Mikuni VM36 dripping fuel

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Luke Turner

Luke Turner

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Within the last couple of months I've had the carb refurbished & I've fitted a new wiring loom along with a new alternator stator plate & a Boyer power box.

I noticed on my first run out that there was a fairly constant drip of fuel from the lower carb vent.

She wasn't running brilliantly either, she seemed okay to begin with but wouldn't idle when warmed up, she kept cutting out so I'd have to push her up a side street & kick her over. Getting home meant blipping the throttle at junctions etc to prevent her from cutting out.

I've had the carb off to measure the float height and it was sitting at 18mm. I read it should be between 17 & 19 so I've taken it down to 17mm. I haven't had a chance to get the bike back on the road yet but is there any other reason why I should be dripping fuel at quite a quick rate?

1974 MKIIa with a Boyer electronic ignition, Boyer power box & Mikuni VM36.
 
Stuck needle, sunken float, particles in the gas/carb preventing full closure. If I understand you correctly the vent you are referencing is the overflow "drain" it keeps a malfunctioning carb from dumping gas into your engine. If gas is coming out of it don't try to ride your bike until you sort it out.
Go back and recheck everything, get some in-line filters.
 
Luke Turner said:
Within the last couple of months I've had the carb refurbished & I've fitted a new wiring loom along with a new alternator stator plate & a Boyer power box.

I noticed on my first run out that there was a fairly constant drip of fuel from the lower carb vent.

She wasn't running brilliantly either, she seemed okay to begin with but wouldn't idle when warmed up, she kept cutting out so I'd have to push her up a side street & kick her over. Getting home meant blipping the throttle at junctions etc to prevent her from cutting out.

I've had the carb off to measure the float height and it was sitting at 18mm. I read it should be between 17 & 19 so I've taken it down to 17mm. I haven't had a chance to get the bike back on the road yet but is there any other reason why I should be dripping fuel at quite a quick rate?

1974 MKIIa with a Boyer electronic ignition, Boyer power box & Mikuni VM36.
Click to expand...


Needs a new float needle, possilble seat. Viton is best if you can get it.

https://www.jetsrus.com/needle_valv...eedle_valve_and_float_bowl_gasket_mikuni.html
 
NPeteN said:
Stuck needle, sunken float, particles in the gas/carb preventing full closure. If I understand you correctly the vent you are referencing is the overflow "drain" it keeps a malfunctioning carb from dumping gas into your engine. If gas is coming out of it don't try to ride your bike until you sort it out.
Go back and recheck everything, get some in-line filters.
Click to expand...

It's all been stripped, cleaned & rebuilt. I'll look at adding an in-line fuel filter on each side but I did also fit 2 new fuel taps with filters.

I took the float bowl off & filled that with fuel to test the floats & they're both buoyant.
 
Steve is fairly confident it's an issue with debris from the fuel tank. He's advised me to take the float needle out of the seat & blow it all through with carb cleaner & compressed air.

I'll also fit fuel filters on both sides underneath each tap.
 
Luke Turner said:
Steve is fairly confident it's an issue with debris from the fuel tank. He's advised me to take the float needle out of the seat & blow it all through with carb cleaner & compressed air.

I'll also fit fuel filters on both sides underneath each tap.
Click to expand...
Take out the float needle and clean it
Then get some metal polish on a Q tip and push it into the float needle seat, then spin it with a cordless drill for a 30 seconds or so and repeat if necessary
I have done this umpteen times with mikunis
If you fit fuel filters make sure the fuel flows freely because some are designed to work with a fuel pump
Ps this could be your Norton's way of telling you to fit a pair of Amals !!!
 
baz said:
Take out the float needle and clean it
Then get some metal polish on a Q tip and push it into the float needle seat, then spin it with a cordless drill for a 30 seconds or so and repeat if necessary
I have done this umpteen times with mikunis
If you fit fuel filters make sure the fuel flows freely because some are designed to work with a fuel pump
Ps this could be your Norton's way of telling you to fit a pair of Amals !!!
Click to expand...
A wooden match stick with Mother’s Mag polish works splendidly as well.
Done it dozens of times. Most times the existing needle works fine as well when cleaned up.

ALSO, the cheap Chinese pattern carb parts/“kits” are shitfull junk, create more problems than they solve.
Use genuine parts on Mik/Keihin carbs.
 
Cheers!

Just a thought, since refurbishing the carb I fitted some new fuel hoses too, they're 5/16". What should they be? 1/4"? If so, it's bigger than it should be so could that be causing more fuel to be dragged in hence the overflow?
 
Luke Turner said:
Cheers!

Just a thought, since refurbishing the carb I fitted some new fuel hoses too, they're 5/16". What should they be? 1/4"? If so, it's bigger than it should be so could that be causing more fuel to be dragged in hence the overflow?
Click to expand...
Nope it won't be that mate
 
Luke Turner said:
Just a thought, since refurbishing the carb I fitted some new fuel hoses too, they're 5/16". What should they be? 1/4"?
Click to expand...

Mikuni is metric. The stub will be for either a 7.0mm or an 8.0mm hose.

Luke Turner said:
If so, it's bigger than it should be so could that be causing more fuel to be dragged in hence the overflow?
Click to expand...

No. A carb doesn't 'drag' fuel in.
 
Do some research on how carburetors work and you will save yourself many headaches. Your floats rise with the fuel level in your bowl, once they have reached the appropriate height they will close the needle valve by pushing it up into the seat, sealing the entry and cutting fuel flow. Take your carb off and poke around, get familiar.

As long as your bowl is filling it doesn't matter the size of your fuel line. I've never had a completely clean gas tank so I've always used in-line filters.
 
I knew it was a daft question but wanted to rule it out!

Will see how things are after a clean through & some new in-line fuel filters.
 
baz said:
If you fit fuel filters make sure the fuel flows freely because some are designed to work with a fuel pump
Click to expand...

+1 on that. Before I knew better I once fitted filters which were not designed for gravity feed to a Honda and it ran and performed terribly.
 
Luke Turner said:
I knew it was a daft question but wanted to rule it out!

Will see how things are after a clean through & some new in-line fuel filters.
Click to expand...
It's not a daft question, no questions are daft
My money is on the needle valve seat needing a hone
And the taper on the needle valve needs a clean/replacement
Or just hit it with a hammer until it stops dripping
Cheers
 
Happened to me once in about 2007 , left fuel tap on with bike shut down , opened shed door and a small puddle starting under bike , tapped carb bowl with end of screw driver and it stopped .... cleaned carb in the morning , never again , no fuel filters .... mikuni VM34
 
Japanese carbs like Mikuni and Keihin are FAR more sensitive to dirt in the fuel than Amal’s. Amal’s will run on grit compared to these!

I had a repeating issue with flooding / overflowing CR carbs once, I stripped them repeatedly and couldn’t find ANYTHING in there. But good in line filters cured it.

This issues needs sorting out, don’t ignore it, as mentioned above this could be dumping petrol into your engine which could cause serious issues...
 
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