Mikuni HS40 smooth bore ??

7 replies to this topic
  • MAX2

Posted April 29, 2005 - 09:53 PM


I have a like new Mikuni HS 40 Smooth Bore carb i took of a Harley FXR. would it be any good on a xr650r or is stock better ??


Posted April 30, 2005 - 02:32 AM


HS40 = TM40

http://www.thumperta...ght=mikuni tm40

  • frankstr

Posted April 30, 2005 - 04:20 AM


Is it a pumper carb... :naughty: If not get a Edelbrock........ :naughty:

  • MAX2

Posted April 30, 2005 - 02:52 PM


yes it is a pumper carb

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  • MAX2

Posted April 30, 2005 - 02:57 PM


the carb is a HS 40 not a TM 40 ? what is the diffrence ??

  • BWB63

Posted April 30, 2005 - 06:59 PM


The HS40 is the mikuni pumper carburetor for Harley's before the Mikuni HSR 42. It is a lot like the TM40. They use the same jets and are all but, the same. I have ran both the Mikuni TM40 and the Mikuni HSR 42 on the XR650R stock and bored to 680cc. The Mikuni HSR 42 seemed to give more power everywhere but, real noticable down low. I thought it ran wet (Not all the fuel burned, that's not the same as ritch) But, I switched to a TM40 and haven't put the Mikuni HSR 42 back on to see what was up. I used pvc for the adapters (worked real good) for both the TM40 and HSR42. The TMR40 Mikuni is the way to go for a stock XR650R out of the Mikuni line up. If the HS40 was off a Harley you will have about a 160~165 main jet and a Y-6 needle jet, a 22.5~30 pilot and a 0.9 air jet. For a stock XR650R (uncorked with a free flowing exhuast) You will need something close to this: 1.2 air jet, 22.5 pilot, Y-4 needle jet, 9Doj-1 needle, and a 135~137.5 main jet.

  • MAX2

Posted May 01, 2005 - 07:25 PM


thanks BWB63, so in your opinion it would be worth replacing the stock carb with this one. dose the air filter boot fit or will i have to make an adapter

  • BWB63

Posted May 01, 2005 - 07:50 PM


You will have to make adapters for the front and back of the carburetor. I used pcv turned down on a lathe to fit. I have seen some real cool aluminum ones made. Sudco and XR's only has them but, I do not know if they sell them apart form the kit. The Mikuni TM40 is the smoothest carburetor I have used on the XR650R (once tuned). I hope your HS40 works out as well. The air jet was a big deal, the TM40 and HSR42 ran real bad at the bottom intill I changed the air jet to a 1.2. Didn't matter what pilot I used there just wasn't enough air to atomize the fuel. I ran a little fat on mine 25 and 27.5 pilot and ran the pump a little late. The needle jet and the needle you run will make a huge difference. The TM40 kits they sell for the XR650R have a 8djh-4 and Y-8 needle jet, some have had lots of trouble with that setup. You might have the Y-6 I found the Y-4 the best. The squirt from the pump nozzle must hit the needle. The pump nozzle moves with needle nose plires on the TM40. It can be a fight to tune a unknow carburetor but, it will be worth it. Pumpers :naughty: rock compared to the stock Vacuum flow carpuetor. Please let me know the out come. I can put the info on my carburetor page.

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