Mikuni problem
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Fast Eddie
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Almost certainly a partially blocked pilot jet, they gum up, presumably due to ethonol.
This might help you find your way around:
http://www.mikuni.com/pdf/vmmanual.pdf
This might help you find your way around:
http://www.mikuni.com/pdf/vmmanual.pdf
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There are a lot of possible reasons...although your description could use more detail.
Pilot jet and the passages through to the pilot screw could be dirty. Many people just clean the jet but the screw should be removed and carb cleaner sprayed both ways until clear...a wire probe might be required if really dirty. You need to insure that the spring, washer and o-ring on the pilot screw are put back in the right order...o-ring should be replaced if possible. If you don't put them in the right way, air will get sucked in and not possible to adjust. The adjustment is also important for idle. Proper way is to get engine up to temp and set mechanical idle up a bit (higher RPM). Adjust the pilot screw until fastest RPM is achieved. Some say to back it off a 1/4 turn but not necessary. Then set the mechanical idle back to spec RPM.
Pilot jet and the passages through to the pilot screw could be dirty. Many people just clean the jet but the screw should be removed and carb cleaner sprayed both ways until clear...a wire probe might be required if really dirty. You need to insure that the spring, washer and o-ring on the pilot screw are put back in the right order...o-ring should be replaced if possible. If you don't put them in the right way, air will get sucked in and not possible to adjust. The adjustment is also important for idle. Proper way is to get engine up to temp and set mechanical idle up a bit (higher RPM). Adjust the pilot screw until fastest RPM is achieved. Some say to back it off a 1/4 turn but not necessary. Then set the mechanical idle back to spec RPM.
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+1 on the idle jet.; there is a possibility that the idle air circuit is also got some blockage. Unless you have some sophisticated cleaning equipment it is a much better proposition to just purchase a new idle (pilot) jet.
You could, certainly, have a mole (Avogadro's number) of other issues; I'm sure that the faithful will enumerate them in due course, and may hit several nails on the head, but the idle jet has been #1 in discussions I have had with scores of customers...
You could, certainly, have a mole (Avogadro's number) of other issues; I'm sure that the faithful will enumerate them in due course, and may hit several nails on the head, but the idle jet has been #1 in discussions I have had with scores of customers...
Thanks for all your replies, today i will take it of AGAIN !!!
thanks fast eddie for the link , i must have found every mikuni guide apart from that one !
If ive missed any blockages i will eat my hat, but shit happens lol
The choke operation does baffle me a touch i have to admit, i will eat the manual for breakfast
One question , is there anything that could be at fault with the choke plunger itself ?
thanks fast eddie for the link , i must have found every mikuni guide apart from that one !
If ive missed any blockages i will eat my hat, but shit happens lol
The choke operation does baffle me a touch i have to admit, i will eat the manual for breakfast
One question , is there anything that could be at fault with the choke plunger itself ?
Fast Eddie
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xxGraham said:Thanks for all your replies, today i will take it of AGAIN !!!
thanks fast eddie for the link , i must have found every mikuni guide apart from that one !
If ive missed any blockages i will eat my hat, but shit happens lol
The choke operation does baffle me a touch i have to admit, i will eat the manual for breakfast
One question , is there anything that could be at fault with the choke plunger itself ?
Your symptoms are ‘it won’t run right without a little choke’.
Now, IF my memory serves me correctly, the cold start device on the Mikuni allows extra fuel into the circuit when ‘ON’.
Therefore, your symptoms seem to suggest that the carb is overly weak, hence you need the choke ‘ON’ to make it run. Hence this is why I suspect a blockage somewhere.
I cleaned out a Mikuni once and was sure all was clear, but only when I removed the pilot jet did I see that it was partially blocked.
Again, IF my memory serves, the choke plunger has a rubber pad which stops the extra fuel getting into the system when the choke is ‘OFF’. Hence, a faulty choke normally means a damaged pad, which causes an over rich mixture, which would then NOT be made better by switching the choke ‘ON’. So… I would doubt the choke is faulty.
Having written all of this, I now realise that incorrect use of the choke would also cause strange symptoms. You don’t say how familiar you are with the bike, could it be that you are using the choke in reverse inadvertently?
Fast Eddie
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xxGraham said:Wooooooohoooooooo , Sorted , looks like i have to eat my hat :roll: , fortunately i dont have one
Good show!
What was it?
Fast Eddie
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Splendid.
Onto the next problem...
Carry on !
Onto the next problem...
Carry on !
lazyeye6
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Fast Eddie said:
Ok. My Mikuni installation developed an air leak between the carburetor and the head. I correctly diagnosed a leak between
the manifold and the head and corrected it with new gaskets and a thing coating of gasket sealer. Then I found I still had an
air leak between the manifold and the rubber mount. I removed it and all looked well, so I coated the mating surfaces with
a bit of gasket sealer. Well it's better, but there is still some air leakage. Any ideas on how to get this joint perfectly sealed?
Fast Eddie
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xxGraham said:
Actually, normal silicone is dissolved by petrol, so is a poor choice in this application.
It's sounds like at least one of your surfaces is not flat. If they are flat, and a correct gasket is used, it will seal. Adding some petrol proof sealant will doubly ensure it seals.
There is no more to it than that IMHO.
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xxGraham said:
Not to be a jerk but there was an earlier comment correcting the terminology used for this jet calling it an idle jet which is not correct.
There can be confusion about this when the terminology is wrong. Mikuni calls this the "pilot jet". The screw that is used to adjust it is the "pilot screw". The circuit in the carb is called the "pilot circuit". While this is essentially the idle circuit as most people know it, it actually overlaps into the fuel delivery beyond idle as the main jet opens. When purchasing parts for a Mikuni carb the proper name is pilot jet.
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