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Discussion Starter · #1 ·
Been messing with my Mikuni carbs over the weekend trying to set the chokes and I think I have the play set right at 1/8". What I am having trouble understanding is, how do they work? There is a hole in the bore, perpendicular to the choke plunger that I can see the choke plunger slide up and down through, but you need a little mirror an flashlight to see it. There is also a hole on the air box side of the carb looking straight down the bore of the carb that goes to the plunger but it tapers to a much smaller hole right at the plunger. In either of these holes you can see the plunger move up and down as you cycle the choke lever. What hard evidence can I get that the choke is at half choke and at full choke? Is it possible? I find the directions a little vague in the fact it is only looking for play in the choke lever. Thanks, Daryl
 

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daryl.. the plunger when bottomed out in the carb hole is basically off / no choke.. as the choke lever is pulled the plunger raises in the hole letting fuel in.. 1/2 choke.. when pulled to full choke the holes are fully open allowing more fuel in to the carbs..

I found the key to setting the round slide is to have the plunger bottomed out with the brass carb fitting with a C hair push into the carb body with the plunger bottomed out.. you want lite pressure from the brass fittin and spring to hold the plunger down bottomed in the hole..

I try and set both brass fittings and plungers the same heights
 

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Discussion Starter · #3 ·
You lost me at hello. Actually the second paragraph. There are two holes I can see into, one drilled perpendicular to the bore, it is plugged on the outside with a brass BB but with a little round mirror and a flashlight I can see in it from inside the bore of the carb. The other is on the big end of the carb looking into the motor, with the carb off and looking straight down the bore in the hole you can see the plunger but the hole is much smaller down by the plunger. Do either of these holes mean anything or are they just tooling passages? I can't believe they are. Could you explain a little better for me?

Thanks
 

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Keep it simple. When the barrel is lifted off the hole in the bottom of it's bore, engine vacuum will pull fuel up through that hole into the intake - IF - you leave your hand off the throttle. Lifting throttle almost eliminates vacuum, making starting process much more difficult.
 

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Discussion Starter · #5 ·
Makes sense but which hole does the fuel travel through? The bigger one that gese side to side in the bore or the little 1.5 millimeter one that runs along the bore starting at the intake? The reason I ask is that when I am at half choke, should half of that little hole be exposed and when at full choke, the whole hole be exposed? I wish I had some pics here to explain, I'll try and find some. I just really want to understand how this works and not just that it works. There must be a visual way to know.

P.S. Nice to talk to ya Al, I've been real busy with some family issues lately.
 

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Im pretty sure the fuel comes up from the bottom through the hole in the center, then travels towards the slide
 

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An actual "choke", does what it says. It chokes the air flow to enrichen fuel mixture for better cold starting. The Mikuni "choke" is really not a choke, it is a "fuel enrichener" because it adds a shot of gas to the existing volume or flow of air. To open the throttle on the fuel enrichener system when starting, negates the effect of adding more fuel.

Fuel is drawn directly from the floatbowl, controled by the "choke" lever and flows out the hole you see at about the 3 o'clock position. If you have the carb off in hand, squirt some carb cleaner through these passages and you'll have a better picture than can be found in print.
 

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Actually, the design of the fuel enrichener allows air to bypass the slide and will actually lean out the mixture when the slide is open. You don't want to be flipping up the choke as you are screaming across the lake or it will get very quiet!!
 

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Actually, the design of the fuel enrichener allows air to bypass the slide and will actually lean out the mixture when the slide is open. You don't want to be flipping up the choke as you are screaming across the lake or it will get very quiet!![/b]
Ive done it very briefly running across the lake just to see what it would do. Didnt hear anything.. no slight surge nor did it fall on its face. Figured what air it pulled in had to pull a slight amount of fuel. Anyone with EGT's have any info on this one or try it?
 

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New here with a similar question... my 98 Arctic Cat Bearcat 454 has had carb problems for a while, with the filter side hose off I see no movement when moving the choke lever. It has the original lever, cable (non adjustable as far as I can tell) and carb. Had the quad to the shop, running rough, hard to start. He cleaned the carb, ran good for a while. Now it’s running rough again, acts like the choke is on(smell has, wet plug).
Any ideas? It’s a great old machine, when it runs.
 

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New here with a similar question... my 98 Arctic Cat Bearcat 454 has had carb problems for a while, with the filter side hose off I see no movement when moving the choke lever. It has the original lever, cable (non adjustable as far as I can tell) and carb. Had the quad to the shop, running rough, hard to start. He cleaned the carb, ran good for a while. Now it’s running rough again, acts like the choke is on(smell has, wet plug).
Any ideas? It’s a great old machine, when it runs.
Not familiar with the Bearcat 454, but a carb is a carb. I work on them all the time.
Nearly all chokes on these machines are adjustable in some fashion. But, not all.
But, it make be more than that.
You may need a new float needle and seat, new float, new gasket/o-ring, etc.
You might have an air leak at the mounting boot. Compression may be too low.
On many 4-stroke engines, valves need adjusted or reground.
I'd get a service manual and read it.
Folks nowadays have forgotten/don't know how carbs work.
Once had a 17 year GM fuel system Engineer come to me for a carb problem on an ATV. It was a simple fix. He knew nothing about carbs. Just EFI.
Good luck.
 

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Same for rebuilding a carburetor for vehicles that we had years ago. None of these auto tech's today know anything about them or how to rebuild one. They have to send them out to a specialty shop that only specializes in them. When it comes to fuel injection sleds i'm lost if an issue.....rather have a carb sled any day. Imo, their so simple.
 

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All fuel delivery systems work on pressure. EFI uses an electric pump and an injector.

Carbs use atmospheric pressure or a variation of it acting on the surface of the fuel inside the float bowl. The slightly reduced atmospheric pressure caused by moving air in the bore of the carb creates a reduced pressure inside the bore that varies with air velocity. Thus, fuel is pushed through the jets by the higher pressure inside the float bowl.

The choke in a typical Mikuni carb is similar in that it has a long tube that reaches almost to the bottom of the float bowl. But, it is activated by opening the choke plunger and subjecting the choke tube to a very strong pressure differential (partial vacuum) from the downstream side of the closed throttle. This combined with float bowl atmospheric pressure acting on the surface of the fuel in the float bowl, forces a relatively large flow of fuel into the bore of the carb for starting.

Applying the choke at any rpm and throttle position with engine running will add extra fuel. More so with higher vacuum inside the bore.
 
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