Author Topic: A word of warning for 500 and 750 owners with dual outlet fuel taps  (Read 1487 times)

Offline Oddjob

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Been doing a bit of work on these recently. The tap fitted to the early 750 and the 500, bowl underneath with metal strainer and the dual outlets etc.

After taking apart a few I've found serious problems with the seals inside. Most likely due to age but also due I think to the ethanol in fuel these days. The 2 orings were brittle and flat on all of them, the one which seals the tap against the tank and the one fitted between the brass strainer and the bowl. Simple enough fix, just fit some new ones in Viton.

However the special doughnut seal behind the fuel tap lever (16955-268-020) is another problem. These are quite expensive to replace, CMS at £5.74 doesn't sound so bad but it's the postage that kills that one or DS at £7.24 plus whatever postage DS deems appropriate, most likely £2.95 which makes it £10.78. That's not cheap but the problem I think is that they are old parts and are most likely made of nitrile so the same problem will rear it head again over time. Namely it's reaction to ethanol.

All the ones I removed were pretty bad, really stiff, hard to even bend, all of them had shrunk in thickness. I measured a new seal against and old and the old had really thinned out, around 3.3mm when new to 2.6mm when old. This seal relies on compression to seal, the 2 screws next to the lever compress the lever against the seal, saying that, why no grease or lube is used is beyond me, it's no wonder the lever can be stiff to move and it also explains why these wear down so fast. If you study a new seal you'll see the 4 holes in it are raised up, they all have a lip around them, on an old seal that lip is gone, worn away. As soon as that happens I suspect petrol starts to leak past the seal and into the carbs, it fails to shut off the petrol as it should. Maybe remove the pipes on yours and see if they are dripping when the taps closed.



So my solution was to try and make the doughnut out of Viton or FKM as it's known, that has proved harder than expected. I can make them but the thickness of the sheet of Viton I bought at 4mm means it's too thick for the tap to close up again. So I need to work out how to thin it down whilst still keeping it flat so it seals, I've just ordered some 3mm Viton to see if that would seal, I'll have to make one and see.

So just a word of warning, you may think your tap is closed and find it's actually not. Which could cause problems with your carbs if the fuel starts to sit there long enough. If it doesn't leak now, it's possible it may do in the next year or so.
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Offline deltarider

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Re: A word of warning for 500 and 750 owners with dual outlet fuel taps
« Reply #1 on: October 27, 2023, 09:03:50 AM »
This is another argument to keep the fuel system in a natural state, which is wet. These seals can shrink enormously when left dry, after they have been exposed to fuel. They do not when they're unused ofcourse, but once they have been in contact with fuel, it's best to keep them wet. If your rings have shrunk, best you can do, is put them in fuel and wait. The plasticizers will do their work. However, once seals have been exposed to fuels with different properties, one can expect issues.
The only seal that began leaking on mine, after I had removed the petcock for inspecting the filter, was the fat one. I had already ordered a new one from CMSNL, PN: 16997-467-000 and, guess what, it's still in its plastic bag. Just a fraction of a twist more, cured the old one after it had been in the fuel again.
A warning: I myself never had them come loose, but some in the international forum have. Whenever I pull the fueltubes from the brass pipes, I twist them back and forth in the process to reduce the strain and prior to fitting them again, I do some vaseline on the pipes.
When you use the right size fueltubes, there's no need to fit those annoying clamps. I have never had one coming off. But... only with tubes that have the right ID, which is 5,5 mm.

Offline Oddjob

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Re: A word of warning for 500 and 750 owners with dual outlet fuel taps
« Reply #2 on: October 27, 2023, 04:55:07 PM »
I fail to see your point. Even with the fuel tap in the off position the fuel tap is still full of fuel, so the rings are always wet, as is the doughnut. Even if your also one of those who advocate running the bike dry by switching off the tap when your near home the carbs will still have fuel in them, just because the engine stops it doesn't mean there is no fuel left, it just means there wasn't enough to keep it going. They'll still be fuel in all the little chambers of the carbs as well.

I advocate using silicon oil on the ferrules to avoid pulling one off, they are only soldered in place.

It's not that the seals have dried out, the orings were flat due to compression, more compression may work but at the cost that they are why not just replace. The doughnut is another thing entirely, that wears away, especially where the ridges are around the holes, the 2 screws holding the plate on can only compress so much, as soon as that limit is reached that seal will fail to seal. There were lots of posts years ago blaming the 2 fibre washers under the screws holding the tap in place being the cause of the leaks, whilst that may have contributed to leakage it's more likely to be the doughnut wearing away which was the cause. Study the internals of the tap, the lever just has a small recess carved into the back of it, all this does is connect 2 of the holes depending on where the tap is positioned, top hole is a blank, it's just a locating hole to stop the doughnut from moving, the next at 3 oclock is the main feed from the tank, bottom hole is the feed to the bowl, 9 is the reserve feed from the tank. So at off the holes at 12 and 3 are connected, no fuel passes, in the ON position 3 and 6 are connected, so fuel flows through the tap, in the reserve position 6 and 9 are connected so fuel still runs until the tank is dry. However the recess in the lever acts like a cheese grater on those lips around the holes in the doughnut, it's very slow as the tap doesn't really get moved that much. But eventually it wears away those lips, it most likely still seals until it wears away enough of the seal underneath to allow fuel to start to weep past.

THAT'S what this post was about, to warn members that this is a possibility, to let them know that even with the tap in the closed position fuel can still pass through it, given certain conditions.
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Offline Oddjob

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Re: A word of warning for 500 and 750 owners with dual outlet fuel taps
« Reply #3 on: November 01, 2023, 09:59:43 PM »
New Viton sheet has arrived. I've now cut some doughnut shapes out of it and I've made a jig to allow the alignment to the 4 holes to be spot on, just needs welding together. After that comes back I'll test the new doughnuts and see if they seal ok.

If I find 3mm isn't thick enough the new jig should allow me to thin down the 4mm. I'll let you all know how this pans out.
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Offline Oddjob

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Re: A word of warning for 500 and 750 owners with dual outlet fuel taps
« Reply #4 on: November 02, 2023, 12:36:40 AM »
Well that was quick. My lad took it to his mate to weld and it's already back. So I thought I'd test it in making some new doughnuts, the rig is supposed to get the holes lined up so they are right everytime, works well enough TBH. Made one out of 3mm Viton and tested it by using air, if when the tap is in the closed position you blow down the tall brass tube it shouldn't allow you to do this. And bingo, test passed. Just needs a practical test now.

I may start to do a service kit for the 750/500 fuel tap, both orings in Viton, a Viton doughnut and 2 stainless steel screws to hold the lever plate on in case you round them off getting them loose, they've been in there a long time after all.
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Offline Laverda Dave

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Re: A word of warning for 500 and 750 owners with dual outlet fuel taps
« Reply #5 on: November 04, 2023, 09:54:01 AM »
Is this the same size as the 400 item Ken or is the 400 smaller?
1976 Honda 400/4
1977 Rickman Honda CR750
1999 Honda VFR 800FX
1955 750 Dresda Triton
1978 Moto Morini 350 Sport
1978 Honda CB400/4 'Rat' bike
1982 Laverda 120 Jota
2020 Royal Enfield Interceptor 650

Offline Oddjob

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Re: A word of warning for 500 and 750 owners with dual outlet fuel taps
« Reply #6 on: November 04, 2023, 02:33:47 PM »
Different tap used on the 400 Dave, single outlet for one, and it screws onto the tank whereas the 500/750 bolts onto the tank.
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Offline Laverda Dave

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Re: A word of warning for 500 and 750 owners with dual outlet fuel taps
« Reply #7 on: November 04, 2023, 02:49:24 PM »
Thanks for the clarrification Ken. I must have this mixed up with the Kwak 400J I restored a couple of years ago, it had the same arrangement and I remember buying a kit to refurbish the fuel tap.
1976 Honda 400/4
1977 Rickman Honda CR750
1999 Honda VFR 800FX
1955 750 Dresda Triton
1978 Moto Morini 350 Sport
1978 Honda CB400/4 'Rat' bike
1982 Laverda 120 Jota
2020 Royal Enfield Interceptor 650

 

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