600R and K&N Filter

Trying to sort out a new to me 600R.  Stock carb, airbox and exhaust.  It does have a K&N filter.  Have replaced the choke plate.  Idle and low speed jet work perfectly.  Easy to start 3rd or 4th kick when cold. Idles like it should, Blip the throttle while sitting still - perfect response.  After warming up coughs and falls flat in upper RPM range under load.  Have totally disassembled the carb and cleaned it twice.  Reading the plug seems to be lean.   Picking up a 155 main jet tomorrow.  My question is-   Could the increased air flow through the K&N filter by itself cause the bike to run lean.

Thanks, Charlie

That shouldn't be enough to make that much of a difference, however running a larger jet would be my first thought. Install, run and check the plug :)

Thanks for the reply.  I'll get the new main jet put in this weekend.  Hopefully the fix will be that simple.  I want to get the stock bike running right before I begin adding modifications such as pipe and possibly a FCR MX carb.  I've been reading about FCR39's and 40's.  Does it really make a difference between the two?

Have you cleaned your airfilter. K&N make a special cleaning kit for the filter

Hey Charlie,


I dont think the filter is the main cause, but probably adds a little to the equation. These always come lean from the factory. Also, if you stil have the intake boot leak, that's the more likely cause. I know you have a stock pipe still, is the plug still in the end of it? By the way, if you ride in a lot of dust, I'd consider a foam filter...


From what I have read, the FCR size differences wont matter.....

Edited by fourstrokin650

Only difference between the 39 and 40 or 41mm FCR is 1 or 2mm  in bore size..on a highly modified engine you might possibly loose a little top speed.....probably not anything you`d notice..get a late model FCR MX though as the AP linkage was updated,amonst other things..



As said, you are most likely lean in the main jet department. K&n filters are not that great and all though it is probably not your problem, i would get a uni or twin air foam filter. If the bigger main jet doesnt solve the issue, start looking for a fuel flow issue.

A 155 main is stock, or close. With basically a stock bike, the jetting shouldn't be so far off as to cause the engine to cut out. The problem is probably something else.

You do know that these bikes, stock, will cough and sputter if the throttle is whacked open quickly?

Have you cleaned your airfilter. K&N make a special cleaning kit for the filter

found the special cleaning kit




Edited by Zimman

found the special cleaning kit





I like that!  I agree 100% . 

I doubt the filter is your problem, yet.  But do use the above cleaning method.

Should I fill the garbage can up with kerosene or gasoline to clean the filter.  Yes, I am joking.  I prefer foam also. 

Yes the alternate method of cleaning is to soak the filter in either kerosene or gasoline.  When fully saturated ignite with an open flame source.

Has anyone seen the K&N ad on the homepage? It's a guy leaning against a truck with the caption "I drove a million miles on one K&N filter."

It doesn't say if he left it behind the seat and never took it out of the box, or how many engines he used while using that rock filter.

They work ok on boats, and snowmobiles.

They work ok on boats, and snowmobiles.


the dust free environment is where they shine  LOL


  but they also let water in unimpeded  and that can cost u a PWC or Boat motor in seconds  

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