I am going to post the details of this small project here. As well as a few questions. I could not find anything like this when I started so I hope it will help someone down the line.

The bike is a 2007 klx250s. The cylinder was bought off Ebay and replated. The piston is a Wiseco standard size for a klx300. Before I started I had decided to increase the compression ratio as much as clearance would allow. I had asked to have .020 thousandths decked off the cylinder when it was replated. I do not believe this was done as the 300 cylinder height measured very closely to the 250 cylinder. If anyone can verify the height of both stock cylinders I would appreciate it. to achieve the higher compression ratio the base gasket was removed, and a thinner copper head gasket was installed. The head gasket thickness is .016. This brought the piston to head clearance to about .030 thousandths, the intake valve clearance to .065 thousandths, and the exhaust valve to .100 thousandths. The valve clearances were measured with clay and the piston to head clearance was measured with solder. It still runs on pump gas no problem. I have premium 93 in it now. I think it would take a lesser octane fuel no problem.

The only issue I am having is with selecting a main jet. I have tried lots of jets from k120 to a drilled jet roughly equivalent to k145. I run it wide open down the highway and pull the plug out as soon as I am off the highway. When cut open none of the plugs show any color around the base of the porcelain. I don't understand this. The two largest of the jets I tried were obviously way too rich. Currently I am using a k132 main that seems fine. If anyone can offer advice I would appreciate it.

 

I need to know the exhaust system components, airbox configuration desired, and current status of the carb internals.. Then I can help you with a main jet, etc..

 

It is a stock exhaust. kdx220 snorkel. Everything in the carb is stock except for the main jet and the fuel screw. I read several guides online about reading spark plugs. Is that a practical way to determine jetting in an engine such as this? Thanks for the help!

 

Assuming the pump gas you're running is unleaded, IMO it's a crapshoot trying to read plugs for fuel mixture. The 351 kit supposedly runs a 12:1 compression whereas the 250/300 OEM setup is 11:1. Since you're not apparently getting detonation, you're probably somewhere in that area. You're somewhat in uncharted territory, but on the other hand it's not totally alien territory. Especially with your stock exhaust you should be able to hear detonation if you know what you're looking for. Otherwise it's some kind of exhaust sensor to give you real fuel mixture numbers or seat of the pants...and ears...testing for your main jet. You have a quiet exhaust and an airbox snorkel...I think you should be able to hear detonation if you're careful...then back off on jetting.

 

No, it is no longer possible to accurately determine fueling issues by reading spark plugs. This is due to unleaded fuel. Alcohol burns very, very, differently than gasoline and leaves no insulator coloring that "old skoolers" can identify. Once retrained, it is possible to determine a little bit of fueling info using magnification on the insulator. But basically, in a nutshell, if the insulator coloring is white, all is "acceptable"..

For near sea level:
Jet size should be a DJ132 or K142 - idle set to 1300 rpm, fuel screw initial setting to 2.75 turns out - but with final adjustment being an unknown.

See the link for a proper main jet conversion table on my signature line.