Which ports to block, which to leave open? (last-ditch effort) - Kawasaki Vulcan 750 Forum : Kawasaki VN750 Forums
Carbs and Fuel System
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post #1 of 7 (permalink) Old 06-01-2008, 05:18 PM Thread Starter
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Which ports to block, which to leave open? (last-ditch effort)

Duck talked me into giving the ol' girl one more try, since the engine is running on starter fluid, I'm just not drawing fuel from the carbs. Here's the thing: Previously, I had the carbs hooked up to the engine block (altho' Duck suggested I get a new hose clamp for the rear boot - done), and that was about it. No airbox, no air filters, no emissions cannister, no hoses, no pipes, no premuffler chamber. At his suggestion, I've put the carbs back onto the engine block and have reinstalled the pipes, goat, and the mufflers, in an effort to see if increasing the back pressure will draw the fuel out of the carbs and into the combustion chamber. As we got to talking, though, we couldn't decide what to do with the reed valve ports and the vacuum ports.

I had blocked off all four - I was either putting fuel directly into the lines, or drawing it from the tank (but had modded the petcock to a gravity feed, so no vacuum was required to pull fuel out of the tank). But, if I put fuel directly into the lines, what do I need to block off to ensure that the engine has enough vacuum to get the fuel out of the float bowls and through the carbs?

So, at this moment: carbs are connected tightly to the engine block; pipes, goat, and mufflers are on; no airbox and no airfilters (slides are, in effect, open to the air); crankcase breather hose is still attached... and vacuum ports and reed valves are not blocked off; and I have no emissions hoses (two to the reed valves, one to the airbox, one ot the rear vacuum port) anywhere near the bike. And I'm just gonna dump fuel directly into the lines on the carbs... what's gets blocked? What remains open? And, just for my edification, can someone explain why?

Thanks.

C
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post #2 of 7 (permalink) Old 06-01-2008, 06:59 PM
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Wish I could help you on this one, Cindy. But you passed how far I've wrenched into my bike over 2000 post ago!!!
Good luck though


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post #3 of 7 (permalink) Old 06-01-2008, 07:40 PM
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Cindy, you may want to try blocking off the vacuum ports from the carbs. If you leave them open, you may not generate enough of a pressure difference to draw fuel into the cylinders. Both are hooked up to a valve or similar so that if you have a vacuum leak there, the engine runs crappy or quits.
The reeds could probably remain as is since they only function on decel.
This is my theory.

Since you don't have anything covering the carb intakes, you may want to try shielding one with the back of your hand to see if you can feel the air going into the carb. If you get a weak airflow, it may indicate a carb problem or a compression problem since the downward stroke of the piston is what created the "vacuum" in the first place.

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post #4 of 7 (permalink) Old 06-01-2008, 07:42 PM Thread Starter
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Ok, so I'll block off the vac ports and leave the reed valve ports open; I do have the boots (from carbs-to-airbox) on, and have just covered them slightly to try to restrict the flow a little.

Hyper, at this point, we're in the same boat!!
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post #5 of 7 (permalink) Old 06-01-2008, 11:37 PM
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If you have a feather or something else highly motivated by little air. It might be easier to see if there is air flow. Just a thought.
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post #6 of 7 (permalink) Old 06-02-2008, 06:58 AM Thread Starter
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I had some light material over the end of the carb-to-box boots - it was definitely pulling in when I turned the engine over; also put my hand over each and there really is good pull there. But still not any fuel draw into the engine. Only blocked the vent ports on the carbs and left the reed valve ports open.

??

C
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post #7 of 7 (permalink) Old 06-02-2008, 08:02 AM
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Don't block the vents, only the 2 vacuum lines ( one goes to tank). The vents have to stay open

85 VN750 basketcase ( yes its a true 750, and no smog controls, its good to be Canadian)
It will have
a rebuilt 96 top end, rebuilt 85 bottom end
degoated, Eared
stripped, lowered, raked, bobbed,
reed valve boxes shaved
ported and polished
balanced
right side dump exhaust
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looking for smoothbore sidedraft carbs
possibly fat bob tanks
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