Should there be a breather hole in the inlet manifold directly under the Carburettor?

[PhilthyPhil]

Hi,

I decided to finally install the vacuum advance on my electronic ignition system.

I removed the Carburettor to install the new insulation plate, that has the vacuum hose connection and noticed there is a small pin hole in the air inlet manifold.

It look like it is meant to be there but I thought this part of the fuel delivery system was meant to be air tight?

Can some please confirm if this is normal?

Regards,

Phillip.

[2CViking]

Some manifolds have that drain hole, earlier A models didn’t. Many people notice after cutting the engine off, that a small amount of fuel is dripping underneath the carburetor. The pump continues to fill the carburetor bowl until the needle valve blocks the flow. The excess fuel ”overflow” the carburetor and drips out through the manifold. Some of the last A models were fitted with a pad sandwiched between the manifold and top of engine to catch the fuel (allowing it to evaporate) Without the drain hole, the fuel would find its way to combustion chamber making hot start rather difficult (extra fuel)

[PhilthyPhil]

Thanks Peter,

You are a wealth of information as always!

Now to put it back together again…

[2CViking]

It is VERY important that the manifold is fitted (bolted) correctly. In some cases, both cylinders heads needs to be loose when fitting the manifold. Loose heads allow the manifold and the heads to mate together securing good contact on the gaskets on the head side and manifold side. Especially exhaust side can cause the gasket to burn rather quickly if the 2 faces are not perfect. Hot exhaust gasses burns the gasket if any gab is present. The inlet side is important too. If not sealed ”false air” is drawn in distorting the mixture. Offer the manifold WITHOUT gaskets and see how well it fits on the head. If too much uneven gaps loosen the heads (allowing them to turn) tighten the exhaust nuts first (only 1 kg or so) then inlet side. Tighten both heads also 1 kg. Remove manifold, fit the gaskets and tighten the manifold using copper or brass nuts (steel will melt) When done, do the heads no more than 3.5 kg. Adjust the valves to 0,20 mm.

[PhilthyPhil]

Thanks for the additional tips.

I will have the covers off anyway checking the clearances, so can will check the fit of the manifold to the heads.

Do you recommend using a copper gasket goo as well to ensure a perfect seal?

[2CViking]

I use cement exhaust gasket seal just a small amount, nothing on the inlet. I also flatten the exhaust flange with a file removing rust etc. Careful on the inlet bolt 1 kg only, very easy to strip the threads. Check all bolts and nuts after 1000 clicks or so.
You need 2 gaskets under the carby. 1 between the manifold and the spacer and 1 between the spacer and carby ( 2 different shapes) If base of carby is warped, it need flattening use a fine file.

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