Petrol supply to carburettor; modifications

[AdamWilkes]

In a thread to do with ignition problems, I found a petrol-pipe modification (also suggested by Renault, I think). It helps to cool the petrol in the engine-bay a touch (reducing any problems experienced with vapour-lock/petrol evaporation when parked-up after a hot run), and eliminates residual pipe-pressure when the engine is not running. The modification is two T-joints (I used brass 6mm types) and the 6mm hose between them, as below:

The calibrated hole size was not known. I soldered over the right-hand brass T-joint exit pipe (the T-joint nearer the carb), and then drilled though the solder. I tried a 1.5mm diameter, but the pump could not pump petrol after it had been stood overnight. Petrol pumped (visible through my clear petrol-filter between fuel pump and carb-end T-junction) seems 'splashy', and may be full of bubbles, which I guess keep on circulating around the new pipework. Bleeding the pipework may help, but it would need doing every time one had a dry pipe; maybe re-routing pipes to eliminate air-traps would help (bubbles would be vented into the carb), but that's not all that easy.

The pump can't make the same pressure as before, as it's outlet pipe is open (through the small calibrated hole) back through to the petrol tank. The pressure needed is that to raise the petrol from the tank up to the carb, and is a lot less than a good pump can produce, so the calibrated hole needs to be small enough to still let the pump do it's work without bleeding the pressure straight back to the tank.

I tried a 1mm hole, and that seems to work, though I suspect a slightly smaller hole still would be better.
Whether this tiny hole size is actually any good for cooling circulating petrol enough, I am unsure, but it certainly de-pressurises the pipes when the engine is off.

 

[angel]

This arrangement was improvised by Renault solely for de-pressurizing the fuel line between pump and carburettor, not for cooling.

Interesting to hear this trick works well - I have never done it but suggested it to a Fiat mechanic, after he told me he had similar problems on a Fiat 500 that he couldn't resolve.

 

[Azazello]

A few days ago I've seen a fuel pump for cleon engines that has a third pipe with a tiny hole... maybe is involved in this game?

 

[mr-reno-139]

Some of the mk2 R5 super cinque had a metal bowl bolted on a bracket to recirculate fuel and nearly all other renaults fitted with carb have got a third small diameter pipe on on pump to return back to tank
Modern petrol seems to be more volatile and designed for injection systems whereby it constantly circulates back to tank keeping fuel from vaporising

 

[orsome]

Hi, i'm still having promlems with my R4. I tried to adjust petrol level in carb but began to wonder if it is writh to adjust the needle upside down(acc Sylvie R4L instructions), when needle drops many millimetres downwards when fitted in?
I have the Zenith 28IF model with only one narrow and long pipe to carb pot.

 

[AdamWilkes]

I have, alas, reverted to the original single-pipe system (with a filter before the pump), as I was needing to crank the engine for about 20 seconds after leaving it standing for a day or two. Seemed like fuel disappearing from the carb, and a lot more difficulty filling it with the return pipe in place. The fuel pump is about 2000 miles old. I guess the pipes & pump could also have emptied in the standing time. I reduced the small return hole to 0.5mm but the same effect recurred.
With the single pipe back on it now pumps fuel more quickly to the carb, and so starts more quickly. I thought of fitting an electric fuel pump to fill the pipes when the ignition is switched on but, hey, the R4 is all about simplicity so I couldn't bring myself to complicate it any more! Sorry to disappoint, but it was fun testing the ruturn pipe system out!
I have 'lagged' the single fuel pipe (which is a rubber pipe now, running past the alternator & over the rocker cover) from the pump to the carb, with larger diameter rubber pipe to try and reduce heating from the engine.
If anyone knows of a Zenith 28if v05083 or v05083A carb, I would be interested as my R4 has the wrong one fitted (I have the F6 van 1108cc).

 

[Bobble]

Mine does the same but only if i leave it for a week or more, if it's used every day it starts first time every time, i to thought of fitting an electric pump and like you, but decided not to as it's not really a problem ( F6 1108 to ).

 

[AdamWilkes]

Good to see another F6 on the road - and looks like the colour mine was originally!

 

[Bobble]

Yes we have to keep them going, and here will be another one reasonable soon but it's taking a little longer than anticipated, i think most of them were red :-)