GL1000 Carb Theory- Multiple Main Jets?

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Lucien Harpress
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GL1000 Carb Theory- Multiple Main Jets?

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Post by Lucien Harpress »

I got to wondering the other day, and excuse me if this has been posted someplace before, but I was curious about the idiosyncrasies of the GL1000 carbs. Mostly the fact that it's the only set of motorcycle carbs I've seen that have a primary and a secondary main jet. Now, I get this is (probably) a holdover from the fact they are repurposed auto carbs, but I'm more interested in the "how", rather than the "why".

What does one jet do vs. the other? Does one kick in earlier than the other, and if so, what kind of rev range are we talking about here? What advantage does two main jets serve over just one?

If anyone knows the answer, or can point me to where it is, I'd be grateful. If only to satisfy my own curiosity.
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Re: GL1000 Carb Theory- Multiple Main Jets?

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Post by pidjones »

FSM starting on page 9-6, plus Google "CV carb theory".
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Re: GL1000 Carb Theory- Multiple Main Jets?

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Post by Whiskerfish »

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Re: GL1000 Carb Theory- Multiple Main Jets?

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Post by Old Fogey »

Something interesting on the plastic buttons. I have a set of 76 tops in this week to get the buttons replaced. Under the plastic there is a thin rubber gasket sealing it to the alloy. Three of them were fine but on one the gasket was distorted so much there was no seal. Now, they are held in pretty firmly by the turnover of the alloy, but that seal wasn't put in there by Honda for no reason so this cap was, in all likelyhood, leaking from new.
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Re: GL1000 Carb Theory- Multiple Main Jets?

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Post by redglbx »

Quick explanation, the idle jets discharge fuel through a small hole metered by the idle mixture screw on the intake valve side of the throttle valve with the fuel being pulled out by vacuum when the throttle valve is closed and siphoned out by the air rushing over the hole with the t-valve open.

The primary main allows fuel to be siphoned/sucked by the air rushing through the Venturi and over those little brass emulsion tubes sticking up in bore And the primary main screwed into the bottom of the emulsion tube when the t-valve is opened.

The cumulative fuel of the pri main & the idle circuit keeps the engine fueled until enough air passing through the Venturi & under the bottom of the slides creates a strong enough vacuum signal through the small communication holes in the bottom of the slides (around 2200-2400rpm) that creates sufficient vacuum on top of the slides to lift the needle in the slide out of the emulsion tube that the jet is screwed into the bottom and siphons fuel out of the bowl.

Keep in mind that all the fuel is cumulative, one circuit does not shut off as the next one turns on. One other thing is that the government allowed all manufacturers to self certify for emissions and most including Honda picked around 2000-2500 which is why the idle jets and main jetting is so lean and once the slides pull up and feed fuel from the secondary mains they go “pig” rich because it reduces warranty claims and is less affected by the weather & barometer. A properly jetted fuel curve will be around 14.7 parts air to one part fuel, but that’s really hard to maintain do to many influences.

On modern engines they can compensate with computer controls to run close to that 14.7-1 a/f ratio. While our goldwings run between 15.5-1 around idle & low speeds to 10.5 -1 a/f ratio once the slides come up & the sec.mains are all in.

So that’s the basics, I’m sure that I’ve probably missed something but if you have questions feel freee to ask !
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