I spoke to a professional mechanic today and showed them my head and piston. He told me that what caused this was very clear and it was because I was simply running way too lean. He said that he had seen this before and there was no doubt about it. The thing is, I was running the richest Dynojet(#132) offered in the kit. I had the needle 3rd notch down with two washers on top. Granted I was still running the stock #35 pilot jet. I would like to know where I can get the #38 pilot jet for when I get everything back together.

Here are the pics for anyone that never saw them:

 

I run the 132 DJ on my 250. Was running the 128 but after cam mod I needed the 132 so if you were big bore and cam mod you were probably two sizes small on your main jet.

 

I call bs. Running lean will not cause the valves to hit the piston.v Just doesn't seem logical.

 

Too lean? Man...that's hard to believe. Granted the mechanic had the benefit of getting to inspect your parts in the flesh, but I'm not sure I'm seeing anything indicating a too lean condition. I'm able to speak very bravely from afar tapping on my little keyboard here...LOL!

 

Yeah I have the 351 and did the cam and exhaust mod. I also have a HMF slip on with the baffles and spark arrester removed. Along with that, I have the airbox lid removed along with the backfire screen removed. I have all smog removal done too.

Chances are, he is right. I was running way too lean. Too much air and not enough fuel. I had the bike really opened up.

I'm just telling you what the mechanic told me. He said that I simply burnt everything up and that's why the valve stuck and I eventually lost it.

 

Wow you really had it opened up. Extreme lean condition causes extreme heat, notice it was the exhaust valve that failed, could it have gotten hot enough to fail?

 

That's what it sounds like. I had noticed that header pipe was glowing red hot at night a little bit. I had the stock instrument panel off the entire time I had the 351 because I was in the process of installing the Vapor speedo and didn't have a temp gauge installed. The fan kicked on a couple of times so it could still monitor the heat but this only happened when I left it idle for too long.

I imagine that the bike will be fine after I reset the camshaft back to normal with either the #128 or #132 main jet. But I need to install the #38 pilot jet. Anyone have a link where I can order that?

 

If you talk to the mechanic again, please ask him for an explanation on his diagnosis. Several of us here would like to hear it.

On the KLX300 with the California needle, if you take the top of the airbox off and remove the butt plug from the muffler, the engine gets so lean that it hardly runs above mid range. I believe you said that yours was running strong when it blew?

Ride on
Brewster

 

I said the plug looked like a fresh plug and that it seemed like it was dogish in mid range. I thought all along that I was running lean but was told by other forum members that it's impossible to really tell with the way new fuel and all the additives in the fuel if I am running too lean or not by just looking at the plug. With all of the air I am getting to the bike, does it really surprise anyone that I was running too lean? I definitely wasn't running rich the way I have the bike opened up. He didn't really provide much of an explanation other then "oh man this sucker really burnt up! looks like you were running way too lean"

 

Interesting - I agree with him that the culprit is very clear, I just can't bring myself to agree with him on what that culprit was though. How a lean situation causes valve dents in the piston is a bit beyond me, but he probably knows a lot more about engines than I. Did he offer any detail on the link between "Lean running" and "valve connection"?