Mikuni Air Screw setting

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obiwaynekenobi

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Sep 25, 2007
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Hello, New guy here so here goes. I have 73 MkIII engine in my Norton Commando Clubman all stock parts except 34mm Mikuni. Ran Ok before but would not charge while running and would die and needed new alternator so installed 200volt and rebuilt the 34 Mikuni Carb. Now after this bike won't start. Put in new resistor plugs and wires Coil is new, getiing good spark and appears to be getting Fuel. When I try to kick it it sputters and sometimes backfires. I believe it is the Carb and I just don't know what initial setting the Air Screw needs. I have tinkered with different settings and I get close to a start but no cigar. I have read the Mikuni manual but it doesn't really make it easy to find the intial Air Screw setting. It is 70 degrees outside and my battery is fully charged. I have not discounted timing (28TDC) but my Norton buddies all think its not the culprit. Can anyone shed some light here? Thanks!
 
The starting point for the air screw is 1.5 turns out from seated.
What size pilot jet are you using?
Are you trying to start it with the choke on or off?
The 200watt alternator is 3-phase? With three phase reg/rectifier?
The plugs are resistor, are the caps and wires too?
What ignition are you using?

If you are starting the bike cold with the choke, (choke all the way on, throttle all the way closed) it wouldn't matter what jets or settings you have because they are bypassed by design, when the choke is being used.
 
obiwaynekenobi said:
Hello, New guy here so here goes. I have 73 MkIII engine in my Norton Commando Clubman all stock parts except 34mm Mikuni. Ran Ok before but would not charge while running and would die and needed new alternator so installed 200volt and rebuilt the 34 Mikuni Carb. Now after this bike won't start. Put in new resistor plugs and wires Coil is new, getiing good spark and appears to be getting Fuel. When I try to kick it it sputters and sometimes backfires. I believe it is the Carb and I just don't know what initial setting the Air Screw needs. I have tinkered with different settings and I get close to a start but no cigar. I have read the Mikuni manual but it doesn't really make it easy to find the intial Air Screw setting. It is 70 degrees outside and my battery is fully charged. I have not discounted timing (28TDC) but my Norton buddies all think its not the culprit. Can anyone shed some light here? Thanks!
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not sure its the carb but - do you have the choke raised? do not twist throttle when kicking. if you must guesstimate airscrew 1.5 - 2 turns is typically close enough to "run"

also have you removed the plugs and smelled gas on them?
 
If you are certain the carb has been rebuilt properly, you have a good spark and the plugs are getting wet with fuel, then simply remove the plugs and dry them with something like a butane blow lamp, refit them, and try kicking with no choke and slightly open throttle. Very poor modern fuels tend to mean plugs track to earth very easily, and wetted plugs which are shorted to earth often mean a bike wont start.
 
Thanks Guy's for the posts, I will try them today and let you know what happens. I am going to recheck static timing as well and dry the plugs as suggested.
 
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