kx 80 ported jetting problem

[dirtbikr99]

I have a kx 80. I tore it down because it was old. I replaced the piston and rings with weisco, stock bore. While I was in there I did some porting. Just some to the exhuast mostly. The nikasil looked good. Put it all back together and it was cutting out on top end. It would start right up then not rev out. I changed the plug and it would rev out again and be good. Then work then not work.

The plug looks ok, more on the rich side. Because of the porting is it now needed to lean out the jetting? I noticed pro circuit used a 122 instead of a 125 main jet.

Any thoughts?

 

[_JOE_]

If it won't respond to jetting I'd think you either ruined the ports or the exhaust valve is malfunctioning. Just a guess. Lower end good?

 

[whenfoxforks-ruled]

Go with what the plug tells you. Pro circuit is not jetting your engine, you are. If its rich, lean it. Start at the pilot, not the main. Hide the "porting" tools, do not do that anymore. Its not porting, making it pretty, making it seemingly flow better, shinier or bigger. Porting truly, involves altering the openings in the cylinder bore. A halve of a teaspoon of aluminum removed from the exhaust, could very well ruin the cylinder. Unless you can put it back to how it was before, down to the existing texture. I have done what you have, before, as have many others. To do it properly, you would need a right angle porting tool, a virtual 2 stroke engine soft ware program, and read up on every paper and SAE paper that Professor Blair, Eric Gorr and Rich Rohrich have written. Catch up work, they have done the hard part, and have already ruined plenty of cylinders. Its not like you figure, there is a specific volume and flow for a reason, scientifically and seat of pants proven. Eric has some very good basic manuals for 2 stroke performance, check out Forward Motion. And 2 stroke MX race bikes are usually engineered to the edge already, anymore and you will have a 10 minute rebuild engine instead of a 5~7 hour rebuild engine. That choice is yours. Vintage Bob

 

[plynn41]

Hey fella,
You've been duly warned about the dangers of self-porting, so I won't go there.

But unless you did some radical cutting, I don't believe you've changed your jetting needs enough to cause the bike to cut out. And I doubt that you've ruined your exhaust port. And if you actually changed the timing or width of the port enough to cause it to run poorly, it would run consistently bad, not on and off like I understand you to be saying.

If I understood your post right, the bike revs out sometimes and then doesn't other times. This is not a porting or jetting issue. This is a mechanical issue--some part that moves in response to rpm is not consistently doing its job--or some part that shouldn't move in response to rpm is now doing so. Since you worked on the exhaust port, I think the first responder to this post was right on to suggest that maybe your exhaust valve is hung up. Did you get the aluminum grit cleaned out of that area good?

Other possibilities include the ignition--but you probably didn't touch that, so I doubt it's a problem. The reed cage and cylinder base gaskets might also be suspects. Is it possible that you didn't get a gasket sealed up good and there is an air leak that comes and goes depending on engine vacuum?

I'd look at the exhaust valve first.
Regards,
Lynn

 

[dirtbikr99]

Thanks for the responses guys.

I have ported other cylinders before. With the correct porting tools and such. With guidance of a master engine builder. All I did was a little to the exhuast. I did not change the ports or the port heights.
I used new gaskets and a touqre wrench.

It runs great! Then runs ****ty.

Also the plug keeps fouling. So I guess the only thing left to do it start leaning it out. Also keep checking the plug make sure its not too lean.