Mikuni carb pouring out fuel when engine swithched off.

Peter Waller · May 17, 2022

Hi. I have a Mikuni single carb conversion on an 850 Commando I have just bought and it runs superb......BUT when the engine is switched off fuel starts to run out of the pink plastic tubes that come out of the carb. It does not do it every time but leaves a mess when it does and the only way you can stop it is to turn off the fuel taps. Any ideas please! Sticking float perhaps????
Thx in advance

baz · May 17, 2022

Peter Waller said:
Hi. I have a Mikuni single carb conversion on an 850 Commando I have just bought and it runs superb......BUT when the engine is switched off fuel starts to run out of the pink plastic tubes that come out of the carb. It does not do it every time but leaves a mess when it does and the only way you can stop it is to turn off the fuel taps. Any ideas please! Sticking float perhaps????
Thx in advance

Try tapping the carburettor with the handle of a screwdriver
It should stop
If it does stop then it's definitely the float needle not seating , very common with a mikuni

concours · May 17, 2022

Needle & seat. What is your skillset? Do you have a talented friend you can bribe with beer to help?

Fast Eddie · May 17, 2022

Yup, needle and seat. Quite possibly dirt twixt the two. Do you have quality in line filters fitted?

baz · May 17, 2022

If it is the needle valve leaking and 99.9% of the time it is
Remove it from the float
Clean and examine it ,if there's any splits etc bin it
Take a look at the needle seat ,is it perfectly clean?
The slightest crud will cause a leak
Take a Qtip with some metal polish ,put it in a cordless drill and lap the needle seat in
This has worked for me many many times
Another problem can be the O ring that goes between the needle seat housing and carburettor body perishing

Schwany · May 17, 2022

Float level check while the float bowl is off might be prudent.

baz · May 17, 2022

Schwany said:
Float level check while the float bowl is off might be prudent.

Always worth checking float height but the problem here is leaking only when the engine isn't running
Float would be same height running or not ?

Schwany · May 17, 2022

baz said:
Always worth checking float height but the problem here is leaking only when the engine isn't running
Float would be same height running or not ?

If the fuel petcocks are open it could leak sitting still, but I get your point it might leak all the time. I also know it is intermittent. I would look at it in addition to looking at the needle valve as standard procedure if my carburetor was leaking out of the overflow tubes. It is most likely a small piece of crud in the float need valve as already mentioned a few times.

Craig · May 17, 2022

Needle sticking , dirt ,Baz has it right , always good to keep fuel taps off if engine not running , the screw driver tapping worked for me both times your symptoms happened to me over 20+ yrs ….