Hi all.

Can I have some advice please

Sorting out a fuel tank problem (sludge) after a long lay off from moving. Fuel pipes all flushed through now and clear and this pipe on top of the float chamber started leaking fuel. Is it due to the float inside sticking open or should it have a pipe on?

Also, where would the engine i.d. number be located. Looked everywhere.

Many thanks

 

That pipe is an overflow from the float chamber, the reason fuel is coming out of it is likely due to the valve sticking in the lid of the chamber due to lack of use. The best thing to do is renew them as the valves develop a ridge around the tip and fail to seal. The pipe(s) should have a tubes attached to them to direct any fuel away from the engine and down below, there should be a couple of brackets attached to the oil block to secure them and just on top of the flange that the oil block is bolted to is where you will find the engine number.

 

Thank you very much Nigel. My workshop manuals don't reference these for some reason

 

You might also have a leaking float that is sinking ~ quite common. Newer needles & seats have viton tipped needles that are less inclined to leak.

Engine number should be repeated on the cylinder head.

 

Hi Glyn

Thanks for this info. I did find a 8 inch length of plastic fuel pipe attached to the fuel tank. I thought it was a breather pipe. Could it be part of a return pipe from the carbs that has broken off?

 

SU carbs do not have a return pipe to the fuel tank as a modern car has. They have a single line from the tank to the carbs and the pipe that Glyn has mentioned that you are missing is just an overflow pipe from the float chamber to the floor. On my S Type it is a slightly different design that has a banjo bolt going down the centre of the float chamber holding the lid on and acting as a slide for the float in the chamber. At the top there is a banjo bolt head with a copper tube leading off down the back of the carbs to the side of the sump where any overflow is dumped straight on to the road.

 

SNG show a pipe with a banjo fitting which connects to the carb. Mine looks like a rubber 90 degree fitting should be attached. Any ideas anyone? Who do I try next?
Still not happy with them just draining to the floor!!!
Ron

 

You might like to contact The Worlds Sole Manufacturer of Genuine SU Parts - SU Carburetters to see what they say but unfortunately the pipes do drain to the road as from the factory. Under normal situations it would be very rare to get fuel coming out of the overflow as this is only caused by a fault with the float bowl. I'd post a photo of my system but as I've fitted HD8 carbs they are not the same as the standard 1 3/4 carbs.

 

Hi Nigel. Thank you for your reply. Ron

 

New gaskets, needles and a good blow through of the float chambers. Adjusted float level, primed system and no joy. Fuel still coming out of overflow pipe as before though not as much. Tweaked the float level again but again still leaking. Also noticed fuel now coming out from another place somewhere under the carburettor. Took carburettor off and it was coming out of one of the what looks like one of two drain points underneath the intake. What are these drain points for? Any idea why this would be happening? Also noticed that a v shape has been cut out of the bottom of the air intake for some reason. Possibly to fit around the carburettor spring mount or something! Looks quite deliberate. Any clues anyone.
Thank you
Ron

 

That only leaves the floats themselves, they are not coming up to shut the needle valves off when fuel is present.

I have those ports on mine too, don't know what they are for, I have mine capped off.
But they are there to probably prevent fuel from filling up the engine in a situation just like yours.

The other thing that will cause this is if the float level has not been correctly set, check your specs and make sure it's for your carb and float bowl type.
If not right the fuel level will always over flow through the main jet and leak out those ports you mentioned.
Once the fuel starts to flow out the main jet, the float will never reach the needle valve seat to shut the flow of fuel off.

The float bowl and needle valve has to be set just to the point where the level of fuel is about 1/8 of an inch below the main jet

Do you have the correct float bowls in there ?

 

Jeff is on target--there is a float or float needle issue. That said the V needs to be repaired as you will pull unfiltered air. The two fittings on the plenum should have lines (plastic) routing to the bottom of the engine bay--it is the same idea as the overflow lines on the SU carb float bowls. All of that said is it possible you are running a fuel pump that has too much pressure?

 

#6
HS Carburetter Description and Adjustments - SU Carburetters

 

Had that problem on my 51 Bentley, someone installed an aftermarket electron fuel pump, and it was always flooding, got worse when the weather was hot of course.

 

Thanks for all of your info.
I guessed it was a float issue but now I'm not so sure. The car was running absolutely fine before hibernation. Thinking about it, the fuel tank and pick up pipe was well and truly gummed up with the brown 'orrible sticky stuff left in the tank over many years. (Oh and this was the filter. Look away if you are easily offended!) It has probably been struggling for fuel for quite a while.
Had the tank and pipes cleaned out.


Whilst this was being done I flushed the system from a can of fuel in the boot which now the pipes are clear could be providing too much fuel/pressure. How do I now check/reduce the pressure? Is this the correct pump?

 

Looks like a correct SU fuel pump, with out checking it put out around 5 to 6 psi.
I'm not sure how to actually check the pressure, but being an SU pump, it will work.
All that gunk in the system; I wonder if that was simply getting caught in the needle valve, although there should be a fine screen filter right after the banjo bolts on both carbs where anything big enough to catch large particles that would affect the needle valves should have been dealt with.

 

Hi Jeff. I've changed all the filters and needle valves and cleared out everything. I looked on youtube and a similar setup had the 2 overflow pipes linked together! Any thoughts?

 

The YouTube video is wrong.
This is from post 12.

The two fittings on the plenum should have lines (plastic) routing to the bottom of the engine bay--it is the same idea as the overflow lines on the SU carb float bowls.

 

Sorry, I meant the 2 overflow pipes from the top of the float chambers.

 

If fuel is running out of the plenum chamber drains then one or both carbs are flooding.

It can only be caused by excessive fuel pump pressure or a leaking needle & seat, float/fuel level too high or sinking floats due to a leak allowing them to fill with fuel. Then they sink or partially sink & do not shut the needle & seat leading to flooding.

SU carburettors work well with fuel pressure in the range of 1.5 psi to 3.5 psi at the inlet to the float chamber. Pressure of 5 psi or higher will likely make the fuel overflow. The original SU electric fuel pump has an internal spring on the operating solenoid that effectively limits pressure to the intended range.

So check fuel pressure with a gauge at the pipe where it connects to the float bowl. Better still if you have leaking needle & seats on both carbs or excess pressure might force both needle & seats open, go back to where the pipe that comes from the fuel filter joins to the metal pipework with banjo fittings to both carbs & take the pressure reading there. (At the flexible jumper pipe after the filter that is attached to the inner wing)