Holed Piston

Last fall I siezed up my 66 YDS3. It has been repaired and is running again. I am concerned because I'm not sure what caused the failure. I am posting a pix of the piston in hopes that some of you can supply possible causes. At the time I was at about 6K rpm and pulling. and had been for about 5 miles. Throttle was wide open but no where near redline as I was passing a cage to get to my exit.

 

Looks like the pix didn't go. Heres another try

 

Bob, your picture files are probably too large. You will need to use a photo editor to reduce the size. I can't remember the max size but it's posted somewhere on the site.

 

You are right. I forgot about resizing the pix. Hope this works

 

what to the psark plugs look like? pure white? i would say thats too lean!

 

how about the crank brearings? any slop?

 

A couple of thoughts on this,first of all no two stroke likes to be held almost wide open for that lenght of tme so it could be from over heating.Second, two strokes like to have the crank seals change in them every few years or risk running lean from an air leak and also the carb/s should be cleaned once a year at least because the wonderful fuel we have these days only lasts for about 6 week at best befor starting to turn into varnish in the carbs and lowering the octain rating both a bad thing.

 

I remember this Bob,,,,,have you cleaned anything on the piston,,,the color an pattern of the stains would help if you did,,, how much history do you have with it running with the tune,,,,crank seals an bearing are a good start if there not done,,,but you got it running so,,,that's something you don't want to hear I sure,,,,,but a real possibility,,,good advice to look into it for sure,,barring that

I would put a cold plug in it an the biggest mainjet it will take for a while,,,an start the leaning out process after about an hour of run time

I we know heat did it,,,,an not just a little,,,

how long was it wide open Bob,,,,she needs to be rich if your going to make her pull that hard,,,,could you have down shifted(trying to see how much this motors got left),,,gearing how's the gearing,,,,asking her to pull a high load without rpm's is asking for it too....interesting project Bob

 

I only have 2 observations to offer.


1.). You had just recently rebuilt the one side due to a holed piston. Was it too early in the break-in regimen to put the pedal down so hard.


2.). I hate auto-lube systems. Any chance this could be part of your problem?

 

If you are going to run that 2-Stroker hard; add a little bit of 2-Stroke Oil or Marvel Mystery Oil to the Fuel.
Slightly increase the Oil~Fuel Ratio so the Mixture isn't so lean it gets to where it will cause the Piston Crown to Weld-through.
How much Oil do you need to add to have the Ratio right.
1.5 -to- 2 Persent additional Oil.
Run the Bike at Full Throttle for a test run. If a companion following you does NOT see your Exhaust Gases containg some smoke from unburned oil ... it's still too lean.
If you have an Auto Oiler ... you have to make sure that the Jets on the Oil Feed tubes are all open and clog-free. Just the slightest blockage will change the Oil-Fuel Ratio a good deal.

 

I dont think this was a lubrication problem as there is no scoring on the side of the piston that I can see from the picture.

 

its definately not from lack of lube, however lack of 2-stroke oil will make the mixture leaner.....possibly pushing it into the realm of MELTDOWN!

 

I should have said not sufficient scoring and the ring grooves are not destroyed.Sorry about that.

 

Definitely lean. I'd suspect a leaking crank seal. It's gonna suck air before it'll draw fuel from the carb if that seal is leaking bad enough.

 

Boy I never thought this would generate so many great responses. Thought I might add some background from my other threads. Bike had been in storage in my basement for about 40 years with occassional (about every 10 years) trips around the block. With the high cost of fuel last summer I thought I would use it to commute to work. Had a problem with timing but with help from this site that was solved. Replaced plugs and condensors. Replaced off road rear sproket with stock road sproket. Got it running pretty well except for a high end burbble. Auto lube was working well and there was blue smoke from both pipes. Temp of both tubes about equal. Rode back and forth to work for a week or two. Hit the freeway, about 5 miles at 55-65 mph and 5-6k rpm. Was passing a car to get to my exit and went to WOT for a little more speed. Bike locked up and slid rear wheel until I could grab the clutch. Got it home and found damage shown. That plug was covered with splattered aluminum and the exhaust header had some in it. Plugs were pretty dark as I remember them. Ran a hone through the cylinder and it looked good, no scorching or deep scratches. Took it to John at NWVCP and he thought it was fine. Tried a stock piston in it and had good fit. When replacing exhaust headers, couldn't find gasket from failed side so had to buy another. Painted and polished during the winter and found nearly plugged vent in fuel tank. Unplugged that. Just waiting for better weather and hoping.

Thanks for the responses so far.

 

Oh Ya! Forgot to add. Bike has less that 2500 miles on it.

 

I think Richard may be onto something here. This old gal SAT a really long time; the crankcase in a 2-stroke is basically dry. If the crank seals dried out, they may work "ok" most of the time but then start sucking wind if you really screw it on. You had no signs of overt leanness (nice smoky exhaust, etc.) but leanness definitely occurred at high speed/rpm.
I know the bike is back together and all, but it might be worth pulling it back apart rather than having to repair another meltdown.

 

Wouldn't you expect some sign of an oil leak if the seals are bad? Since it was cleaned up there is no sign of a leak except around the Autolube hose and that one has been fixed.

 

plugged fuel vent could cause the fuel level in the bowls to drop...leaning it out!

 

I used to have two stoke Yamahas in the early 70's and holed a piston in an AS3 - the (very neat) hole lined up with the tip of the spark plug.

It was due to using too hot a plug - a 7 instead of a 9 which was essential for motorway driving.

Looking at the photo the heat was very localised here and not and general overheat imo.

 

You don't have the luxury of running an Air Cooled Machine critically Lean for very long. The heat from the lean mixture accumulates rather than dissipates.
It leads to failures.
You have to know how you are running or you WILL get into trouble.

 

+1 on this(I don't have any AS3 ex) ,,,,may very well be just a hot plug,,,,,,,in any case put a 9 in it,,,,nkg's,,,,for sure,,,,no an's if's or but's,,,,,

what plug was in it? I would almost bet it was not a 9,,,,need to take a serious look into it if it was,,,,,


I agree that the hole on top came from a hot plug,,,,,I believe if it lean seized the piston skirt would have more damage,,,,these look like very heavy pistons an probable took a lot of heat to get the hole in it,,,,an this lead to the lockup,,,,,,hole an a lockup,,,have to say the hole came first,,,which is a hot plug,,,,you should be able to lean sieze the motor without a hole in the piston with a 9 in it,,,,is the angle I would aproach it

bad crank seals may not leak oil,,,(I have to admit I've never had this problem but have seen it),,,get the crank bearings out of it as soon as they get loose),,,,,Dan

 

Interesting feedback on the plugs. I had just changed the plugs and went from an NKG B7HZ to an NKG B7HS because that was all the local Yamaha shop had in stock. It sounds like I should be running an NKG B9xx. When John at NWVCP first looked at the piston he commented that it was caused by the plugs.

The pix shows the piston after I cleaned the melted aluminum off it. There was very little scratching on the skirt but some melted aluminum on the ring grooves. The cylinder showed only one slight scratch which honed out easily so I suspect the siezing may have been due to the metal in the ring groove.

It also sounds like I should keep this bike for the next time I decide I want to go track racing and get myself an XJ for just catching the wind.

 

was it pinging? i dont think the hole in the piston is caused by an incorrect plug. i have seen pistons like that before....usually after they leaned out on a dyno!

 

No Ping, No Knock, No Warning. Just like a woman, got my adrenalin pumping and left me standing alongside the road wondering what happened.

 

we see that quite a bit up here especially with snowmobiles. That looks like a classic case of " the engine sat so long that that seals dried out and we not replaced so it sucked a lot of air and made for a real lean condition which will burn up a cylinder in no time at all if you're horsing it". Maybe bikes are different, but two stroke is two stroke, and summer is hotter than winter here. FWIW.

Dave

 

I had a buddy with a newer 1980 something yamaha two stroke and had a similar issuse when he put new plugs in. Are you sure your plugs were the right heat range?

 

Try a colder plug and please check your ignition timing. It may be too advanced for that cylinder.

Good advice about the crank seals. Smoke may also be from middle crank seals esp. if your gearbox oil starts going low.