THROTTLE PLATE and those tiny BYPASS HOLES (butterfly blues)

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Paul S Wilcox

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Hi Gang,

When the throttle plates (butterfly valves) are completely CLOSED, should they completely cover those three teeny bypass holes in the upper carb bore? Or should 1 or more holes still be uncovered by the edge of the butterfly?

Thanks in advance,

Paul
 

Vinmax

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Those holes are where the idle mixture comes through. If you block those, then no idle mixture. And of course no idle. So when setup correctly, and looking from the 'bottom' of the carbs, you should see the holes. That's where the idle mixture will come from and feed the engine when the butterflies are closed against the idle speed adjustment stop screw. If the idle speed is closed a bit too much, then the butterflies will block those holes and the engine will die.
...you'd have to hold the throttle open with your hand to keep the bike running.

Vinnie
 

Paul S Wilcox

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Thanks Vinnie. Y'know how the #2 carb is the non-adjustable one and we sync #1, 3 and 4 to match that one? How many of those tiny holes in carb #2 should be visible when the throttle plate is completely closed? I'm just trying to establish a baseline.
 

Vinmax

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Sorry about that one, I'm not too sure off hand. I'd have to look my mine.

But what the holes do is, the first uncovered one (or two?) is keeping the bike idling, Then as you open the throttle, there is a transition to provide my fuel to raise the revs. So fuel will come from more of the holes, and in greater quantity as the vacuum rises and pulls more through those holes.

As you open the throttle more, then the slides will start opening due to vacuum and provide more fuel through the needle jet. The jet needle's taper (*there's a 'needle jet' and the corresponding 'jet needle') will adjust the quantity let out of the jet needle.

Then when wide open, the needle will move almost completely out of the way on each carb, and let the maximum fuel get sucked into the carb from the needle jet,,,, via the main jet. That's when you're running pretty much on full main jet (although the idle holes will still be providing fuel, but their contribution to the total percentage of fuel flow will be less of course. They only can supply just a small portion of fuel flow to keep the bike idling and provide for the raise in revs until the slides/needles can take over).

Vinnie
 

maleko89

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The pilot outlet is different than the three bypass holes. Those should be completely covered by the throttle valve at idle.

Mark
 

mavgrab302

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I assume your syncing carbs? Got to do it with sync gauge. Anyways that said start off by just bottoming them out so they ‘just’ close then use idle knob until It cranks/idles/then you like where it idles, then use sync gauge. I wouldnt pay much attention to the holes really. It is mechanical you cant move location of holes or move axis of butterflies so it doesnt matter
 
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