I have just fitted a single 34mm Mikuni to my 1972 Commando. After 200 miles the left plug colour is correct, i.e light brown, but the right one is sooty black. Before fitting the Mikuni I used the bike for 1500 miles since purchase on the original twin Amal Concentrics. I had a similar situation, i.e right side running rich, which I resolved by unscrewing the pilot screw more on the right carb than the left. This got the plugs a similar colour. I thought the problem was due to the carbs being worn, so didn't expect the same situation with a single carb. I have swopped the coils which made no difference. The valve clearances are correct at 6 inlet, 8 exhaust. Please does anyone know what might be the problem and how to fix it? Thanks, Simon, Wiltshire, England.
Mikuni running rich on right cylinder
- Thread starter septamus
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If you have no air leaks that usually means:
1. Loose needle jet in the carb or enrichener piston that does not seal. Either of these can cause raw fuel to end up in one cylinder or the other.
2. leaky valves or worn cam
3. a buildup of carbon on the RH piston which will raise the compression ratio and cause that cylinder to pull more fuel from the carb. Likely if that side has been running rich for a while. Jim
1. Loose needle jet in the carb or enrichener piston that does not seal. Either of these can cause raw fuel to end up in one cylinder or the other.
2. leaky valves or worn cam
3. a buildup of carbon on the RH piston which will raise the compression ratio and cause that cylinder to pull more fuel from the carb. Likely if that side has been running rich for a while. Jim
I have same problem with VM36 but not all the time.Does any one know what is air jet purpose.I wondering why top of the slide was gasoline and today it took out airfilter and started engine and let it running on idle speed.Then i saw sometimes airjet hole came gasoline drops out-what that means?.i have heard that some engines has airjet blinded...
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Hey Mrnorton,
The Air jet, or Air correction jet, located at 6 O' Clock in the rear of the Mikuni is there to help atomize or
emulsify the fuel at the Needle jet as it rises up from the Main jet.
The Air jet corresponds with the hole in the side of the Needle jet, near the top.
Theoretically it doesn't do much at anything under 3/4 throttle, but after that it does start to work
in conjunction with the Main jet.
I wouldn't remove it or blank it off. This is a common falacey bred from the 70's, when people were guessing a lot,
because the VM Mikuni was designed for 2 strokes.
The Carb will work without it, but will be hard to get right.
Here is a quote from my 'MIKUNI CARBURETA TUNING & SECRETS' book I've had since 1974 when I fitted my first Mikuni
to an English bike. A 500CC Single.
" Air jet size is critical to operation of the high speed fuel system, and is a major reason why a carburetor intended for a different
engine size or type is not always immediately succesfull."
The mixture curve, ie. the combination of Air jet, Main jet, Needle jet, Needle and Air intake must suit the breathing requirements of each
individual engine, and the more I read about Norton twins on this Forum, the more interested I get.
So that's the Air jet.
AC.
The Air jet, or Air correction jet, located at 6 O' Clock in the rear of the Mikuni is there to help atomize or
emulsify the fuel at the Needle jet as it rises up from the Main jet.
The Air jet corresponds with the hole in the side of the Needle jet, near the top.
Theoretically it doesn't do much at anything under 3/4 throttle, but after that it does start to work
in conjunction with the Main jet.
I wouldn't remove it or blank it off. This is a common falacey bred from the 70's, when people were guessing a lot,
because the VM Mikuni was designed for 2 strokes.
The Carb will work without it, but will be hard to get right.
Here is a quote from my 'MIKUNI CARBURETA TUNING & SECRETS' book I've had since 1974 when I fitted my first Mikuni
to an English bike. A 500CC Single.
" Air jet size is critical to operation of the high speed fuel system, and is a major reason why a carburetor intended for a different
engine size or type is not always immediately succesfull."
The mixture curve, ie. the combination of Air jet, Main jet, Needle jet, Needle and Air intake must suit the breathing requirements of each
individual engine, and the more I read about Norton twins on this Forum, the more interested I get.
So that's the Air jet.
AC.
But he states this is a single Mik install. So if the carb was FU'd, both plugs should read rich. I'd say he's got an ignition problem or oil burning problem on the 1 cylinder. Pretty hard to have a single carb force 1 cylinder to run rich and the other perfect.
Again, my 2 cents, not from any large amounts of knowledge or scientific testing, just personal experience. Agreed, it does not seem possible with a single carb that this situation would happen, but it does, for reasons way beyond my understanding. I had the exact same condition a while back, and it drove me crazy. turns out, it was an air leak at the manifold, so the plug that looked good, was actually running lean. When I fixed the air leak, both plugs settled down to the same brownish condition. Doesn't seem possible, but it's true in my case.
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