Kx85 going thru cranks

[85#371]

My son has a 2014 kx85 we bought the bike from Garrett marchbanks it has pro circuit cylinder carb and head the bike was set up to run mrx-O2 fuel and we swapped heads to run a cheaper c-12 fuel my son ran the bike roughly 15 hours after purchase and it started missing at higher rpm I cleaned carb,replaced top end,replaced reeds,swapped Cdi,stator,coil and it still had a miss on higher rpm' only thing left was replace the crank and that worked now 15 hours later it's doing it again I called PC today and they said it's because I used a hot rod crank and I need to use oem. I Connor keep putting cranks every 15 hours in this bike unless I get lucky and win the lottery. Any answers would be much appreciated

[Foxx4Beaver]

Hot Rod cranks are the worst quality you can get.
My buddy was replacing them every 4 races in his kids RM80...and every 6 races in his YZ250...and they're only mid to front pack amateur riders.
Once they switched to the more expensive OEM cranks, he hasn't had any issues.

Try Wossner...I'm not sure if they make kits for the 85...but maybe our members Alward25 or Greenkaw24 can assist on that. Wossner is high quality from Germany...check them out...

http://www.wossnerpistons.com/

Have you done any wide open plug chops to be sure it's not leaning out on the higher revs?

[dan580,95 moto]

i brought a hotrod conrod kit for my 95 kx500, got it installed and built my bottom end but after buying a brand new cylinder decided to take it all apart again and remove the rod. i never ran it after everyone telling me that it would end up through my crankcase or worse my new cylinder. 

[Foxx4Beaver]

i brought a hotrod conrod kit for my 95 kx500, got it installed and built my bottom end but after buying a brand new cylinder decided to take it all apart again and remove the rod. i never ran it after everyone telling me that it would end up through my crankcase or worse my new cylinder. 

EXTREMELY wise move

[sandblaster]

i brought a hotrod conrod kit for my 95 kx500, got it installed and built my bottom end but after buying a brand new cylinder decided to take it all apart again and remove the rod. i never ran it after everyone telling me that it would end up through my crankcase or worse my new cylinder. 

EXTREMELY wise move

+1

[85#371]

Have done plug tests using three brand new plugs and its definitely not too lean I'm going to da a compression test on it Friday and see how that goes if I was to guess it seems as maybe it's sucking air from the motor and leaning it out but it doesn't show on the plug I have the old crank from the first time and I'm no expert but it seems fine but after replacing it ran great

[85#371]

I'm not an engine expert but it boggles my mind how the crank would be the issue what would the crank haveto do with it sputtering at higher rpm'a unless it's sucking air leaning it out not giving it enough fuel. I just know for fact I replaced everything and last ditch effort put a crank in it and ran great only for 15 hours though

[k2000x]

Nothing wrong with hot rods had one in my yz125 for 5 yrs racing now a local kids still thrashing it
More like its you crank seals leaking

[Foxx4Beaver]

I'm not an engine expert but it boggles my mind how the crank would be the issue what would the crank haveto do with it sputtering at higher rpm'a unless it's sucking air leaning it out not giving it enough fuel. I just know for fact I replaced everything and last ditch effort put a crank in it and ran great only for 15 hours though

Have you checked and or set your float height?...it could be set too low.
Another possibility that I never used to believe, until it happened to one of my friends bikes...is the fuel cap vent. If it gets clogged, the fuel won't flow correctly...there's even some threads on fuel cap issues on here too.
Have you checked for excessive play in your crank bearings?
Does it smoke excessively when running?

**EDIT**
check the magnet on the flywheel for cracks or chips too.

[85#371]

Have checked float height when we first got the bike it would leak fuel from overflow and we cleaned and adjusted the float.never have checked the cap or vent hose but will do Friday amongst other things that I will post. Thanks for your help

[DoldGuy]

Where did the "C12" head come from? P-C has a tendency to deck the cylinders, possibly your MSV is causing rod failure. Just thinking out loud here.

DoldGuy

[SHLEPY]

any one ask what oil ratio....just askin.

[85#371]

Yeah the head and cylinder came from PC it came with a head made to run mrx-02 but at 30$ a gallon we got a different head to run c-12 from them.

[85#371]

Symptoms of worn crank seals? Why and what would make it sputter at higher rpms that would be crank related?do these bikes have a keyway on the crank for timing that could be worn? I have the same bike stock that we can swap parts with if there's any suggestions. I would like to find the root of the problem instead of just throwing cranks in it every 15 hours. I don't beleive the last crank was bad neither did my mechanic that changed it. How to tell if there was a problem with the last crank? Maybe it was just seals and I didn't need to change the crank out?

[85#371]

What is MSV? What about crank breather on the 85 could it be clogged I doubt it because we just put a new clutch in it before this happened this time and all the tranny oil leaked out and then we replaced it? I'm trying to think of all possibilities