Jetting for new cam


What are your experiences about jetting when changing cam on a XR650R? Did you have to increase jet sizes or not ?

Which jets and how much?

I increased the main jet from 152,5 to 157,5 but it still feels a bit flat on top (doesn´t rew fully). This gets me into thinking about changing the other circuits also so it would be great to get some info.

BTW Great forum, many thanks to all of you.


That seems a bit lean to me. I run a 168 for altitude of 7500+ feet. I think they recommend somewhere between 172~175 for sea-level. Oh, that's for the uncorked bike. I don't really know about what changes for the cam though. I'm sure someone on here knows more than I do.

Sorry, forgot to tell you I run a Mikuni TM40 carb. 157,5 is quite rich and this is why I´m getting a bit curious.


Ah-ha. Not sure about that carb. Sorry about that.

Good Luck

It could be that you're too rich on top as well. I don't run a TM40, but there's a few people around that do and I recall them using significantly smaller main jets. Check with "BWB63" here on TT cause he's got a TM40 and has it dialed in pretty well based on reading some comments from a group of people who were trying each others bikes that said his bike felt really good in terms of power delivery and throttle response and ripped compared to their stock uncorked bikes. From reading BWB63/Bruce's posts a while back when he was tinkering with his carb, it sounded to me like he put some time into finding out what works and what doesn't and that fact that other people felt his bike ripped compared to their's tells me he's doing something right :)

Based on his signature here on TT, he's running the following setup with great results and I think he's usually riding at altitudes of about 1,000 to 3,000 feet in 50F to 75F temperatures, but he can confirm that for you incase I'm wrong.

*** Mikuni TM40 Specifics ***

Main Jet = 140

Air Jet = 1.2

Pilot = 25

Needle = 9ojy1 neadle

Needle Clip = 3rd position

***Bike Specifics***

UNI Filter

Side Panel cutout/holes

White Bros. R4 exhaust cannister

XR's Only header

As far as jetting for a new cam, it really all depends on the specific cam profile because some cams make their power early on while others make their power up higher, etc.

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