Basic but serious mikuni question

[WillMatrix]

I have a genuine mikuni but I keep reading on it and getting different answers

please tell me which way the choke lever should be (up or down) for choke on or off.

i have included the exact carb I have

 

[panchothedog]

I don't have the real deal but I have two knock off Chikunis sold by Go Power Sports and others. On them the choke ( not really a choke but a fuel enrichment
circuit ) is with the handle down. Once it fires and warms up for a few seconds pull the handle up into the run position. I would be very surprised if the real one
is any different. They are such a copy they even have the Mikuni name on them. I think on the other side of the body it says China and not Japan.

 

[WillMatrix]

I don't have the real deal but I have two knock off Chikunis sold by Go Power Sports and others. On them the choke ( not really a choke but a fuel enrichment
circuit ) is with the handle down. Once it fires and warms up for a few seconds pull the handle up into the run position. I would be very surprised if the real one
is any different. They are such a copy they even have the Mikuni name on them. I think on the other side of the body it says China and not Japan.

Ok perfect! Thank you for the information. I’m not sure of the difference between the two but if I knew all I had to do was make a gasket to seal up the intake correctly I wouldn’t have bought the genuine one. If there any pointers you can give me for setting the idle screw and air screw?

 

[panchothedog]

once I got mine to quit sucking air ( leak ) they worked OK. Never had to mess with either of the screws. The way to get the carb sealed to the manifold so it doesn't leak is to put them together off of the engine, so you can look into the manifold from the back and make sure it is perfectly centered on the O ring.
Then bolt the assembled unit to the engine. FWIW on another engine I built I
put on a real Makuni 24 mm flat slide. Quite a bit more money and worth every penny. I will NEVER buy another 22 mm round slide, real or fake. No comparison. Night and day difference.

 

[Rat]

Ok perfect! Thank you for the information. I’m not sure of the difference between the two but if I knew all I had to do was make a gasket to seal up the intake correctly I wouldn’t have bought the genuine one. If there any pointers you can give me for setting the idle screw and air screw?

The rule of thumb for both is ONE simple rule.
Start with an already heated up and actively running engine where possible

Turn the idle in until it bottoms out, but don't crank them it (soft seat) Now back it out 2 full turns, and leave it alone to tune the air bleed.

Turn the air screw to soft seat and then out 2 turns, this is where you need the engine to be actively idling (due to the idle screw it will be extremely high, but necessary for now)

You need to turn the idle screw out 1 full turn at a time giving 30-45 seconds for the rpm to fully respond. You will listen to the idle and continue to turn the air screw out until the RPM stops climbing (KEEP COUNT!) then turn it back in by a half turn. If the idle responds to half, turn it back out a quarter.

NOW... if the air screw was turned out 3 or more turns, your pilot is too rich and you need to go down a size maybe 2 and start the air screw tuning from a soft seat.
If it won't run with more than 1 turn out, it's too lean and you need a bigger pilot jet.

The ideal air screw position is between 1¾ to 2¾ out. Once that is set turn the idle screw out until the engine begins to stumble and then turn it in slowly until it smooths out (about a quarter turn usually)

You cannot properly tune the main jet if the pilot is not set correctly because an excessively rich pilot will cause the main circuit to also be rich at no fault of its own. This is notable by a flat sounding bog out that often only clears up with a wide open slide, but can also be an almost instant flood stall just off idle or just after 1/3 where the pilot and main are both in the breeze.
A too lean pilot will be touchier than a scared squirrel and lack a controlled or predictable response wanting only to either rev hard and fast, or rev straight into a stall
Very common rookie mistake is reading a rich plug and assuming the main is the problem when the carb has never been dialed in on all circuits systematically.

 

[WillMatrix]

The rule of thumb for both is ONE simple rule.
Start with an already heated up and actively running engine where possible

Turn the idle in until it bottoms out, but don't crank them it (soft seat) Now back it out 2 full turns, and leave it alone to tune the air bleed.

Turn the air screw to soft seat and then out 2 turns, this is where you need the engine to be actively idling (due to the idle screw it will be extremely high, but necessary for now)

You need to turn the idle screw out 1 full turn at a time giving 30-45 seconds for the rpm to fully respond. You will listen to the idle and continue to turn the air screw out until the RPM stops climbing (KEEP COUNT!) then turn it back in by a half turn. If the idle responds to half, turn it back out a quarter.

NOW... if the air screw was turned out 3 or more turns, your pilot is too rich and you need to go down a size maybe 2 and start the air screw tuning from a soft seat.
If it won't run with more than 1 turn out, it's too lean and you need a bigger pilot jet.

The ideal air screw position is between 1¾ to 2¾ out. Once that is set turn the idle screw out until the engine begins to stumble and then turn it in slowly until it smooths out (about a quarter turn usually)

You cannot properly tune the main jet if the pilot is not set correctly because an excessively rich pilot will cause the main circuit to also be rich at no fault of its own. This is notable by a flat sounding bog out that often only clears up with a wide open slide, but can also be an almost instant flood stall just off idle or just after 1/3 where the pilot and main are both in the breeze.
A too lean pilot will be touchier than a scared squirrel and lack a controlled or predictable response wanting only to either rev hard and fast, or rev straight into a stall
Very common rookie mistake is reading a rich plug and assuming the main is the problem when the carb has never been dialed in on all circuits systematically.

Dang!!!! This is the most detailed set of instructions I have seen in years. I will do everything to the letter I’m just nervous about the part where I start with the idle all the way in. I will hate for it to blow lol

 

[Rat]

Dang!!!! This is the most detailed set of instructions I have seen in years. I will do everything to the letter I’m just nervous about the part where I start with the idle all the way in. I will hate for it to blow lol

I didn't even give 100% of the full tuning instructions lol. I only gave you the first half because the second half is useless until you can start it easily and hold a good idle.

I'd did forget to mention since you're using a motorcycle carb, the standard 1200rpm idle does exactly not apply to you... you can probably pick up a smooth idle at around 900-1k

If you still have a governor, there's absolutely nothing to worry about... it will hit the 3500 rpm wall.
. If your governor is deleted it will not rev past valve float, so depending on stock springs that's maybe 5k still not much to worry about but if You're that concerned you should be able to still accurately dial it in with 3 turns out on the idle to start from as a base...there's always the kill switch if you feel like it's out of control

The OTHER half of the tuning process is dealing with the main circuit and needle which requires taking it for test runs and paying close attention to the throttle response AND how the engine sounds. Putting it on jackstands will not provide an accurate tune whatsoever.

Any further instructions are dependent on if this is a kart or a minibike... I've got a Keihin PWK 24 PJ on my 208, same tuning procedure and the biggest difference is I've got a D-slide instead of a round (less turbulence/more efficient)

 

[Denny]

Governors don’t work on engines with aftermarket carbs.

 

[Rat]

Governors don’t work on engines with aftermarket carbs.

I had a moment

 

[WillMatrix]

I had a moment

I don’t have a governor and I have billet rod and flywheel and 22lb valve springs

 

[Rat]

I don’t have a governor and I have billet rod and flywheel and 22lb valve springs

LMMFAO nothings going to explode then

 

[WillMatrix]

The rule of thumb for both is ONE simple rule.
Start with an already heated up and actively running engine where possible

Turn the idle in until it bottoms out, but don't crank them it (soft seat) Now back it out 2 full turns, and leave it alone to tune the air bleed.

Turn the air screw to soft seat and then out 2 turns, this is where you need the engine to be actively idling (due to the idle screw it will be extremely high, but necessary for now)

You need to turn the idle screw out 1 full turn at a time giving 30-45 seconds for the rpm to fully respond. You will listen to the idle and continue to turn the air screw out until the RPM stops climbing (KEEP COUNT!) then turn it back in by a half turn. If the idle responds to half, turn it back out a quarter.

NOW... if the air screw was turned out 3 or more turns, your pilot is too rich and you need to go down a size maybe 2 and start the air screw tuning from a soft seat.
If it won't run with more than 1 turn out, it's too lean and you need a bigger pilot jet.

The ideal air screw position is between 1¾ to 2¾ out. Once that is set turn the idle screw out until the engine begins to stumble and then turn it in slowly until it smooths out (about a quarter turn usually)

You cannot properly tune the main jet if the pilot is not set correctly because an excessively rich pilot will cause the main circuit to also be rich at no fault of its own. This is notable by a flat sounding bog out that often only clears up with a wide open slide, but can also be an almost instant flood stall just off idle or just after 1/3 where the pilot and main are both in the breeze.
A too lean pilot will be touchier than a scared squirrel and lack a controlled or predictable response wanting only to either rev hard and fast, or rev straight into a stall
Very common rookie mistake is reading a rich plug and assuming the main is the problem when the carb has never been dialed in on all circuits systematically.

Part two with the air screw, do I turn it out 2 full turns as well?

 

[Rat]

Part two with the air screw, do I turn it out 2 full turns as well?

That's the default starting point yes... but it will point you towards what size your pilot jet needs to be by idle response as you turn it out more.

If you get the highest revving idle at 4 turns out you need to go up at least one maybe 2 pilot sizes

 

[WillMatrix]

That's the default starting point yes... but it will point you towards what size your pilot jet needs to be by idle response as you turn it out more.

If you get the highest revving idle at 4 turns out you need to go up at least one maybe 2 pilot sizes

I ask because you specified “full turns” for idle and not the air screw

 

[Rat]

I ask because you specified “full turns” for idle and not the air screw

Procedurally speaking you have to tune the air screw to identify proper pilot size. Once that set you can then adjust the idle screw to the best lowest idle setting (900-1200) then cones the road testing and response tuning to the main jet and lastly the needle position (it is the fine tuning element to the main circuit like the air screw is for the pilot although functionally different

 

[WillMatrix]

Procedurally speaking you have to tune the air screw to identify proper pilot size. Once that set you can then adjust the idle screw to the best lowest idle setting (900-1200) then cones the road testing and response tuning to the main jet and lastly the needle position (it is the fine tuning element to the main circuit like the air screw is for the pilot although functionally different

Ok one more question.for the first step, Turn the idle screw in the direction that raises the slide?

 

[Rat]

Ok one more question.for the first step, Turn the idle screw in the direction that raises the slide?

Turning it all the way in (soft seat) will give maximum slide lift...since your running a lot of billet give it 4 full turns out. You want the idle high because you want room for the engine not to stall if it suddenly drops off, and also in a range where it's going to be more sensitive to the air screw adjustments; obviously screaming wide open is totally useless.

Then begin tuning the pilot circuit: start with soft seat and 2 back out.
Anything over 3 out jet the pilot up, anything less than 2 drop a size.

Ideally you want the peak idle response at 2 turns exactly, with 1¾ being the point the idle begins to fall off or become erratic and the 2¾ turn mark would be unresponsive... this is an impossible benchmark for 100% of everyone using any given carb at various altitudes, on various sizes of engines which is why the standard rules to follow puts the peak rpm is ANYWHERE between those two positions as acceptable.

On another note, you will find yourself jetting that carb obscenely lean to compensate the negative intake pressure being too high for such a small carb. Even the 24mm on my 208 is tuned towards the leaned side because according to a particular formula I should be using at the least a 28mm. A 212 would ideally use a 30mm, but 28mm would probably be easier to work with in terms of BAC's (Big @$$ Carbs)

√(D)×2=X
D (your displacement)
X is approximate ideal carb size

 

[WillMatrix]

Turning it all the way in (soft seat) will give maximum slide lift...since your running a lot of billet give it 4 full turns out. You want the idle high because you want room for the engine not to stall if it suddenly drops off, and also in a range where it's going to be more sensitive to the air screw adjustments; obviously screaming wide open is totally useless.

Then begin tuning the pilot circuit: start with soft seat and 2 back out.
Anything over 3 out jet the pilot up, anything less than 2 drop a size.

Ideally you want the peak idle response at 2 turns exactly, with 1¾ being the point the idle begins to fall off or become erratic and the 2¾ turn mark would be unresponsive... this is an impossible benchmark for 100% of everyone using any given carb at various altitudes, on various sizes of engines which is why the standard rules to follow puts the peak rpm is ANYWHERE between those two positions as acceptable.

On another note, you will find yourself jetting that carb obscenely lean to compensate the negative intake pressure being too high for such a small carb. Even the 24mm on my 208 is tuned towards the leaned side because according to a particular formula I should be using at the least a 28mm. A 212 would ideally use a 30mm, but 28mm would probably be easier to work with in terms of BAC's (Big @$$ Carbs)

√(D)×2=X
D (your displacement)
X is approximate ideal carb size

Let’s go!!!! I’m about to try it now