99 yz400f getting harder to run when cold...

I am new to the h20 cooled motorcycles scene (came from a 86 TT225 which was last run in 91).

Anyways, as the temps get cooler the bike is getting a LOT harder to keep running when cold. Is there anything I should do? Rejet or anything like that??

1st thing would be to check the Valve clearances. I haven't had to adjust mine yet but my son's Honda has the same problem and that is what is was...

I plan on swapping a 03 450 cam & thus was going to do it all at once. However the issues I am having started when the temp went from the 80's+ to 50-60's

You should get into the habit of checking your valve clearances at least twice a season. Also, be certain you change the timing chain out when you install the exhaust cam. They are very inexpensive and are the key to a good running engine.

Very easy to see a badly worn chain as soon as you remove the valve cover. While the tensioner is still on, grab the chain between the two cams and see if you can move it up and down more than a little bit. If it is way thrashed, you will notice it actually sags between the two cams before you even touch it.

As said above check the valve clearances. If you have never done it I would suggest replacing timing chain before it starts jumping timing. That chain has about reached the end of its service life.

Now if that is all good. install a 68 starter jet. NOT the pilot jet the starter jet. Its the other little jet next to the pilot. You have 3 jets in the bottom of the carb, the starter, the pilot, and the main. The pilot and starter are on either side of the main.

The starter jets job is to richen up the choke circuit. Does your bike start and stall a lot when you are are trying to start it in cold weather?? After about 15 attempts at this it finially stays running? this is a lean choke circuit. A bigger starter jet will be a huuuge help in getting it running.

having the proper main, pilot, and needle position is also important. But as its only 50 deg out I would say that your stock jetting is pretty close still. Maybe a littlel lean but probably not enough to worry about yet. When it gets to 40-30deg and below you have to start worrieng about jetting.

if the temps drop below 30 install a 72 starter jet.

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