I have a 08 yfz 450, it has the cam mod done to it. FMF slip on. 15 tooth sprocket, k&n filter and outerwear with the air box lid on. I rejected it at 048 pilot, 165 main, 4th notch on needle. mixture screw about 1-1.5 turns out. It runs great but is still has a low end bog to it. What you guy think? my elevation is 1817ft by the way.

 

Low end bog sounds like maybe rich. Remove that airbox lid and see if it straightns up. If it don't backfire terribly I'm betting your rich.

 

ok thanks i'll give it a try. The quads kinda outta commision at the moment. I messed up the threads on the rear axle. I just need to get a die and run down the threads with it. After I get it going again I'll pull the lid off and let you know if there was any difference.

 

Ooo, that sucks. Keep us updated.

 

ok got the axle all fixed up. I pulled the lid off and took it for a ride and it kinda made it worse. when i nailed the throttle with the lid off it would stall out. With the lid on it wouldn't stall out but it would back fire slightly. Not sure if maybe my mixture screw isn't set where it should be. Whats your thoughts?

 

If the lid off made it worse, sounds like you are lean, have you pulled the plug after a long run for color? you are looking for a brown to tan porcilean on the plug, black is rich, very light tan to white is lean. Do a steady speed run, pull in the clutch, hit the kill switch and check the color, that will get you in the ballpark for jetting. Keep us posted on what you find.

 

Try a 170 main, 2 1/2 truns on the screw

 

I pulled the plug and it was black, so i cleaned it off and took it for a ride. Pulled the plug a second time and there was a little black around the plugs threads. So I'm guessing I'm a little rich.

I was looking through other topics and came across an answer to kinda the same problem i'm having with my yfz.

I think your problem is in your pilot system, which is from idle to 1/4 throttle. Especially if adjusting your air mixture screw did nothing. Try going back to the stock pilot jet. I have never had to increase my pilot jet even after significant changes to my intake and exhaust. Also, try changing only one thing at a time as a rule of thumb. This will help you find your problems much quicker and easier. I would go back to stock on everything, and start by changing the main jet. I don't know what other mods you have done like to the intake or if you have a jet kit so you have many jets to choose from, but I would start with 2 jet sizes up from stock and make sure you are running slightly rich. When you get that right, work backwards. Good luck! I'll keep checking back...oh, and send me some of that warm weather! I'm tired of the cold here!

What you guys think. Should I give it a try?

 

im running a jet kit in my 660 grizz and when i had it installed i was told that when i change the plug it should b a golden brown almost like toast black is rich and white is lean u may be off on one ot the jets also u might want to check to make sure the oring isnt caught between the groves

 

If its backfiring your definitely lean. Don't pay attention to the threads on the plug, they mean nothing when reading the plug.

So what exactly is going on again? Is it backfiring while your just riding or when you let off the throttle? If it's when you riding along its lean on the main, if its when let off its in the fuel screw/pilot jet.