Probably last ride this year

[RGP750]

It has to be a tap problem ,tank on bench,attach a length of tube 6", to both pipes
on the tap,put tap on reserve,fuel will flow from rear pipe but not front,put finger
over rear pipe and fuel flows from the front pipe.
As Lobo says you would think when the two carbs being filled by the rear pipe are full the fuel would flow from the front
but this dosen't happen for some reason.
 as for breather pipes the carbs were rebuilt this year and if i swap the fuel pipes over whichever pair is fed by the front pipe is starved. weird!!!

[Lobo]

Rich,

A possible explanation...

The tank fuel enters a common chamber within the petcock, and from here exits straight out the 'aft' nipple, and via a couple of 90 degs bends the forward nipple. With the petcock ON all is well...

When set to RES, fuel only flows out the Aft nipple (easiest route), but not out the forward. However, if the Aft is restricted, it'll then flow normally out the Fwd nipple...

So... this implies (to me) that there is a restriction in the reserve tank feed, ie it is unable to supply enough fuel to BOTH forward & aft nipples simultaneously.
Ie... The reserve filter is likely clogged, or it's fuel gallery clogged prior to the common fuel bowl.

Perhaps a new filter, sonic clean or air line?

Simon

[tom400f]

I would agree with Simon that the flow to the forward outlet must be in some way restricted (analogous to an electrical resistance) and the fuel then (only) takes the easiest route. The fact that full carbs then does not seem to promote the restricted flow is certainly odd and not easily explained. So we'll park that

Don't know about the 750 but the reserve mechanism on the 400 is very simple. You doubtless know all this I guess but anyway... the tap has two inlets - one for main tank and one for the reserve. The reserve level is achieved by the fact that the inlet pipe is about three inches long sticking upwards from where it mates with the tap. There is a gauze that envelopes this as the fuel filter. When the level drops below the pipe the reserve opening (being at the bottom) allows the remaining fuel out. The pressure will be that much lower as the level in the tank drops - may be it's not really at reserve - can you tell?

If it is then one assumes the reserve opening in the tap is partially blocked. I guess you have had the tap apart and cleaned it/checked the orifices.

[Lobo]

...found this helpful pic.  Hope it helps...am sticking to the above theory!

(ie the reserve feed (one of the red arrows) is partially clogged)

Simon

[RGP750]

Hi Guys
Thanks for all of your input.
I'll take the ruddy thing off again.
I have blown out tap tubes with an air line and blown down the brass tubes with my mouth
resistance seems the same with both.
I could be a bit technical with this and try the flow by measuring the amount of fuel
flowing from the rear pipe over a measured time and then try the front pipe and see if they are the same.
maybe  I feel a new tap coming on.
cheers Rich

[RGP750]

Thanks Odd job I'll pm my address.
Be good to give that a try

[Lobo]

Hi OddJob,

That pic shamelessly 'stolen' off the net.... I'm not near my bike (or inclined!) to dismantle it to this extent despite being very interested in the problem. (sorry Rich!)

If you're prepared to strip it all down again why not simply boil (the metal parts) for a few hours with a dishwasher tablet (whatever) - there's likely 40 years of varnish furring its arteries. Elsewhere, and recently, on this forum is a thread dedicated to such cleaning techniques.

Treat it to a new fuel strainer.... or better still leave this part out for the various bench tests.

Keep us informed...

[RGP750]

Morning LoBo
Strainer is new, I'll try the boiling technique tonight.
The misses is away house sitting so shouldn't get grief 
Rich Looking at your post time have you got insomnia?

[Lobo]

.... yea posting times... not 100% sad! just work abroad (Asia) & 7 hrs ahead of the UK... which often gives me plenty of time to alter my dafter posts before making a complete arse of myself...