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FAQ Big bore vs. high performance kit?

Should I install the big bore or high performance kit?

Q:  I'm trying to decide between just the big bore kit or the complete kit with the high performance cam. What is the difference in power & performance between the two? Or, am I comparing apples to oranges to some degree. Should I be thinking of the big bore vs. the high compression, and figure the cam should be used for either?

  • Also, should I assume that I need to upgrade the carburetor and exhaust as well for both these upgrade routes?
  • Lastly, for a carburetor, I need to sort out whether I should go with the Mikuni TM40 pumper upgrade or try to go with the Keihin FCR-MX? How does the Mikuni perform vs. the Keihin?


A:  It is somewhat a case of apples and oranges. The high compression piston and cam will give you crisper engine response and a motor that is more willing to rev and makes more power everywhere with a better top end pull. The big bore kit by itself will give you a motor with a powerband similar to stock but with the biggest gains mostly in the low end and mid-range. For the best of both worlds add the cam to the 790 kit.

  • Upgrading the exhaust and carburetor are not required but will definitely help make the most of the engine modifications.
  • Performance wise, the TM40 and FCR are nearly equal. The TM40 will flow slightly more air so might have a small power advantage in a well built and tuned motor. The choice between the TM40 and FCR comes down to what is practical for you. Our TM40 kits are a 'plug and play' setup. New carb, jetted for your bike, no tinkering necessary. Most people setting up an FCR will start with a used carb from a Yamaha YFZ450 ATV and have to make their own modifications to make it work. IF you can find a GOOD used carb and get it for a good price the FCR can be done for less money than our TM40 kits. At this point there isn't anyone offering brand new FCR carbs kitted up for the DR650. New FCRs are very expensive. One technical advantage to the FCR is that there is an air cut valve that decreases deceleration popping noise from the exhaust. Both the stock carb and the TM40 will cause popping noise from the exhaust during deceleration with closed throttle.