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ray 316
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Joined: 11 Dec 2009
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PostPosted: 15:37 - 22 Dec 2009    Post subject: vacuum fuel tap Reply with quote

Can someone explain how these work because of the info l'm getting off here and elsewhere they seem to think the honda vision might have a vacuum fuel trap and this may be the cause of the petrol leak l have when the bike is stationary.
If it works like a on/off tap for fuel and l find that the vacuum fuel trap is faulty could l take it off and just put on a on/off tap.
Also if it works on vacuum when the bike is running and should shut off when its standing.....Say it is faulty and petrol is going through would'nt the carb float value shut off the petrol at a certain height so why would it leak petrol still... Confused Confused Confused
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temeluchus
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PostPosted: 15:54 - 22 Dec 2009    Post subject: Reply with quote

take a look at this

http://www.gpzzone.co.uk/gpzzone/technical/fueltap.jpg

you have a plunger (I) that blocks off the fuel supply until it is pulled back by vacuum supplied by the engine, via a vacuum pipe from the carbs or throttle bodies to the far side of plate K. When the engine stops spring J pushes the plunger back into the tap body (G) shutting off fuel supply. The PRIME position manually pushes the plunger back and allows fuel to flow.
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finpos
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PostPosted: 16:09 - 22 Dec 2009    Post subject: Reply with quote

The plunger is operated by a diaphragm, and the most common failure is that the diapragm cracks with age, then petrol can flow throught the crack and down the vacuum pipe. Where it goes from there depends on gravity - it'll either fill up your engine or it'll dribble out of the carb mouth. What is pretty unlikey is that it will come out of the carb overflow pipe. If it does that, you most likey have a double failure - the plunger is not cutting off the fuel supply from the tank and then the float valve in the carb has failed as well. You'll need to fix both.
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finpos
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PostPosted: 16:26 - 22 Dec 2009    Post subject: Reply with quote

...In the diagram above, part I is the diaphragm - you need to check it is intact and that it blocks the fuel passage when it is pressed in by the spring. If the tap is more than a simple on/off tap, i.e. it has main/res/prime, then you also need to check part F which is a rubber disk with holes in it that connect the various fuel passages together as appropriate, if it's damaged it'll allow fuel to bypass the plunger/diaphragm, and also part C which is a spring washer which sometimes looses it's spring. The simple way to test what's up is to pull off both the vaccum and fuel output hoses and see which one the petrol comes flooding out of!
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ray 316
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PostPosted: 16:31 - 22 Dec 2009    Post subject: Reply with quote

finpos wrote:
The plunger is operated by a diaphragm, and the most common failure is that the diapragm cracks with age, then petrol can flow throught the crack and down the vacuum pipe. Where it goes from there depends on gravity - it'll either fill up your engine or it'll dribble out of the carb mouth. What is pretty unlikey is that it will come out of the carb overflow pipe. If it does that, you most likey have a double failure - the plunger is not cutting off the fuel supply from the tank and then the float valve in the carb has failed as well. You'll need to fix both.
So if he vacuum fuel trap was faulty and fuel runs down to the carb will the float valve shut off at a certain level and stop the fuel from leaking out or will it still leak and if so where would it be most likely from. This is presuming the carbs alright but the vacuum valve is not ..
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finpos
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PostPosted: 16:39 - 22 Dec 2009    Post subject: Reply with quote

That's the double failure. Even given that the fuel tap has failed - in that it is not shutting off fuel when the engine is not running, rather than it has failed by allowing fuel down the vacuum line - then the float valve in the carb on it's own should be sufficient to prevent the carb overflowing.
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finpos
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PostPosted: 16:45 - 22 Dec 2009    Post subject: Reply with quote

...then, in the case of a faulty float valve, petrol will either dribble out of the overflow if the carb is fitted with one, which in turn is usually connected to a dangling pipe. If there is no overflow, then it'll fill up the carb and then either go into the engine or out of the carb mouth and into the airbox, where it might escape out of the airbox drain if that has one and the plug is missing. Start at the top and work down!
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ray 316
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PostPosted: 17:06 - 22 Dec 2009    Post subject: Reply with quote

finpos wrote:
...then, in the case of a faulty float valve, petrol will either dribble out of the overflow if the carb is fitted with one, which in turn is usually connected to a dangling pipe. If there is no overflow, then it'll fill up the carb and then either go into the engine or out of the carb mouth and into the airbox, where it might escape out of the airbox drain if that has one and the plug is missing. Start at the top and work down!
Thanks for that info mate l will start at the top and work down to the carb and see if l can find this leak, if l can get the dam side panel off.

If on inspection the fault is the vacuum valve trap could l replace this with a in line on/off valve , if l can any ideas on ones to use...
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yen_powell
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PostPosted: 17:31 - 22 Dec 2009    Post subject: Reply with quote

I had a vacuum fuel tap on my GPz750 that didn't have enough oomph in the spring to shut it off totally. I stuck a penny in behind it and it was fine for the next few years.
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finpos
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PostPosted: 00:15 - 23 Dec 2009    Post subject: Reply with quote

ray 316 wrote:

If on inspection the fault is the vacuum valve trap could l replace this with a in line on/off valve , if l can any ideas on ones to use...


Yes, and no. You can do that and it'll stop the thing peeing petrol over your garage floor as long as you remember to turn it off at night, however it'll never run right until you fix the float valve as well. If the tap is fixed directly to the tank, you'll also have the problem of finding one that has the same mounting pattern. Bottom line is that the (potentially) failed float valve is more of a priority that the fuel cock.

You can usually fix the vacuum taps by taking them apart, cleaning them and putting them back together - broken diaphragms excepted.

f.
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ray 316
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Joined: 11 Dec 2009
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PostPosted: 09:12 - 23 Dec 2009    Post subject: Reply with quote

finpos wrote:
ray 316 wrote:

If on inspection the fault is the vacuum valve trap could l replace this with a in line on/off valve , if l can any ideas on ones to use...


Yes, and no. You can do that and it'll stop the thing peeing petrol over your garage floor as long as you remember to turn it off at night, however it'll never run right until you fix the float valve as well. If the tap is fixed directly to the tank, you'll also have the problem of finding one that has the same mounting pattern. Bottom line is that the (potentially) failed float valve is more of a priority that the fuel cock.

You can usually fix the vacuum taps by taking them apart, cleaning them and putting them back together - broken diaphragms excepted.

f.
Ok will look today and if the carb float is at fault for the petrol leak what are the possible causes (most likely) and are these's carbs easy to work on... Any chance someone can give me some advice/info/diagrams/ step by step instructions if possible as this will be my first attempt... Thanks
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ray 316
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Joined: 11 Dec 2009
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PostPosted: 08:27 - 24 Dec 2009    Post subject: Reply with quote

Had a inspection of the fuel line coming from the tank down to the carb without taking the side panel off as l dont know how to get the seat lock barrel off. Anyway l could see up inside the panel and saw the fuel line with the vacuum trap right near the tank and l still had some petrol left in the tank but no leak coming from the vacuum trap or fuel line. l started the bike up and left it running for a minute then switched off but no leak from the area near the air filter Confused Confused So if the carb is dirty/needs cleaning if sticking l need to get the panel off, l'm just going to have to work out how to get the lock barrel off. l can see it has a clip on the inside of the panel holding the barrel and a bar from the barrel to the lock mechanism but l cannot work out what how to get the bar dismantled from the lock mechanism...
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27cows
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PostPosted: 13:49 - 24 Dec 2009    Post subject: Reply with quote

Can you post a pic or two of the problem panel?

Haynes made a manual for the Vision...and I think that might be your only means of getting tech info on your bike, as material seems extremely thin on the ground online.
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