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Thread: Oil burner/pitted piston

  1. #11
    Senior Member RaZed1's Avatar
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    Hmm, there must have been some pretty serious crankcase pressure to be blowing oil out of the clutch lever seal. Was the crank breather capped off or otherwise obstructed?

    Oil burning on acceleration is generally the rings/cylinder being worn, and tends to get better as the motor warms up(expands, tolerances close up a bit). Oil burning on decel (high vacuum) is generally valve seals, and tends to get worse as it warms up (thinner oil is more readily sucked through the worn seal).

    It's kind of hard to see in the photo but if you can still see remnants of the factory cross-hatch pattern, you should be good to go with a stock bore (67mm) and just knocking the glaze off with a dingleberry or 3 block hone. Most auto parts stores rent these. If it's worn to the point that's no longer visible- or has scarring or other damage, you'll want to bore it.

    I'd say go with a Wiseco piston since they're actually cheaper than a Yamaha OEM, and you get a bit more oomph in the low to mid range since it boosts compression- I believe stock is 9.5:1, and the Wiseco is 10.25:1. Not an earth shattering difference, but it is noticeable. I'm not familiar with the Venolia but it looks like it has a bigger hump than the Wiseco, so I'd expect it to be even higher.

    The Wiseco piston does seem to prefer at least mid-grade gasoline. On 87, loading it up at low RPM can cause some minor knock. 89 or higher seems to be enough to stave it off.
    Fred likes this.

  2. #12
    Junior Member liverup's Avatar
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    Thanks for all the good info! I have a 70mm wiseco piston coming.
    I live at 1100 feet. All our trail riding is at 6000 feet. The 114 jet was too rich. I couldn't find a 112 jet so I went with a 110. It ran great. After going with the 70mm piston will I want to change my jetting?
    Fred and jtstdub like this.

  3. #13
    Junior Member liverup's Avatar
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    elime thanks for the pics and info! I appreciate it. I’m looking forward to getting my cyl bored and the new piston installed. Is it common to have the valves ground when one does a piston and rings job or is it not necessary unless there is a valve issue?

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  5. #14
    Senior Member Fred's Avatar
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    While the head was off I would go ahead and inspect the valve seats and lap or grind as necessary. However more importantly I would inspect the valve guides for the possible source of the oil leakage into the combustion chamber. Maybe others can tell at a glance whether the smoke issue and carbon build up definitively were due to worn rings but I sure cannot.
    Was diagnostic clues in post #11 helpful in determining that rings were likely the cause of blow-by thus causing oil bubbling at clutch shaft seal and not a clogged breather tube assembly nor leaking valve guides? If so a light lapping of the valves would be good practice while awaiting completion of cylinder and piston work.
    That the breather tube assembly did not seem to vent excess crankcase pressure as designed into the air box housing remains an issue to address. I do not think bubbling at clutch shaft seal should not have occurred even with a marginal seal.
    If just the cylinder work is done and engine still smokes after a good break in period and procedure then it is not that much work to remove just the head and have the valve guides replaced at a later date.
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  6. #15
    Senior Member elime's Avatar
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    I like removing the valves and cleaning the carbon from the back side. I like to lap the valves too though some feel it is a pointless exercise I personally think it does some good and is a good idea. Another thing to replace is the valve stem oil seals. If the ones on there now are bad that could contribute the smoke.

    And I am with Fred about the breather vent tube might be blocked and not venting enough.
    Long live the internal combustion engine!

  7. #16
    Senior Member RaZed1's Avatar
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    When I swapped in a Wiseco piston along with a stage 1 cam from WebCam, initially jetting required no adjustments. It still started up and ran perfectly fine. Previously (and after) I also had a couple holes punched in the airbox lid, a FMF muffler, and XT head pipe (same as stock, just shiny). I had a 32.5 pilot, 2 turns on the mix screw, and a 132 main. Sea level riding. Starts and warms up quickly, pulls cleanly through the rev range, and still returns about 70mpg on average.

    I ran that for a while. One day for some reason I felt the top end wasn't quite as crisp as it could be, so decided to toy around with the main. I tried a #130 first, which I thought was about the same, maybe a touch worse, hard to tell. Then I tried a #135 and it was noticeably better, and top end power was stronger. Power in the 8000+ range was much better even with 15/47 gearing, and maintaining 70mph (true, or about 73-74 indicated) was easy, even up grades. The effect of snapping from 50 to 100% throttle was definitely more pronounced with the 135 than the 130, so I'll be leaving the 135 in. It actually pulled to 8700rpm on flat ground, which was close to 80 indicated, previously I'd never exceeded about 8400 without a downhill.

    No idea what the redline on these is, most similar 2v thumpers don't like going much past 9k or so. Downhill I've had it flirt with 9000 and let off at that point.

    I attribute it's thirst for a bit more fuel in the top end to be more to the cam than piston, since it actually can breathe up in the 8,000+ range with a cam and opened intake/exhaust.

    110 seems really lean even for higher elevation. Stock is a 126, which is pretty lean at sea level and decent for the 3-6000 ft range. Are you using Mikuni jets? They physically fit fine but the numbering scale is different. A 110 Mikuni equates to about a ~125 Keihin/Teikei which would seem more in the ballpark.

  8. #17
    Senior Member elime's Avatar
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    Older carburetors the stock high speed jet was #114.
    Long live the internal combustion engine!

  9. #18
    Member wildhorse1966's Avatar
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    it sounds like a head problem not a cylinder problem to me remove the valves and lap it replace the valve seals put it back together and you will be good to go

  10. #19
    Junior Member liverup's Avatar
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    I got my 110 jet from Bike bandit. I have the stock 114 in order. Not sure where I put the original one that I took out. The 114 stock jet was definitely too Rick at 5000-6500 feet.
    elime likes this.

  11. #20
    Junior Member liverup's Avatar
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    My new wiseco 70mm piston showed up today. Came with a good diagram on how to align the rings and ring gap specs. What kind of piston clearance should I shoot for?
    elime likes this.

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