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Old 10-26-2011, 06:08 AM   #11
Easy Rider 2
Join Date: Dec 2010
Location: North Central Florida
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Originally Posted by jturn View Post
Would it be worthwhile to drain the fuel out, fill it with fresh gas and try again with B12?
Maybe but at this point it's doubtful.
If you have not drained the float bowls yet, you definitely should do that.

If the bike is in good condition otherwise it would be a shame not to get it fixed. No doubt that the idle jets are plugged up and a manual cleaning should fix it right up.......if done correctly.

Depending on the number of carbs and how they are mounted, that's about 2 hours of shop labor and would run around $200.

Have you check for vacuum leaks yet?

If you do take it in, don't TELL them what to do; describe the problem and what you have done and let them diagnose it. It might not be carbs at all.

P.S. Have you checked the settings of the idle air screws yet ?
'11 Kawasaki Vulcan 900 C
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Old 10-26-2011, 01:20 PM   #12
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The reason the bike dies when you "take off the choke" is due to the way the fuel-circuit is set up in the carb(s).

When the bike is at "normal" temperature, choke/enricher off, at idle, in a typical Constant-Velocity (CV) carb, it is drawing fuel through the "pilot" or "idle" (sometimes called "primary") jets. As you roll-on the throttle, you are opening a butterfly-valve mounted downstream of all the actual "workings" of the carb (but still in the carb body, typically) - this allows a vacuum to be drawn (via small ports) on the top-side of the slide-diaphragm (in the top of the carb) that lifts the slide, allowing more air to flow through the carb. As it does, more and more fuel is drawn through the idle jet, up to the the max it can flow, as well as being drawn out of the main jet.

Attached through the center of the slide is a tapered needle, which acts as a metering-valve through the "main" jet (or "secondary" jet). The main is WAY too-big for "transitional" (or "mid-range") power-demands, so the needle is the "regulator" and adjusts the open cross-section of the main, as it is withdrawn. Once there is sufficient air drawing fuel from the main jet, usually no fuel is being drawn through the idle jet. It's not that it CAN'T come through - it's that the vacuum signal is enough that is sort of "dead-heads" the idle jet. The smaller the jet, the more-pronounced the fuel-cut (that's why the "dip" or "flat-spot" in power you feel on some bikes in lower RPMs can be reduced with tuning of idle-air bleed screws, or larger idle-jets).

The idle jets have tiny orifices, in the nature of anywhere from .005" to .010", depending on the size/volumetric-efficiency of the cylinder(s) it is feeding, so they can plug quite-easily with deposits if the fuel is allowed to age to where it breaks-down, or evaporates entirely.

When the engine is cold, it requires many-times more fuel than when warm, as the gasoline does not vapourise as easily. It used to be that you would "choke" a bike by dropping a sort of "gate" across the inlet of the carb's venturi, effectively decreasing the "size" of the carb, which allowed the jets to add more fuel per unit of air.

With the common-usage of the CV carb, an air-bleed valve was installed to bypass-around the carb butterfly - this bleed lets a metered amount of vaccum to pull the carb-slide up a bit, allowing the main jet to provide the required fuel for cold-start. Little or no fuel is drawn through the idle-jets at this point. This is why adding carb-cleaner to the fuel to "unplug" the idle jet(s) often does not work - if you can't get the cleaner THROUGH the jet(s), it won't do anything. Fuel does not flow through the idle jet "on choke".

A complex explanation, to a simple problem.

When you pull the carbs off the bike, take care not to lose any tiny screws or springs, or mess un-necessarily with the adjustment of the enricher/choke assembly.

Use glasses/goggles/faceshield and carb-spray-cleaner in each and every orifice on each carb - in the fuel bowl AND throat/venturi - all of them go someplace, so what goes in one hole MUST come out another. Take EXTRA care NOT to get the cleaner on any rubber-parts (gaskets, o-rings, PARTICULAR care not to get it on the slide-diaphragms). The rubber used in these carbs swells and stretches with contact, and diaphragms are getting exceedingly stupid-expensive, if you can find them at all.

The idle jet is the smaller of the two, it will be offset to the inlet-side of the carb, you will need to remove it to clean it. If it's particularly crudded-up, a small, thin, COPPER wire through it will get the gunk out. Then spray-through with the cleaner. YOU MUST SEE LIGHT THROUGH IT, OR IT IS NOT CLEAN.

The procedure is not difficult, but it IS Technical, and you CAN break things easily (including yourself) if you don't have a care. I hope you have a shop manual for the bike. If not, it's a Wise Investment.

Good Luck, you're gonna need it.
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