I would love if some of the ones with knowledge re 2 strokers could have a look at this and give me your opinions:
I have just bought a 2009 KTM 85cc MX bike for my son. He has been riding KTM for 4 1/2 years and we have not had one big end fail. (OK, so the bikes have been small ones: 50cc and 65cc). I have rebuilt the big ends at approx 40-50h use before a failure occured. Top end gets a new piston etc. at approx 20-25h use. He does relative much racing: We have this year been racing every w/e since beginning of February and will do the same in 2013 with the 85cc one. We use 40/1 mixture and use quality full synthetic 2 stroke oil.
The 85 I got had 2 big ends fail inside 2 months before I got it. After 20min on the beach (to get a hang of the bike) and 2 1/2 lap in practice before the race, the big end again failed for us. The motor had been repaired both times by the same bike mechanic that has a good reputation and is used by many in the HB MX circles. The crank and conrod are not original KTM bits but by Hot Rods. Tends to be the only aftermarket ones that can easily be found for the KTM 85's.
The KTM 85's have a little of a reputation as a bike that blows big ends. On the web you will find many "experts" who have their own solutions what the best fix is. When talking to a KTM shop they obviously tell you that we should have a original KTM crank and conrod set up with the tolerances as specified by KTM. So I have bitten the bullet and a original KTM setup is now on the way for just under $700.
When talking to the mechanic who repaired the big end twice he blaims the jetting. Too lean he recons. (Bike came from factory in 2009 with a 118 jet. He fitted a 125 jet at last repair). Todays 2 stokers have a reed valve through which the fuel mix goes in to the crank chamber and then at bottom stroke is transferred to the top of the piston. Initially I disagreed with him as in my world lean jetting meant a seized top end, but the top/piston is in perfect shape. The spark plug is also nicely chocolate brown. But after some more thinking he might have a point as the fuel mix will pass the big end before getting to the top of the piston.
Clearly I have no interest in rebuilding the big end after every race meeting, so I am trying to figure out a way to make sure the big end survives 40-50 hours (that it should do) w/o exploding.
Also: When I got the bike it was running like crap: Plug was black and oily and she was smoking heaps (recon it was the jump in main jet size that caused this). I spent some time on this and got it better by dropping the needle one click. (from middle to dropping the needle one click)
Here the options I have come up with including pro's and con's I can think of:
1. Have the crank and conrod sent to one of the overseas engineers who recon they have the solution.
- Before I would know if their solution is actually "the one" I would not consider this. And there would be the additional costs...
2. Re-engineer the reed valve inlet setup. This has been discussed as one solution (There is an opinion that KTM got this part wrong on this engine).
- Probably not an option as the costs would be high and it would be based on trial and error and guess work.
3. Fit a modified Yamaha crank assembly.
- Others have done this with good results. But it would not be my first option as it would again add costs to a bike that was initially cheap but soon would become a money pit...
4. Re-jet bike with bigger main jet.
- If this will save the big end until I do the rebuilds anyhow (40-50 housr of use) then this clearly would be the preferred option. But I have a suspicion that the motor would be running rich and splutter (it was doing this when we got it) and even if I could get it to rev properly at full throttle, the mid range would be a problem and I would end up dropping the needle. This could mean that there would still be same issue at 1/4 - 3/4 throttle. And the chance would be that the big end would again blow.
5. Change the fuel/oil mix to say 30:1.
- I am not sure how this would work in practice. Clearly the jets would have to be bigger (as there would be more oil and less fuel coming in to the motor). But perhaps things would get oiled up and plugs would start to fail?
- If this was the solution, why has KTM not advised it's re-sellers to recommed this, as it would be a easy fix?
6. Use one of the racing fuels available overseas.
- Would be costly to import (don't even know if I could do this). And I have no idea if this would fix things?
That is abaout as far as I have got. The new crank/conrod assembly will arrive 7 November and I plan to have the bike up and running before the w/e 10/11 November as the bike will be raced then.
Any help appreciated!
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Originally Posted by DEATH_INC.
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