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Mikuni RS36 fuel overflow issue

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Zomerset Zed
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Mikuni RS36 fuel overflow issue

#1 PostAuthor: Zomerset Zed » Tue May 14, 2019 3:25 pm

I am guessing that this is a known issue as there is a bit of chatter about it on various forums.

When the bike is on the side stand, fuel pisses out of one of the overflows, i'm guessing no.1 but i will investigate further tomorrow and no matter what i do (banging carb float bowl side) the only remedy is to get the bike upright or disconnect the fuel supply to stop the deluge. So i dont think its a sticking float or valve.

Carbs are 500mls and 12 months old, float heights are set to 17mm +/- 1mm as per Mikuni manual and are clean as inside with tank filter plus 2 inline sintered fuel filters.

Some suggest setting float heights to 21 or even 22mm, but this may compromise fuel delivery at WOT?

Does anyone have any EXPERIENCE with this issue and have a solution they have tried?

Will be investigating properly tonight/tomorrow.

Thanks in advance

Julian_Boolean
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Re: Mikuni RS36 fuel overflow issue

#2 PostAuthor: Julian_Boolean » Tue May 14, 2019 3:45 pm

My RS34s do this, I store the bike upright, the other option is to turn off the fuel before stopping the engine so that it drains the float bowls.
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Al
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Re: Mikuni RS36 fuel overflow issue

#3 PostAuthor: Al » Tue May 14, 2019 5:19 pm

As above from Julian but got pizzed off with that so bought another alloy bottom bung, the one with the 17mm hex on it. Drilled a hole in it and rivetted / glued a short bit of brass tube into it. Attach a bit of clear tube and use it whilst its running to set the fuel height dynamically which is far more representative of real world scenarios. Cant say what height my floats are because i dont need to measure them, just to know when set that the fuel level is 3 - 4 mm below the gasket face. Its also better at getting them all the same. Bung was about £15.
The main jets are well down in the float bowls so lowering the fuel height fractionally will not affect WOT. All you will notice is that you may need to turn out the fuel mixture screws a little more if the idle is weak and 'hunting'. As a guide; if the fuel screws are in further than the standard specs then the fuel height is probably already too high. The reverse is also true; if the fuel screws are too far out then the fuel level is too low.

The other problem i had with a much older set was the 'O' rings round the brass float needle seats were perished and compressed, (flattened out) and passing fuel round the outside even when the float valve was closed.. I bought four new seats with 'O' rings when there was nothing wrong with the brass seats so could have saved a bag load of money there and just bought the 'O' rings!
As far as i remember there was a choice of seats. They vary in the size of the hole the fuel flows through so if youre chasing HP you might want the larger diameter ones.

AL

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Zomerset Zed
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Re: Mikuni RS36 fuel overflow issue

#4 PostAuthor: Zomerset Zed » Wed May 15, 2019 1:46 pm

Thanks guys,

I will definitely make up a new float bowl screw with a clear tube fitting for setting up the actual fuel level, but for now i think i will lower the fuel level on no.1 first to try and eliminate the problem, then set all the others to match.

Was bloody embarrassing yesterday filling with fuel and it all pissing out on the forecourt :blush

Suppose i could put in a small catch tank as well?

Cheers,

Mike


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