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TOPIC: Mikuni BS32 Jet Sizes

Mikuni BS32 Jet Sizes 30 May 2010 21:08 #372225

  • WABBMW
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My 1982 KZ650 CSR has the original Mikuni BS32 carburetors, stock air box and almost stock 4 into 2 exhaust. The original and present Main Jet size is 107.5. (My service manual shows main jets being 110.0 for non-US models). The jet needles only have one groove. Overall, the bike runs fairly well, but is weak on throttle response when running fast on the highway at 1/3 to 1/2 throttle.

I changed the front sprocket from 13 to 14 tooth, and retained the original rear sprocket with 33 teeth, which results in the engine turning 5,400 at a true 70 MPH.

In order to improve power output with the throttle open say 1/3, does anybody have a jetting recommendation for:
(1) Retaining the stock exhaust, and then also for
(2) Using the 4 into 1 exhaust.
Thanks guys.
Bill Baker
Houston, Texas
1982 KZ650 CSR
2008 Yamaha FZ1
2006 Yamaha FZ1
1977 Honda Supersport 750 four (sold)
1984 Honda Nighthawk 650 (sold)

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Last edit: by WABBMW.

Mikuni BS32 Jet Sizes 31 May 2010 07:18 #372262

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Bill Baker
Houston, Texas
1982 KZ650 CSR
2008 Yamaha FZ1
2006 Yamaha FZ1
1977 Honda Supersport 750 four (sold)
1984 Honda Nighthawk 650 (sold)

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Last edit: by WABBMW. Reason: I was far too wordy.

Mikuni BS32 Jet Sizes 31 May 2010 07:36 #372264

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WABBMW wrote:

I'll now try to be brief. 1982 KZ650 CSR; Mikuni BS32 with stock 107.5 main jets; stock 4 into 2 exhaust; stock air box. (My service manual shows main jets being 110.0 for non-US models).

In order to get the best throttle response while cruising in high gear at a steady 6,000 RPM on the highway, is there any benefit to increasing the main jet size? If so, then how much?

If I change to a 4 into 1 exhaust with a free flowing muffler, then what main jet size? Thanks.


For stock pipes try the 110 jets and raise your needles by using washers from the hardware store (4 mm ?)Make sure you measure the thickness so you are raising them all the same amount.

With the 4/1 try 112.5.
2002 ZRX1200R
81 GPz1100
79 KZ1000st daily ride
79 KZ1000mk2 prodject
78 KZ650sr
78 KZ650b
81 KZ750e
80 KZ750ltd
77 KZ400/440 cafe project
76 KZ400/440 Fuel Injected

www.dotheton.com/forum/index.php?topic=39120.0


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Mikuni BS32 Jet Sizes 01 Jun 2010 06:39 #372560

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Nebr_Rex:
I raised the jet needles by 4 mm. as you recommended. Haven't changed the Main jets yet, but what an improvement in throttle response in the 1/8 to 3/4 throttle position. The engine puts out more power, so the throttle does not need to be open near as much for a given amount of power. I must have really been running lean.

I am now considering raising the jet needles perhaps another 4 mm., or a total of 8 mm. from stock. I'll provide feedback in case anyone else might have a similar situation.
Bill Baker
Houston, Texas
1982 KZ650 CSR
2008 Yamaha FZ1
2006 Yamaha FZ1
1977 Honda Supersport 750 four (sold)
1984 Honda Nighthawk 650 (sold)

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Last edit: by WABBMW.

Mikuni BS32 Jet Sizes 07 Dec 2018 05:47 #794763

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I just bought a set of cv carbs for my ‘81 KZ650 CSR from your year model bike. My original mechanical vm carbs were destroyed by a moron. I’ve installed a MAC 4 into 1 header and will be using K&N pod filters. Any idea what size pilot and main jets I would need? I assume I have to drill out the brass retaining pins to remove the emulsion tubes, correct? They’ve never been out and I can only assume that they need cleaned or replaced. Thanks for any help you can give me. I’m also a Texan, by the way.

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Mikuni BS32 Jet Sizes 10 Dec 2018 08:28 #794882

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It's been so long since I disassembled these type carbs that i forgot about the brass washer under the jets. Glad I didn't whack it too hard before I remembered! The needle jets (emulsion tubes) came out relatively easily. I am almost done cleaning the carbs. I used a cheap Chinese ultrasonic cleaner that worked better than Chemdip with a lot less stink. I have two of the float bowls polished and most of the bodies cleaned up, just need to order pilot jets, main jets, and air mixture screws, along with the orings and the rubber choke plunger boots. Anyone know what jet sizes to use and air mixture screw settings? Getting excited about riding this old Kaw again. Next set of carbs to do are from the KZ1000 I just bought. Fun, fun!

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