Breather pipe restrictor- why?

[Malc Holmes]

There is a small metal pellet with approx 1.5mm hole drilled through it, acting as a restrictor, inserted in the breather pipe between the fuel tank filler on the RHS and the roll over valve. After the roll over valve, the pipe goes to the charcoal canister and from there, on to the inlet manifold.

What is the purpose of the restrictor? Only fuel vapor travels down this pipe on its way to the charcoal canister.

Take a look at the picture below from the pats book. The restrictor is part number 21. You may have never seen it unless you have needed to replace your breather pipes and even then its very easy to mis since its pushed down inside the pipe.

I have my reasons for asking, but want to see if anyone else understands why its there.

[andydclements]

Pure guess (mine has the mechanical roll-over valves), could it be that its' designed to restrict the amount of fluid that can go through, so that in the event of the car going wrong way up, the roll-over valve isn't the only thing stopping fuel getting through?

[lotus4s]

I agree with Andy, I think it's to restrict fluid flow which can also happen if the tanks are full and sloshing around.

[sailorbob]

It is to limit the amount of fuel vapour getting to the combustion chambers during purging.

[andydclements]

Wouldn't have thought so, the fuel is collected in the cannister and the air is drawn over the cannister on its way to the engine from the fresh air pipe.

[sailorbob]

The fuel vapour does not have to pass through the charcoal before being vented to the inlet manifold which is necessitates the need for a restrictor to prevent an overly rich condition.

[andydclements]

True, but that is only for a very very short time while the tank is slightly higher air pressure than the atmosphere and the solenoid on the top of the canister opens. Once the tank pressure is equal to atmospheric then there will be no significant quantity of air movement from the tank, and the venting will be via the free-to air pipe over the charcoal, which can put quite a bit of fuel vapour out itself.

[sailorbob]

When the solenoid valve is open and the inlet manifold is under vacuum there will be negative pressure in the fuel tank. This is why some manufacturers refer to the restrictor as a "vacuum restrictor".

[andydclements]

But with the tank at a lower pressure than atmosphere the air will come from the higher available source, the free-to air pipe.

The restrictor will reduce the effect the pressure reduction has on the fuel level, so stop it sucking neat fuel through.

[Malc Holmes]

I Kind of hope Derek is correct, and this is my reason for asking the question. I went to a small car show yesterday and noticed the smell of fuel in the cabin. Breather pipes of course where the normal suspect, so decided to check them all when I got home. Sure enough a couple of big splits and a smaller one. The small one was between the restrictor and canister. All pipes now replaced, and restrictor re-fitted.

The split between the restrictor and the canister would have allowed unrestricted amounts of air in and , I kind of hoped, could account for the stumble I get on very low loads, under gentle acceleration before any boost kicks in (after the boost kicks in there is never any problem). My theory is that this unlimited air is flowing into the manifold and messing up the mixture to some degree when there is manifold vacuum present.

Unfortunately as soon as I finished the pipe replacement last night the heavens opened, and I wont get a chance to try it again till the weekend.

If, by any chance, its improved, it could be a good pointer for many more folks out their who have a similar stumble (I know there are several) and have either not checked the pipe for splits, or replaced the pipe and not noticed the restrictor in the old pipes they have thrown out.

Views, opinions? Are my hopes well founded or will I be disappointed come the weekend?

[sailorbob]

You're part way to seeing how it works Andy. By acting as a vacuum restrictor, yes, there is a bias of flow from the fresh air inlet. This is needed because the fresh air flowing into the canister 'purges' it of the stored fuel vapour.

[Jonathan]

I've just disassembled the whole system to find any leaks - pressurised it and tested it in many places. Mine has a stumble and a fuel smell but the tanks are very much sealed as well as the piping.

I actually put it down to the fact the charcoal canister fresh air pipe is always open to atmosphere + venting from the main tanks - I put a non return valve in there to force the fresh air pipe to breathe IN only...not had time to test.

Malc, what you're saying is the charcoal might be saturated ?

That was one of my theorys as well with mine - sounds plasuable, might check mine tomorrow.

[Malc Holmes]

...

Malc, what you're saying is the charcoal might be saturated ?

That was one of my theorys as well with mine - sounds plasuable, might check mine tomorrow.

I was thinking the split would possibly be allowing large amounts of air to flow into the manifold rather than small amounts of fuel vapour as it should be if it where drawn in through the restrictor rather then the down stream split. In summary, I thought it could be leaning the mixture before boost came on.