CARB QUESTION : what the f*ck does this do?

stirlinggaz · World Chat Champion Joined: 22 Jul 2007 Karma :

hi all,
i have been trying to increase my knowledge.understanding of carbs.(sad, i know)
i know most of the basics & know what to look at, when it comes to my 2 strokes not running right etc.
anyway, i have a box of acquired carbs & bits, & looking through it today, i came across this mikuni carb.
it has all the usual jets in the usual places & is pretty much like any other carb (except for being extremely light. im thinking its made out of mangesium or some alloy as it really is ridiculously light)
but it has an "extra" jet.
the mouth where the air filter goes has what looks like a brass pin coming down through it.
within this pin is a jet.
the "pin" has a screw cover & is linked to the carb bowl by way of a tube.
can anyone explain , in laymans terms, lol, what the hell this does?
cheers,
GAZ

Kickstart · The Oracle Joined: 04 Feb 2002 Karma :

Hi

Looks like a power jet. Basically a jet that is uncovered on large throttle openings.

All the best

Keith
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stirlinggaz · World Chat Champion Joined: 22 Jul 2007 Karma :

thanks Keith.
so how does it work?
as i cant see how it would come into use??
cheers,
GAZ

Kickstart · The Oracle Joined: 04 Feb 2002 Karma :

Hi

Full throttle the slide is open enough to uncover the jet and the pressure drop should be enough to draw the fuel from the float bowl up the pipe and to the jet. Hence richer mixture on full throttle.

All the best

Keith
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stirlinggaz · World Chat Champion Joined: 22 Jul 2007 Karma :

ahaa....
think i have got it now,
it only comes into play when the throttle is wide open & the slide is all the way up.
pressure "sucks" fuel up from the carb bowl & into the mouth of the carb,.
up through the tube & exiting out the tiny little hole on the end of the "pin".
increasing the amount of fuel at wot.
making the mixture richer.
that about right?
cheers,
GAZ

Kickstart · The Oracle Joined: 04 Feb 2002 Karma :

Hi

Yep, although the pressure drop is how pretty much all the jets in a carb work to draw fuel. Just there position affects how much of a pressure drop there is by them at various throttle positions.

Look at an old style Weber twin choke carb on a car and there are no slides. Just a throttle butterfly, and depending on how open it is different / more jets are in a position to work.

All the best

Keith
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temeluchus · World Chat Champion Joined: 01 Oct 2008 Karma :

Does that look to be original, or added later?

Reminds me a bit of one of these:
https://www.thunderproducts.com/thunder_powerjet.htm
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stirlinggaz · World Chat Champion Joined: 22 Jul 2007 Karma :

Hi,
just looked at that ad & i reckon mine is standard mikuni.
the carb body doesnt look like it has been drilled & where the powerjet goes through, its countersunk.
& you have to remove the cap & reach down inside the thing to get to the jet.
i havnt a clue what the carbs off.
looks like 34/35mm,usual random markings (5F6 OO at the choke & T0 at the manifold end) & not a lot else.
cheers,
GAZ