I bought a 2003 KFX 50 and it starts perfect but when I give it gas is just bogs down. I'm pretty familiar w/ working on bikes but thought I would see if anyone else has had this problem. The guy I bought it from said he had the rebuild done then it wouldn't rev when he got it back. Here is what I have done: cleaned carb, tried 60 and 65 main jet, tried removing stacked jet, cleaned all lines, tank, petcock, etc, cleaned magneto, etc. To me spark seems a little week. Orangish rather than bright blue. Maybe coil problem? I didn't see where I could adjust the timing. I'm checking the compression tomorrow. Any help would be appreciated. Thanks.

 

If your running a 60 or 65 main jet you probably need and an aftermarket K&N or UNI filter for it to breath right. (depending on elevation)
We live in south western PA, and this is what worked for us: 65 main jet with the piggy back jet IN. The intake and exhaust restrictors were removed (spark arrestor too), and I grinded the top of the carb slide about 1/8" so that it would open completely. Start with the needle position in the middle, and move it up or down to where you need it, ours worked best second from the top.
If your still running the stock air filter and/or the restrictors try a 55 main jet. (I think that was the stock size)
We also went with a cooler plug (BP5HS), and pre-mixed 32:1.
Some people say the CT pipe really helps, but we moved up to bigger quads before we got that far. Good luck.

 

I have the stock airfilter on it. After trying different jets I went back to stock. Exhaust resistor is out, intake is in. I'll try your set up. Thanks.

 

To remove the intake resrictor, take the intake elbow off, and heat it with a propane torch to melt the glue that holds it in place, the restrictor looks like a washer with an oval hole in it. Once you've heated it, stick a screw driver in the oval hole, and pluck the restrictor out. It made the biggest difference in the way ours ran.

 

Thanks, I'll remove that restrictor. I did a compression test and it was only came up 60. I took off the head and no head gasket. The base gasket was old and the piston was still a suzuki piston. More than likely it was not rebuilt or a terrible job if it was. Who forgets to put the head gasket on? So I'm just going to go ahead and rebuild it.

 

I'm surprised you could get it to start at all. Sounds like you found the problem. Good luck.