problem? big end connecting rod failure? and other ????s

Discussion in '2-Stroke Engines' started by thuginfc, May 31, 2010.

  1. thuginfc

    thuginfc Member

    i just got this shakyness on my bike today i think the conecting rod bearings are failing. it usuallt runs up to 7000rpm and could go even more but on a bike with a 36 sprocket thats fast enough for me till i buy a super good quality bike. today the highest i could get it is to about 6200rpm.

    what does it sound like when they fail or wear out?

    i also think it may be the chain i did notice there was a little more movement in it than before.

    how much slack should it have in it? i keep it about 1/8 inch of up and down on the top side

    i have about 45 hours on this motor is that normal for failure of internals?

    also can someone check my rpm/speed for me ive riddin next to cars and passed the police radar a few times and at 43-4400 rpm im going 25(i think)

    thanks in advance for any help.

  2. Fabian

    Fabian Well-Known Member

    I have had (including one engine with a replacement crankshaft and connecting rod) 5 replacement engines in my motored bike.
    Every single one of them died from big-end connecting rod bearings failing.

    The solution turned out to be simple.
    Reason for the failures was due to incorrect jetting causing detonation that was pounding out the connecting rod big-end bearing.

    Typically the engine started to sound like it had more than the usual rattles and the vibration increased, till it got to the point where the rattles were so bad and the vibration so noticeable that it would be stupidity to continue on riding as the result would be total engine failure.

    In my case the solution was to swap out the standard jet in the 80cc engine and replace it with a genuine Dellorto No 84, but the engine ran smoothest and made the most torque with a No 83 and made the best power with a No 82.

    Using a No 84 jet reduced the engine temperature quite a bit, though it runs too rich at wide open throttle at very low rpm, i.e. just off idle to 2,000 rpm.
    After that it's fine, when the engine is hot.

  3. thuginfc

    thuginfc Member

    so it would rattle all the time or just at real high rpm?

    also chain tension should be what?
    would a spring tensioner make a huge difference on chain slop or no?

    im bout tempted to get a full suspension and build one with shift kit and all upgraded bearings so i dont have many problems outa it just keep it low rpm
  4. GearNut

    GearNut Active Member

    A spring loaded tensioner would help out alot. Is should push against the lower chain run.
    Make darn sure that it will never fall into the rear wheel spokes though. The stock tensioners are notorious for doing that, and ruining your rear wheel, and causing loss of control.
    With a fixed tensioner I recommend 3/4" of chain freeplay checked at the tightest spot. Sprockets are never completely round, and as the wheel turns, the chain will get tighter and looser.
    A connecting rod in the midst of failing sounds like rattling/ ticking from deep within the engine and will cause excessive vibration. Depending on the exhaust, this can be best heard at specific engine RPM's. Sometimes it is at lower RPM's sometimes it is at higher RPM's.
    Check that all of your motor mounts are tight as loose mounts will also cause alot of vibration. Check the exhaust mount too.
  5. Fabian

    Fabian Well-Known Member


    Lets go through this together: take a plastic jar and place a golf ball inside it and screw on the lid. Now shake the jar around. Does it make a rattling sound?
    Ok, lets try again: Take the jar with golf ball and shake the living daylights out of the jar. Does it still make a rattling sound?

    You've just answered your question with regards to the big end connecting rod bearing.

    Last edited: Jun 2, 2010
  6. thuginfc

    thuginfc Member

    so if it was the big end connecting bearing there would always be a rattle?

    i think alot of it has to do with my chain i watch it while accelerating and the top seems nice and tight until i get to a good cruising speed when i release the throttle a little it starts jumping around.

    think i might try one of those manic hub adapters till i can afford a new bike for the shift kit.

    can you get the hub adapter with a 34 or 32 tooth?
  7. 210061741

    210061741 Guest

    I wouldnt run less than a 36T

    36T is good.

    And honestly i haven't seen any big end crank bearings fail on me.
    I have beat the **** out of them.

    Now what i do notice is there is too much clearence on the small end.
    And it seems like hard running causes the bearing cage to get eggshaped.
    Then it starts to vibrate and rattle alot.
    Soon to follow the wristpin breaks in half and the bosses inside the piston get broken.
    At that point in time the engine seizes up.

    I pulled off the top end and cleaned the junk out of the crankcase.
    Put on a new piston and a new bearing and it runs smoothe again.
    When i had it apart i checked the con rod for play in the big end.
    It's still pretty tight.

    I also am rebuilding another PK that was sent to me where the same thing happened.
    I think chances are the top end will go before the bottom end.

    And i had about 700 miles on the engine that did that to me.
    There is just way too much play between the wristpin the bearing and the conrod.
    Things are definatly bouncing around up there.

    As far as the rattle goes a lean running carb will casue the Ping or Detonation that rattles so loud you think the engine is going to come apart.

    If it rattles the most when it gets hot the carb is too lean.

    I'm with Fabian on that one.
    I run my carb a little on the rich side to keep the engine cooler.
    And to avoid the ping caused by detonation.
  8. 2stroker

    2stroker New Member

    Hmmm. You were saying 35,000kilometers outta one of these engines. After you blew up 5 right. Was this before or after you decided to start going through the toughest muddiest off road conditions? ROTHFLMAO
  9. Fabian

    Fabian Well-Known Member

    As i have said before 2stroker, you comprehension skills may be lacking and as i have also said before: do you wish to engage in an ad-hominem verballing? for i can provide if you so wish for it.

    ps don't tempt me!
  10. dodge dude94

    dodge dude94 Member

    Talk about a friggin ancient thread.
  11. Aladdin91

    Aladdin91 New Member

    Hey can you explain more about jetting im in the process