I am wondering if and what effects going from a 140 main jet to a 150 main will do? Will I need to make any modifications anywhere? The bike an '03 650 bone stock with 7500 miles. I have drilled out the brass cap and went ½ turn more out and that made a noticeable difference. Thanks in advance.

Do yourself a favour and get a Dynojet kit, which includes a variety of jets and a decent needle to replace the stock one. Pilot screw mainly influences the slow speed running and the main jet mainly influences 3/4 to full throttle, so you need to take care of the mid range as well, hence raising the needle (either by shimming the stock needle, or fitting another custom needle e.g DJ) You'll also need to open up the airbox to balance out the need for extra air with the supply of extra gas that bigger jets, higher needles give you. Either cut the top open a la Jesse's photo on his site, or take the side cover of the airbox off, making sure you don't lose the captive nuts. Dave.

adventurebob2004 said:

he said don't lose your nuts!! ha!

You can put in a 150 main and pop the top plug off the airbox. I have not used the DJ kit. My bike has a 160 mikuni main jet the top plug off and the side cover off. Put the screws back in the airbox side cover or the nuts will get lost. The slide has two holes in it instead of one and the needle is shimmed. I think the DJ main jet of the same size is a 170. The bike runs great at all RPMS with the stock pipe, not bad for a $2.00 main jet. You will need to drill the brass plug covering the pilot adjuster and open the pilot screw till it runs well (2.5-3 turns approx.). I am sure there is more to be had as far as performance but it has more than enough for the dirt and it goes 100MPH on the road. Dropping a tooth on the front sprocket really helps in the dirt. It will take about 5 MPH off your top speed and probably won't help your miliage. Jetting won't help your miliage either but the gain in fun factor is worth it.

Perry

MapIt said:

If after doing what you describe, you have developed a "flat spot" just before
you get over 3/4 throttle, then all you need to do is to shim up the needle
about 1.5mm and go up to maybe a 150 Mikuni main jet.
The bike will run well set up like this.
If you still have a little flat spot left you will have to try a needle with a bit
more of a taper on it, allowing more fuel via the Main/needle jet a tad earlier.

A DJ kit has everything you'll need in it, so if you don't mind shelling out the
$55... or more for it, it's well worth your while.

Check out this thread, it has lots of usefull info in it on this subject:
http://www.thumperta...80&parentpage=1