Brand new single Mikuni fuel puking issue

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67n15cs

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Just getting round to putting the 1971 Commando on the road. I got a single Mikuni conversion kit several years ago and am now setting things up. The carb has been sitting in a box and not exposed to dust or what-not. With fuel in the tank and one of the petcocks open, fuel is puking out of the carb overflow; but only if the petcock on one side is open. The other side stays leak free when open.

This indicates either that there is crud in the float chamber holding the float valve open, or that the float height has been incorrectly set at the manufacturer. Both of which seem odd for a quality built Japanese product..... anyone had the same issue?

But neither would account for the fact that the overflow leak only occurs when only one of the petcocks is open...... :wink:

Does anyone know the correct air screw 'turns' out setting for this application?
 
The floats are probably just stuck down from sitting, just give the float bowl a couple of light taps with the plastic end of a screwdriver. Have you checked that the fuel taps/lines arent blocked as well, this could be why it only leaks with one side on and not the other.
 
I'd drop the float bowl and make sure the needle is free to move. Sometimes they get stuck and don't move. Also check that the float moves freely and is not stuck on its axle.
As far as petcocks go,you may not have a full tank and only the reserve valve flows.
Jaydee
 
Mikunis are famous for this problem. I had a new set on a bike, tried a number of things but it never totally went away. Every now and then the float would stick, usually if the side stand was used. These were 30mm Mikunis. I replaced them with new 34 mm Mikunis and they have been fine. Several of the people in the club run Mikunis and a couple have the sticky float problem, usually if the bike has been sitting for a few months though. The float valve will usually start to work ok after the bike is ridden some, but not always. When it does it from new, like mine did, or yours does, it does make you wonder.


Glen
 
DO NOT tap on the carb, pull the bowl off using a JIS #2 screwdriver, visually inspect the float needle and seat. Check the viton tip of the needle, crud may have formed. (I heard ya', no gas) The seat may have some fuzz from the rubber. Report back. The one petcock thing may be as simple as.. low fuel? Only reserve flows fuel?
 
67n15cs said:
Just getting round to putting the 1971 Commando on the road. I got a single Mikuni conversion kit several years ago and am now setting things up. The carb has been sitting in a box and not exposed to dust or what-not. With fuel in the tank and one of the petcocks open, fuel is puking out of the carb overflow; but only if the petcock on one side is open. The other side stays leak free when open.

This indicates either that there is crud in the float chamber holding the float valve open, or that the float height has been incorrectly set at the manufacturer. Both of which seem odd for a quality built Japanese product..... anyone had the same issue?

But neither would account for the fact that the overflow leak only occurs when only one of the petcocks is open...... :wink:

Does anyone know the correct air screw 'turns' out setting for this application?
Click to expand...

Seems like I read 1 1/2 some where for the flat slide 34 if that's the one you have.
 
seaguy said:
67n15cs said:
Just getting round to putting the 1971 Commando on the road. I got a single Mikuni conversion kit several years ago and am now setting things up. The carb has been sitting in a box and not exposed to dust or what-not. With fuel in the tank and one of the petcocks open, fuel is puking out of the carb overflow; but only if the petcock on one side is open. The other side stays leak free when open.

This indicates either that there is crud in the float chamber holding the float valve open, or that the float height has been incorrectly set at the manufacturer. Both of which seem odd for a quality built Japanese product..... anyone had the same issue?

But neither would account for the fact that the overflow leak only occurs when only one of the petcocks is open...... :wink:

Does anyone know the correct air screw 'turns' out setting for this application?
Click to expand...

Seems like I read 1 1/2 some where for the flat slide 34 if that's the one you have.
Click to expand...

Yup, 1.5 turns out from lightly seated is the standard baseline. It's a fuel screw, counterclockwise richens the mixture. Clockwise to lean.
 
Yup, 1.5 turns out from lightly seated is the standard baseline. It's a fuel screw, counterclockwise richens the mixture. Clockwise to lean.
Click to expand...

respectfully disagree, and someone will correct me if I am wrong please for the past 40 years

it IS an "air screw", and NOT a fuel screw, and the above quote has it backwards

turning the air screw IN riches the mixture and turning it OUT leans the mixture, and yes 1 1/2 out from seated is the standard setting
 
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