Engine Damage Due to Lean Burn...your opinions please

Discussion in 'Engine' started by KLR650, Oct 9, 2010.

  1. KLR650

    KLR650 New Member

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    First off, I am new to Blasters, have some 2 stroke experience and lots of experience with 4 strokes. I have done some reading here to familiarize myself a bit.

    Borrowed a '98 Blaster from a friend for my g/f to use while I rode my KLR. She bombed around on it for about 1/2 hr when it quit.

    Fast fwd...tore down and found piston failure which shows classic symptoms of a lean air fuel mixture. Exhaust side of piston is melted with a hole burned through top of piston toward exhaust side. Obviously the top end needs to be rebuilt, but I would like to shed some light on the cause of the failure so I can prevent it from happening again.

    Neither of us knows too much about the bike, but here are the details as I can tell:

    1) what carb do you have??? Mikuni--measures 28 mm on engine side

    2) What type of aftermarket exhaust do you have? Stock AFAIK

    3) What type of airfilter do you have? foam

    4) What kind of air box set up do you have?? stock with 8--1" holes in lid

    5) What size pilot jets are you using? 32.5 (stock)

    6) What size main jets are you using? 230 (stock)

    7) What needle position? 2nd from top

    --It was a 50F day, and we are approx. 900 ft above sea level
    --Spark plug was a BR9ES, which is 1 number colder than stock I believe
    --Checked the intake boot closely and it looks ok.
    --Oil injection is hooked up and seems to work, however I did add approx 200 mL of oil to a full tank of fuel "just in case". After doing some reading, I now realize that wasn't smart since adding more oil actually leans out the A/F mixture.

    Here's my plan of attack:
    1. Over bore cylinder with new piston etc, flush debris from bottom end
    2. Install correct spark plug
    3. Leak test to verify no air leaks
    4. Try a larger main jet...maybe 240 or 250 and lift the needle 1 notch
    5. CHECK SPARK PLUG COLOUR OFTEN!

    Also I'm debating on whether to keep or ditch the oil injection. Is it known to be reliable? If I decide to pre-mix, what jetting is recommended.

    Also how much ball park is this thing worth with a good engine...has steel rims with some dents, good tires, plastics are not cracked, seat is not torn, needs front brake shoes...fair to good cond in my opinion...worth maybe $1000?

    Any ideas/advice are much appreciated.
  2. chevymuddman

    chevymuddman New Member

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    id ditch the injection. id rather be safe then sorry.
  3. phragle

    phragle Member

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    What was the result of the leakdown test prior to tearing it down??
  4. KLR650

    KLR650 New Member

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    Hindsight being what it is, I now realize it would have been smart to do a leak test before I tore it down. Now I will have to get it all back together and if the crank seals are leaking it will have to come all back apart! 8-|

    I haven't actually done a leak down test on a 2 stroke before. Have read here to use an expansion plug in the exhaust port then pressurize through the intake boot to 5-10 psi?

    Thanks
    Ian
  5. slickerthanyou

    slickerthanyou New Member

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    First of all, thank you for doing research before you post, That helps out alot! as far as a leak test here is the linky from the God father himself >http://www.blasterforum.com/do-yourself-20/build-use-leakdown-tester-10663/. Before you fire that up do this test and if ya need a tester Pm HOLESHOTMAN, he make a great cheap set-up and ships fast. Also after you do the break in properly linky> http://www.blasterforum.com/engine-13/how-break-two-stroke-must-done-after-top-end-rebuild-4399/ As you said do quite a few plug chops to check tghe jetting. her's a short jetting guide linky> http://www.yellowdogracing.com/techstuff.htm, But in short with 28mm carb stock exhaust no lid 270-290ish would be a starting point in my opinion, yes move needle up one notch and if ya premix ( most recommend> 32:1 ) -Bob
    Last edited by a moderator: Sep 27, 2014 at 5:14 PM
  6. scotj77

    scotj77 Member

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    wow the most awesome new guy ever. you are on the right track. i would do everything you said plus the removing the oil injection, as said earlier. but this isn't your quad so if you do everything you said it should run like a top. you could replace the flywheel side crank seal just to make sure that wasn't the problem. you would have had hella smoke if it was the clutch side.
  7. scotj77

    scotj77 Member

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    is there some way to make all new guys read this post before they can post their first post. by the way dude that is a stock 26mm carb they just measure 28 at the outside edge but really cool that you actually measured it.
  8. KLR650

    KLR650 New Member

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    Thanks to all for the replies! I'm no stranger to internet tech forums and know how useless it is when someone basically says "this is broke, how do I fix it".

    Good tip on replacing the flywheel side crank seal proactively. I might just do that.
  9. royalt67

    royalt67 New Member

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    Yep, nice to read from a guy that has his s__t together. Frankly KLR650, I would be very proactive and replace the right crank seal as well. When mine was bad it didn't sn=moke a lot, but it sure made a diff in how it ran afterwards. You can get a complete seal kit off ebay for about 25 bucks vs 13 each at the dealer.
  10. brandoz28

    brandoz28 Member

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    royalt, i'll be the hater, but i'd rather use the yamaha seals, the ones i got were double lipped on the crank, it was worth the money since the stator side one i put in leaked from the get go
  11. scotj77

    scotj77 Member

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    good info brando!
  12. brandoz28

    brandoz28 Member

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    the seals i used the first time are the ones that came with my wiseco kit. the yamaha ones even feel better in your hands

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