Lousy Mileage - Rich Mixture?

Bonwit · Sep 21, 2009

My 850 with a single Mikuni conversion is getting lousy milage (27 to 30 MPG!). Other indications, like sooty plugs, point to too rich a mixture. Today I pulled the needle and found the e-clip set to the middle position. Is this a reasonable default? In changing it, should I only move it a notch at a time?

Any other ideas what may be causing this bad mileage. The bike runs okay, but this fuel flow is killing me!

ILLF8ED · Sep 21, 2009

Needle height affects 1/8 to mid openning of the slide. If you have the needle in the wrong position you will notice while slowly accelerating the power is not smooth. You didn't mention what venturi size Mikuni.

ML · Sep 22, 2009

I got a VM 34 on my 850 Mk2. Make sure the air filter is OK. Also fit the biggest you can. Try dropping the needle a notch at a time. Also check the float level. There is a circle mark on the main jet body that the arms of the floats should align with. Also the main jet should be 240 to 260, not more than that. It may be possible the needle and or needle jet is the wrong type. See if you can verify these settings -

Main Jet 240 - 260

Needle Jet P2 or P4

Needle 6DH3 or 6DH4

Pilot Jet 35 - 40

Air Jet 1.0

Slide 3

BrianK · Sep 22, 2009

I run Keihin FCRs, but I had a similar experience. Carbs came "prejetted for [my] application." Bike ran fine, but lousy mileage and black plugs. I ended up going from 155 to 140 mains and raising the needle clips several positions, as well as tinkering with the slow fuel and air screws.

I started with about 35 MPG and now get above 50. On extended cruises, I have topped 60 MPG.

Best I can say is one step at a time, lean it out a bit and see what happens. Keep an eye on your plugs.

Bonwit · Sep 22, 2009

ML said:
I got a VM 34 on my 850 Mk2. Make sure the air filter is OK. Also fit the biggest you can. Try dropping the needle a notch at a time. Also check the float level. There is a circle mark on the main jet body that the arms of the floats should align with. Also the main jet should be 240 to 260, not more than that. It may be possible the needle and or needle jet is the wrong type. See if you can verify these settings -

Main Jet 240 - 260

Needle Jet P2 or P4

Needle 6DH3 or 6DH4

Pilot Jet 35 - 40

Air Jet 1.0

Slide 3

I pulled it apart today and found a 260 Main, 35 Pilot, 2.5 Slide and 6DH3 needle. Not sure how to get to the Air Jet or the Needle Jet, but given the other numbers, I suspect they're in the ballpark. I raised the e-clip a notch and will give that a try. I think BrianK's advise is sound. . . One step at a time.

Thanks

splatt · Sep 23, 2009

Make sure your timing is correct for what ever type you are running ,undo the main jet and the needle jet should just push out through the top.

Lousy Mileage - Rich Mixture?

Bonwit

ILLF8ED

ML

BrianK

Bonwit

splatt

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