe and a boysen reed block . when i got the kart i was told to run it on 95-98 octane .
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Permalink Reply by chris foster on -
i'd put the barrel in the same rubbish bin i'm putting my barrel with a cast liner in , after it sized at street races also.
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Permalink Reply by Darryn WAUGH on -
Hi Jamie ..... its hard to entirely answer that question without really being there when it happens. I can tell you from experience in the past that i have resleeved a cracked nikasil KX250 bore with a cast iron liner and that i had no problems with it. However it did have more clearence though but the motor lost no real power. 1094 d/f EGT is not hot .... but what sort of temp range you can run in depends a few things, particularly your timing. As far as clearence goes i can only tell you that until recently i used to run Rotax 257's and new pistons had about 4 and a half thou clearence. I also never used to load up the motor until the water temp was 100d/f and ran ELF HTX976 at a 30/1 ratio. My guess would be that it was too tight? Hope that helps. Also im not sure who told you to run on 95 - 98 octane ...... leaded race gas is the way to go!!!!!!!
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