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Junior Grue
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@hleseidel : If I were you I would do a little studying on how the siphon effect works, the relation of the fuel level in the tank to the float valve level then the internal plumbing of your carburetors.
Only then can you decide if bypassing the fuel shutoff valve other than for testing is a wise idea.
 

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ALEX BERECZKY wrote:
... GL1500 has Vented Float Bowls, so it's not possible for any Siphon Effect to occur, pulling fuel Into the float bowls.
:wtf:Good to know that the vented fuel bowls of the GL1500 are NOT lower than the fuel level in the vented (vacuum relieved) gl1500 fuel tank...:ROFL:where fuel would be transferred by siphon acting due to the higher potential energy in the tank (and not by 'horror vacui' as suction from the carbs...)

Since the idle passages of the Keihin down-draft carbs is fed through the slow jet which is submerged below the float-bowl's fuel level, there's nearly always a little drip from the idle passage (even without vacuum signal) -- Managing the warm-start emissions from this "drip" is one of the reasons that initial production of the 1500 used such a tiny slow-jet.On the scale of the 1500, which isfed from 2-carbs this idle drip is pretty insignificant and led to manageably-rich-mix starts due mostly to the small fuel bowl of the down-draft Keihins. You see, once the float-bowls' fuel level drops to the point where surface tensions manage the fuel drip - the drip stops reliably; this is a good thing... You only drop a few CCs down a 36mm bore and that fuel amount is dumped into the long intake runners and/or up to half of the 1500's 1532cc stroked volume. Yeah ... nothing to see here... move along....

... Unless the few cc's that did drip through the idle passage changed the float level enough to open the float's valve... This is where a vicious, albeit slow, siphon can begin. As mentioned, the float-bowls of the 1500's are vented (as with all carbs) - this is to allow the known fuel-level (as controlled bythe float levelsetting)above the jets to be at atmospheric pressure; neither a vacuum, nor pressurized. (I digress...)

With the float now opened by a reduction in the fuel level in the float bowl, fuel from the fuel inlet hose is allowed to pass into the carb's float-bowl until once again, the float presses against the float valve to close-off the fuel flow. All good, right?

... well... unless the idle passages drip... (see all of the above)

In practice the float valve is not a digital "fully-open or fully-closed" device andwill transition from allowing no flow to allowing grater (full) flow as some inverse function of the fuel level in the bowl (compounded, of course, by being on a side stand or other un-level condition), and this float & float valve has some mechanical hysteresis from frictions and other sticky things.This kinda combines to allow fuel to fill the float-bowl as quickly as fuel leaves the float bowl with a little slop on either side.

  1. IF you have a idle passage drip (for whatever cause), the floats will empty to the level where flow resistance in that idle system stops the drip.
  2. IF this 'drip stop' level is below the threshold for fuel to flow through the float valve, and there is a fuel source on the other side of that float valve, thisfuel flowwill continue unabated (except for the absence of venturi-drawn vacuum, this is how your bike idles)
Since Pascal demonstrated that the siphon works based on atmospheric pressures, which the 1500 has at the vented carb float-bowls,and also in the vacuum-relieved fuel tank... The 1500 does, in fact, have potential energy imbalance in the connected fuel system to allow transfer of fuel from the tank to carb by siphon since the fuel level in the tank is above the fuel level in the float bowls.

If the venting a fuel bowl stops a siphon, then disconnecting the fuel-line at the carb's feed point should have the same effect; effectively venting the fuel line at the carb. So, I would offer that if you can disconnect the fuel hose at the carb, without a fuel valve being in place, and observe no flow, and no continued flow, then it would be safe to say that fuel cannot be fed from the1500's fuel tank to a point lower than the tank by siphon.

:?Is anyone in a position to remove the fuel line from their carbs, and defeat/open the fuel valve to see that no fuel drains out?



:gunhead:
 

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Bottom line here.. carbs (and injectors for that matter) have a basic function of delivering fuel to the engine. When they get clogged up or leaking seals, they can fail in so many ways. It's not too much of a stretch to get an unwanted path of fuel into the cylinders.
I'm quite sure the engineers at honda didn't sit around a table and say "hey, let's put this valve in the gas line 'cause it looks good". :rollingeyes:
 

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Fireman here. Don't screw with this. It's not a matter of "IF", it's a matter of "WHEN". You don't want to always wonder if today is the day I destroy a connecting rod or piston cause the float finally stuck open? Nah. JMO jimsjinx
 

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Im going to bump thread as the info is needed, i am going to bypass the fuel shut off and add a manual shutoff for testing.

I am going to be playing with a Weber two barrel 38DGAS synchronous carb. on my 88' GL1500 as i hate the delay/performance of the OEM Vacuum diaphragm carbs. and am looking for performance, Not to be smog clean.

The stock OEM carbs work Great, there are No issues with them at all.

Other than the OEM carbs are VERY smogged down, and very Slow to react, as it is there inherent nature.
 

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I would think the best way to bypass the stock system is to put a manual fuel shutoff valve in the system. No different than any of the other older carbed wings. Should be easy to mount in the existing place of the stock vacuum unit.

If I was having problems with the stock unit I would look at plugging the vacuum line and installing a manual fuel shut off valve. Simple and effective.
 

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It aint rocket science
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I would think the best way to bypass the stock system is to put a manual fuel shutoff valve in the system. No different than any of the other older carbed wings. If I was having problems with the stock unit I would look at plugging the vacuum line and installing a manual fuel shut off valve. Simple and effective.
Why and where? The older machines were not totally encased in plastic, drill a hole in the plastic and install valve? Open the fuel door and squeeze your fingers in to open and close a valve on every start and stop? No, thank you.

I like mine as designed to be automatic and when/if it fails will pay the $20 in parts for another 20 yrs. of likely service.
 

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life is good
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why and where? The older machines were not totally encased in plastic, drill a hole in the plastic and install valve? Open the fuel door and squeeze your fingers in to open and close a valve on every start and stop? No, thank you.

I like mine as designed to be automatic and when/if it fails will pay the $20 in parts for another 20 yrs. Of likely service.
lol.:rofl::rofl:
 
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