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Kx85 eats cranks

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My son has a 2014 we bought from Garrett marchbanks and shortly after we bought it would miss at high rpm. The bike has PC head cylinder and carb it was made to run vp mrx-02 and we got a different head from PC to run vp c-12 so we replaced reeds,cleaned carb,new top end,changed stator,Cdi,coil and had same results I had my buddy put a new hot rods crank and bearings with seals in and it ran great but 15 hours on the bike later were in the same boat I called pro circuit and they said it needs to be oem crank that's my problem I don't want to throw another crank in and have the same problem in 15 hours money doesn't grow on trees at my house any answers would be awesome

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The stock KX85 has always been problematic. It has much larger needle rollers in the rod bearing compared to other 85cc bikes. Though this may sound better it actually isn't. High rpm and large rollers are not a good combination. Hotrods uses a more standard size rod bearing in thier KX85 crank. This is why the Hot Rods crank has a good reputation in this bike. What is hard on a crank is compression. Don't know if your new head raised compression or not but if it did most of your accelerated crank usage could be attributed to the head.

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Yes I believe it has higher compression and per pro-circuit if I run any fuel besides c-12 I will cease the bike within 15 minutes. PC also says to only use oem parts and stated by them that's the reasoning behind the failure. I'm not a motor expert but what would a crank haveto do with the bike cutting out at high rpm? Other than seals I don't understand it. What should I do?

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Cutting out at high rpm is typically an ignition problem. Loose wire, loose motor mounts, or bad coil?

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I'll try changing the coil on Friday but I swapped it out the last time this happened with no change. My question is what made it change with the new crank?if you have bad crank seals won't you have sporadic idle and throttle? I have the same bike but stock that I can try any parts with if there's any suggestions and is there a timing keyway on the crank that may get worn? I'm really lost and I don't think its a crank issue but possibly bearings or seals

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I agree with the above. There are alot of bad ways to install a 2stroke crank, and only a couple good ways.

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C12 is a higher octane than the mrx02 why raise the compression with the new pc head? Where is the squish at? And what jetting are you running? Elevation?

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The mrx02 is PC top power producer but expensive I was told by tech at PC if I wanted to run different fuel I had to change heads or the bike will blow in 15 minutes. My buddy went to school at mmi and is legit. With that being said he told me everything looked fine with the crank the first time he changed it. I looked at the crank for the first time today and took it to work and everyone said the crank looks great. I don't think I have a crank issue. But I don't understand how it fixed it last time. I live in billings mt I think elev is around 3500 we only ride in higher elevations from here. The mSV sounds good but wouldn't it have the problem right from the get go? Why is it taking 10-15 hours to develop the problem? What about a flaw in the cases where they join and after splitting them and puting new gasket in they're sealed and after 10-15 hours they're is a leak? I'm off tomorrow and will check any and all suggestions and really appreciate all the help thanks

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Ok guys wanted to make sure the bike was fixed today so I went through everything. Pulled cases apart inspected everything and it looked great did replace seals while I was in there. Put a new top end in did have a little blow-by on the exhaust side of piston. I did notice that one of the stud pins going thru the head gasket was gone and in the mrx02 head so put it back in this head not sure if that was the culprit or not but it's made to be there. Cleaned power valve, cleaned carb and inspected. Put back together and it once again rips don't know for how long time will tell I'll just plan on every 15 hours puting a top end in for sure. Thanks for all your help guys

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Also I thought was kind of weird did a compression test before and after before was 150 after 150 is that not odd? Also when I called PC I asked them if compression should be the same with their head and cylinder as stock they told me they have never used a compression tester in their shop ever and they wouldn't know what it should be. Wtf ever right I'm not at all pleased at the customer service at PC they will give you no info at all this bike is the last one that will get PC work done for us next year going with husky and maybe sooner. Thanks for the help guys

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Also I thought was kind of weird did a compression test before and after before was 150 after 150 is that not odd? Also when I called PC I asked them if compression should be the same with their head and cylinder as stock they told me they have never used a compression tester in their shop ever and they wouldn't know what it should be. &%$#@! ever right I'm not at all pleased at the customer service at PC they will give you no info at all this bike is the last one that will get PC work done for us next year going with husky and maybe sooner. Thanks for the help guys

why would a shop not have a compression tester? 

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