YZ450 2008 hard starting when hot...

Recently i put a new Merge racing fuel screw and also i changed the jetting with a new JD Jetting kit. As the instructions recommended i changed the needle and i removed the clip in 3d position as i use a stock exhaust, also i changed the main jet to 170 as also recommended by the kit instructions. The bike runs better that ever but when hot it's really painfull and difficult to start. What can i do?

Does it start any easier if you put it in neutral first? I found that my '09 starts in 1 or 2 kicks when hot if I pop it into neutral first.

are the valves adjusted to spec.?

I always put it on neutral before starting and yes the valves are adjusted.

I have found that the diameter of the tip on the fuel screws is not very accurate. I found that with the zipty fuel screw the bike runs richer than than the stock fuel screw at the same setting. So I would recommend adjusting the fuel screw and see if that helps the issue. I also think that a 170 is a bit rich, I run a 165 year round and a 48 pilot jet. It starts better with a 45 pilot but it feels a bit lean to me. I'm in southern California and ride that jetting both in the desert and in the mountains.

With a stock exhaust I would start with a 162 main, 45 pilot with your JD stuff.

Good luck

I also think that a 170 is a bit rich, ...
JD kits tend to require larger jets do to the custom needles.

Regarding the fuel screw, you have a point as to the direct interchangeability of the settings. It's wise to trim the screw by ear rather than by counting:


Well, i adjusted the fuel screw and the problem is gone. Thank you for your help.

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