Burnt up another piston and cylinder

[DRoZ]

With about 2 hrs on a new piston, my daughter did this within 1/2 hr at the dunes. It had overheated to the point where the rubber spacers in the cylinder fins were smoking. Would an exhaust leak at the flange cause this. There is not a flange gasket in there.

 

[scotj77]

if you are not doing a leak down test after assembly it looks like an air leak. you dont have an oring gasket between the flange and the cylinder?

 

[DRoZ]

I did a leakdown. Held 7# for 20 min. Not sure about the oring cause my machinist attached both intake and exhaust flanges on at his shop. Im talking about where the expansion chamber fits up against the flange, held there by springs. There is a lot of oil right below the pipe.

I'm running a B1 pipe w/310 main jet on a 67.75 bore. It idled and ran like a champ, but Im not sure if the prior owner milled the head or not. I am going to check for timing advance also.

 

[scotj77]

not really gonna cause too much trouble havin a leak there. most aftermarket pipes leak there. I ran a 330 with that setup. better to be fat than rich. you mixin oil or usin the oil injection?

 

[xKroniK13x]

I did a leakdown. Held 7# for 20 min. Not sure about the oring cause my machinist attached both intake and exhaust flanges on at his shop. Im talking about where the expansion chamber fits up against the flange, held there by springs. There is a lot of oil right below the pipe.

I'm running a B1 pipe w/310 main jet on a 67.75 bore. It idled and ran like a champ, but Im not sure if the prior owner milled the head or not. I am going to check for timing advance also.

No, I don't think the flange area would effect your engine like that. My B1 is all assembled correctly and I get a little bit of oil as well.

 

[darederek]

that was def from overheating. did you do a leakdown before you tore it down?

 

[DRoZ]

that was def from overheating. did you do a leakdown before you tore it down?

I did not. probably should have.

 

[darederek]

its a good thing to do that way if cases are leaking or your crank seals are leaking you dont fry a second top end when you rebuild. just be sure to leakdown with the new top end

 

[Kgibson]

IMO that is lean. Anytime you blow a hole its lean. Why its lean is the question....my guess main jet, but what do I know :}

 

[darederek]

i thought that but 310 with a toomey shouldn't be THAT far off. idk his altitude and temp situation but dunes im guessing low with both

 

[DRoZ]

was at sandlake or. about 50 degrees. My daughter was probably running the piss out of it also she is either stop or go.

 

[darederek]

yeah but they are made to run like that. i doubt it was her fault. although if you feel it that hot on your legs it makes sense to stop and see whats going on

 

[Kgibson]

I agree that does sound like enough jet, but usually heat (from an air leak} is a 4 corner siezure and a hole is jetting. But with everything there is more to this story we dont know lol

 

[darederek]

i completely agree. id say rebuild. leakdown. step up a jet or two for breakin. then after break in run some plug chops to see where you need to be jetting wise

 

[toytech]

also what spark plug was in it? like Kgibson said, I think lean not an air leak

 

[DRoZ]

also what spark plug was in it? like Kgibson said, I think lean not an air leak

It was a B8ES plug

 

[slickerthanyou]

That really is a classic sign of leanness. You could always go up to a colder plug b9es that will drop it at least 30*-50. But has to be something.

 

[99LRDblaster]

I've seen more than a few guys blow on the dunes simply because it's at the dunes. For prolonged riding it typically helps to go up 1 main because of all the WOT running at load.

Whether it be lean and/or overheating, the result is the same with predetonation from the piston/spark plug being too hot and igniting the air/fuel mixture too soon........hence the hole in the center of the piston. It could possibly be that the engine was sealed up, but once out riding it an air leak started.......sometimes sh*t just happens. Other than that, I suppose it would be safe to go up 2 mains, rebuild, do a leakdown test.......break it in through heat cycles and get it broken in, do another leakdown and go from there with plug chops. There a lot of times where you can end up with a lost expression on your face as to why things are happening but after a lot of autopsy research and plug and playing, you gain more knowledge out of it in the end. Live and learn as they say.......even if it is random. I:I

 

[Larry's Shee]

Might be a pain to do, but may have to do leak down at operating temp, maybe something opens at high temp (case crack, etc)

 

[Awk08]

i blew an exact hole in a piston, it was a now classic "vito's and k&s" style green paper base gasket failure, causing a very lean condition, this happened cruising about 35mph half throttle in 6th gear, about 10 minutes into the ride up the hard road,
wot, main jet nor overheating had anything to do with it, extremely lean from sucking air did

was everything retorqued after the heat cycles, give that base gasket a good look while tearin it down, and replace with a factory yamaha base gasket only
the head gasket looks ok in the pic