Question about Phazer 480 Mikuni B38-32 butterfly carburetors.

yamadogger01

yamadogger01

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I have scoured the internet for what the pilot/mixture screw on top of the Mikuni B38-32 carburetor does. I have read that screwing it out gives more fuel to the carb but I have also read that screwing it out gives more air to the carb. So which is it? I read a manual by "midwestmanuals" which said that this pilot screw gives more air when screwed out. But as I was doing some real life testing the engine increased in RPM when I turned the screw in, which points at turning it in gives a leaner mixture. Which points at that this screw is in control of fuel flow and not air flow.

But I don't really feel comfortable coming to a conclusion to what function it has since so many sources speak against one another.

I would like someone who KNOWS to tell me what it does. Some kind of carburetor guru would be nice.

Thanks for any and all inputs.
 

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yamadogger01 said:
I have scoured the internet for what the pilot/mixture screw on top of the Mikuni B38-32 carburetor does. I have read that screwing it out gives more fuel to the carb but I have also read that screwing it out gives more air to the carb. So which is it? I read a manual by "midwestmanuals" which said that this pilot screw gives more air when screwed out. But as I was doing some real life testing the engine increased in RPM when I turned the screw in, which points at turning it in gives a leaner mixture. Which points at that this screw is in control of fuel flow and not air flow.

But I don't really feel comfortable coming to a conclusion to what function it has since so many sources speak against one another.

I would like someone who KNOWS to tell me what it does. Some kind of carburetor guru would be nice.

Thanks for any and all inputs.
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That looks to be a air screw 🪛. Turning it out will lean the fuel mixture and in will richen . It doesn't appear to be a fuel mixture screw which would work opposite of what I told you.
 
littlejohn said:
If the screw has a pointy needle end it is a fuel screw if it's more rounded it's a air screw
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It looks like this. A pointy end. Does it really have to mean it is a fuel screw though? I have now searched the internet for Mikuni air screws and they all seem to have a flat or blunt tip. This does seem convincing for me, but I find it odd that a well-known Phazer 480 manual would be wrong about this detail.
 

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74Nitro said:
I think that screw lets in both air and fuel that have already been mixed through the pilot jet.
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Interesting. I really have to inspect the piping in the carb next time I take it out and figure out what goes to this pilot screw. Also where it is drawn from. Could very well be what you say.
 
yamadogger01 said:
Interesting. I really have to inspect the piping in the carb next time I take it out and figure out what goes to this pilot screw. Also where it is drawn from. Could very well be what you say.
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I can see how you would be confused 🤔 as I am as well typically the air screw is located towards the back of carburetor as well . I still feel this is a air screw
 
littlejohn said:
I can see how you would be confused 🤔 as I am as well typically the air screw is located towards the back of carburetor as well . I still feel this is a air screw
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yamadogger01 said:
Interesting. I really have to inspect the piping in the carb next time I take it out and figure out what goes to this pilot screw. Also where it is drawn from. Could very well be what you say.
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May I have yr of phazer?
 
And I was wrong that infact is your pilot adjusting screw ! I second guessed once I realized it was infact on the engine side
 

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So it does appear to controll fuel flow then. Then the midwestmanuals manual is incorrect about this.

I did some more internet searching and found that the guy called yamadad4 (who can be considered a guru when it comes to old yammies) also said it was a screw which controlled fuel.

I guess this pilot/mixture screw mystery has been solved then?
 

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Sorry ....my apologies for steering you in the wrong direction. That is infact is a fuel mixture screw "pilot screw" . No doubt , placement ,shape and of course the parts manual confirming this . Out will be rich in would lean it . Daddy was
correct 🤣 and the G.O.A.T would have to agree
 
Loosen = Leaner per the manual. It's an air screw!

With it all the way closed it will be pulling the max fuel the pilot jet can supply. The reason the RPM went up when you loosened it, is because you just performed an idle drop test. When the RPM goes up, you found the sweet spot, now turn it in 1/2 turn.
 
From the actual Phazer Service Manual..... In = leaner. The photo you found above is not from the manual.(which was my previous comment).
phazer Pilot.png
 
"pilot" not air ...meaning pilot circuit kind sir you are metering fuel with this screw NOT air.
 

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bird said:
From the actual Phazer Service Manual..... In = leaner. The photo you found above is not from the manual.(which was my previous comment).
View attachment 67618
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Ohh thank you for this image. It confirms that the pilot/mixture screw DOES regulate fuel and not air.

As turning the screw inwards makes RPM go higher, the screw must be restricting fuel. In a carburetor, only a higher air to fuel ratio should be able to give higher RPM.

I saved the image. Thanks again :)
 
bird said:
From the actual Phazer Service Manual..... In = leaner. The photo you found above is not from the manual.(which was my previous comment).
View attachment 67618
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Wait a second. The part where your official Phazer service manual mentions "Turning in = Idle speed becomes higher" etc. that might as well be about the throttle stop screw and not the pilot screw. So because it is so unspecific, it can't be used to determine what function the pilot screw serves. I jumped the gun on that one, hehe...

However I am still rather convinced the screw is regulating fuel because of what littlejohn and others on other forums are saying.
 
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