I think I found the reason for engine failure

[eprovenzano]

1st a little background.

99 400ex purchased 4 years ago. HMF pipe with the top of the air box drilled. It always ran well, but smoked and burned oil. Over the years, the lack of prior maintenance was corrected. Last winter I decided it was time to redo the top end. The bike was still on the OEM top end. I installed a new piston, rings, and replaced the valve seals. I put it back together and it didn't want to idle right, it would surge and stall. After 4 hours, the piston seized to the rod. My son said it was as iF it got hot...

I bored out the cylinder again (2 over), new piston, rings, gaskets etc. It idled a little better, but still surged. This time it lasted 10 hours. Again it seized to the rod.

This time I split the cases, replaced the crank, main bearings, timing chain with a CRF 450 chain, and timing chain tensioner. The cylinder is being bored out 4 over (406). I was cleaning everything while I was putting the engine back together and decided to open the carb to check the jets. When I bought the quad, I was told the jets were changed when the HMF exhaust was put on. Well what I found surprised me. The main is a 152 with a 38 pilot air screw at 2 turns. hmmm I think I found the issue... I know that a worn cylinder can mask jetting issues but damn... talk about running lean... If I'm not mistaken I should be running a 160 - 165 main with a 40 - 42 pilot.

I ask the jetting Gods if I'm on the correct path with the jetting settings. I'm thinking I should be at 160+ with a 40 pilot. Since everything is apart, I am going to check the needle also. Where should the needle clip be set at?

[CJM]

Usually the headers will be super hot and turn blue if your running lean. Clear indication really.

http://www.exriders.com/vbb/showthread.php?s=&threadid=445695&highlight=jetting+placement this chart should help.

[supertrooper90]

what did your spark plug look like? white and chalky would be a good indication of running too lean I would say the 160 main and 40 pilot 2 turns out would be a very good starting point. have you checked your oil pump or any of the sceens that are inside the motor? i think theres one at the bottom of the oil tank and one inside the case on the clutch side. Im not talking about the oil filter itself.

[eprovenzano]

I can't remember the color of the plug, as I've completely dismantled the engine, I'll look to see if its still in the box of parts waiting to be reassembled (I'm thinking it was not tan, but lighter). I’ll check the screen in the tank, but I can assure you the oil passages in the engine are currently pristine... I've scrubbed the interior of the engine, and cleaned every part while looking for worn parts.

The original crank had some play in the connecting rod. I don't think it was too bad, but the piston had seized to the rod twice. I felt it better to replace the crank and bearings than slapping it together with another new top end and the engine failing... again.

It may have been a combination of a worn crank with the lean jetting, but each time it seized was after a sustained 3/4 to WFO run... The 1st time was after a series of hill climbs, the 2nd was on a fire road. I suspect the lean jetting on those sustained WFO runs, is the culprit.

I'd rather be a touch on the rich side. I've jetted my two strokes to be a touch rich, but I've never had an issue, nor do I foul plugs.

I think I'll order a 40 and 42 pilot, plus a 160 and 165 main. I believe (based on what I've read) the 40 and 160 will be the right jetting set-up.