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Author Topic: Fuel vacuum valve failing?  (Read 6286 times)
Cannon
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« on: March 15, 2010, 01:33:52 AM »

Hello again. I'm making a new thread on this subjeckt.

This is the issue; ny fuel vacuum valve (-95 3. gen) was stuck and i had to fix it. While doing the manual tests before putting it back I discovered that the valve opens as it should when applying vacuum. What I did not expect was that the valve remains open after cutting off the vacuum. There is actually a tiny one-way air-valve in the housing, in the part where the vacuum hose enters the valve, and this air-valve restricts air from flowing back into the vacuum camber. The fuel valve spring inside the chamber is for this reason not able to shut off the fuel when vacuum is cut off.

Anyone know why this is? I left the thing for 2 hours and it did not close during this period. What would be the idea with a vacuum valve that opens once and never closes?

The inner part of the housing, where the vacuum hose enters, has a small aluminum "lid" with a tiny hole in it. Inside that hole I can se a small "rubber ball" or something similar, and this is the "return valve" restricting air from flowing back into the housing. Why is this thing there, if not to restrict the returning air? I was thinking, maybe it's there to delay the returning air, and not to totaly restrict it? Anyone?     

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lragan
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« Reply #1 on: March 15, 2010, 09:55:03 AM »

Found this paragraph in the general shop manual from Honda.  Hope it helps.
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Lawrence
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« Reply #2 on: March 15, 2010, 02:09:19 PM »

You are talking about this, correct?

I had an issue with mine not allowing gas through to the carbs, have you clean it well?







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Terry

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74 GT750 - 75 GT380 – 01 & 97 Magna - 03 KX 250-01 – 01 WR 250 – 04 WR 450 - 74 T500 Titan
Cannon
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« Reply #3 on: March 16, 2010, 02:51:07 PM »

Yes this is the one. And I have taken it all apart and cleaned t.
You can see the aluminum circle shape inside the rightmost part of the housing in the 2 last pics. There is a small hole in the aluminum part, where the vacuum is applied. Inside that hole there is a rubber ball (on mine anyway), and that is what restricts air from entering the vacuum chamber when vacuum is discontinued.

The question is; should I take the rubber ball away?
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lragan
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« Reply #4 on: March 16, 2010, 03:08:09 PM »

So you are telling us there is a check valve inside the housing where the vacuum line attaches?  The diagram does not show this "feature".  I do not understand why it is there.

To provide a delay before shutting off the fuel valve?  Why would one need this?  I don't get it.
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Lawrence
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« Reply #5 on: March 16, 2010, 04:54:29 PM »

I've got a replacement valve set (part # 16953-MN5-023) which consists of the three parts in the last picture that Terry posted (flapper diaphragm center, spring and vacuum cover) that I have yet to install.  The vacuum cover does appear to have some sort of one-way valve in it (only allows suction).  I don't know that this would prevent the diaphragm from shutting off fuel flow though once suction is removed.  There does appear to be a vent in the center diaphragm part, perhaps yours is clogged?
« Last Edit: March 16, 2010, 04:58:54 PM by LIMagna » Logged

Charlie
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Cannon
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« Reply #6 on: March 17, 2010, 01:26:32 AM »


So you are telling us there is a check valve inside the housing where the vacuum line attaches?  The diagram does not show this "feature".  I do not understand why it is there.

To provide a delay before shutting off the fuel valve?  Why would one need this?  I don't get it.
Me neither Confused

The vacuum cover does appear to have some sort of one-way valve in it (only allows suction).  I don't know that this would prevent the diaphragm from shutting off fuel flow though once suction is removed.  There does appear to be a vent in the center diaphragm part, perhaps yours is clogged?
You have discovered the same thing on your part as I have. Have you tested it to see if it closes when suction is removed?
The vent in the center is open and ok. It can be seen in the shop manual page posted by Iragan.
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LIMagna
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« Reply #7 on: March 17, 2010, 05:36:09 AM »



The vacuum cover does appear to have some sort of one-way valve in it (only allows suction).  I don't know that this would prevent the diaphragm from shutting off fuel flow though once suction is removed.  There does appear to be a vent in the center diaphragm part, perhaps yours is clogged?
You have discovered the same thing on your part as I have. Have you tested it to see if it closes when suction is removed?
The vent in the center is open and ok. It can be seen in the shop manual page posted by Iragan.


  No, I haven't tested it.  I purchased the parts a while back after reading that the diaphragms can become problematic with age but have yet to swap them out.  I'm not sure if holding them together will be enough to simulate installation but I can try it when I get home later today. 
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Charlie
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« Reply #8 on: March 17, 2010, 05:21:10 PM »

Okay, as best I can tell (holding the parts together with my fingers), the valve slowly closes when vacuum is removed.  It doesn't snap shut but within two or three seconds, it's extended the plug which should shut off the flow of fuel. 
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Charlie
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« Reply #9 on: March 17, 2010, 07:01:47 PM »

That is what one would might expect it is supposed to do.  There must be an orifice somewhere that admits air at a slow rate into the vacuum side of the chamber.  If you can identify where it is, perhaps Cannon can clear his and he will be back in business.
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Lawrence
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« Reply #10 on: March 18, 2010, 06:25:43 AM »

I would bet it just leaks back through the check valve.   The check valve prevents pressure above ambient from being applied to the diaphragm but does not completely seal the unit in the absence of vacuum.  I don't remember seeing any other means of venting short of the center section vent. 
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Charlie
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« Reply #11 on: March 18, 2010, 07:12:14 AM »

Here I go again -- speculating about something I have not experimented with, and really don't know much about.  So I beg your forgiveness in advance. Cool

As a designer, I don't understand how one could be assured that all the produced units would release in a timely manner through the check valve -- unless maybe you created a series of grooves in the ball?  If I want the device to close within a prescribed time frame, I am going to have to create a path with a controlled leak rate.  Is there not a tiny hole somewhere that establishes this leak path?

The designer had to care that the valve would shut off -- otherwise, there would be no need for a valve at all.  How about in the diaphragm?  Is there a hole in it's center, maybe?
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Lawrence
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« Reply #12 on: March 18, 2010, 07:45:05 AM »

You may very well be right ... I'll need to inspect the diaphragm a little more closely when I get home.  It may draw air slowly through the center of the diaphragm from the vent in that center section.  By the time this is done, we're going to understand exactly how this thing works Smile
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Charlie
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96 VF750C Magna - Pearl Shinning Yellow
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« Reply #13 on: March 18, 2010, 05:05:03 PM »

Okay, I've looked this thing over nine ways to Sunday and can't see any deliberate path to vent the vacuum side of this device.  I thought that there might be a small hole in the middle of the diaphragm but if there is I can't see it.  I've attached two pictures I took but I don't think they help much.  The vent in the middle section appears to vent only the center section between the two diaphragms.   
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Charlie
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Chad in Michigan
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« Reply #14 on: March 18, 2010, 06:11:33 PM »

from what i know about that device, this is my take on it:  when the bike is off, there is no vacuum applied to the 'in' side, aka the little tube that connects to a small black vacuum line that connects to cyl#4 (right front) when the bike cranks over & fires, it creates a vacuum and pulls the diaphram tighter and tighter allowing fuel to start to enter the other side of the diaphram. the fuel is held back by the large black rubber stopper on the opposite side. the purpose of the little rubber 'ball' i think is to block the flow of vacuum, or to keep maximum vacuum applied to hold back the diaphram during operation. if it were not there, i would assume that the diaphram would pulsate with each vacuum pulse from the engine and cause sporatic fuel flow. when the engine is turned off, there is no more vacuum source, so the diaphram closes off the fuel flow via the return spring, and blocking the fuel from flowing down to the carbs. i've tested mine (when i actually rode with it on the bike, have since removed 2 years ago) and it did not shut off instantly--it took like 4 or 5 seconds to completely shut off the fuel flow. if yours does not shut off after 5 seconds or so, something is wrong, either a torn diaphram and fuel is leaking into the vacuum chaimber and allowed to drain into the engine (i've read about this last year) or the return spring is weak, not shutting off the fuel, or the vacuum 'in' port is obstructed in some way. this is just a saftey precaution valve, you don't actually need it for the bike to function.
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Chad Schloss

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