question about pilot circuit

[twiggy2cents]

okay i know there are 3 discharge holes for the pilot circuits the pilot hole bypass and something else, maybe throttle hole? (i know hole is not the correct term but i dont wanna get fancy when hole explains it)

What does the bypass hole do?

I imagine the throttle hole is the midrange stuff but and pilot is for the idle mixture but im not sure about the bypass hole

[Steve]

OK, is what you are talking about in this picture?



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[twiggy2cents]

there is a hole the the throttle blade covers but i thought there were three holes

[chef1366]

All this technical jargon is confusin me

[Steve]

there is a hole the the throttle blade covers but i thought there were three holes

There are three holes. Look at the diagram. One has the mixture adjust screw in it, the other two are near the throttle blade. The one just downstream of the throttle will admit mixture all the time, the mixture adjust screw can add to that. The one just behind the throttle comes into play when the throttle starts opening a little bit and might be called the transition port. Those three holes are what puts gas into the engine until there is enough air flow to pull gas through the mains.

Where are these bypass and throttle holes you were asking about in your first post?

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[twiggy2cents]

the back hole that the throttle blade doesnt cover does i

[GQROD]

In the factory manual it shows how to bench sync the carbs and which holes if any are covered by the throttle blades.

[twiggy2cents]

yes but i wanted to know the theory of the bypass hole

[chef1366]

the back hole that the throttle blade doesnt cover does i

If your bike has EPA bull that is the hole that goes to the vacuum nipple on each carb that goes to a charcoal canister. (CA only)

[Steve]

yes but i wanted to know the theory of the bypass hole

Still wondering which one you are calling the "bypass hole".

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[twiggy2cents]

The other outlet is call the "bypass outlet" and is located right at the point in the bore where the bottom of the butterfly comes to rest when closed. Typically, the butterfly is set a tad open to permit just a slight amount of air to pass by at the bottom to support idle and decel, and most of the fuel for these functions is supplied through the pilot outlet. But as the butterfly is opened, more air flows past it, and the venturi effect starts to work on the bypass Additional fuel is now drawn out of the bypass to support low speed running and cruising at small throttle openings. (Note that if the butterful valve is adjusted to 'fully closed' the engine will probably not start or idle. It needs to be open a tad. As mentioned above, these outlets continue "giving" throughout the rev range, but their contribution to the overall mixture dimishes as the slide rises.)

An addition to this circuit is found on later Hitachi and all Mikuni carbs. This is the "coasting enricher". A typical problem in earlier carbs was the fact that when you chopped the throttle (closed the butterfly) on deceleration, there would not enough fuel in the mixture at the (at that moment) high revs to allow the engine to fire consistently. You would then get a "lean misfire". That is, the engine would fail to fire, and the unburned mixture (lean though it was) would enter the exhaust header. Then when the engine next fired, you'd get a backfire. (So backfiring on decel is typically a lean condition, and not "loading up" as some people think.) The solution they came up with was to reduce the amount of air in the "airbleed" circuit by about half, meaning the fuel content hitting the bore from the pilot oulet was much higher than the normal idle fuel mixture you get on closed throttle. Once the revs came down, the full air bleed would be restored for proper idling. The "coaster enricher" is activated by the strong vacuum created in the carburetor holder (intake stub) byhigh revs when the butterfyl is closed on decel.

On Hitachis some external piping was added to service these diaphragm driven valves. On Mikunis the needed passages were drilled into the carb bodies.


not our carbs but all cv carbs are basically the same