Carburetor Manifold Fuel Drain Pipe

[ALC 68A]

I mentioned having replaced a missing one of these with a section of brake pipe on the earlier thread about changing a tacho to electronic operation. Now having read Jim Stone's reply, I am having second thoughts. As it is an open ended pipe, will that affect the manifold vacumm pressure, and therefore the operation of the brake servo? Would I be better off blocking up the hole after all? (I'm running Zeniths, though, not a Weber). Thanks.

Steve

 

[tony perrett]

The hole in the drain tube is so small as to have no adverse effect. Blocking the hole can cause a build-up of raw fuel which will find its way past the pistons and contaminate the oil in the sump. Some engines had a wider tube fitted with a ball valve.

 

[Tom H]

But since he replaced the original drain pipe with brake line, that hole is much larger and WOULD be a problem, no?

Tom

 

[Andrew]

Another note, I used a manifold from a Rapier. The drain hole was large enough to cause a problem with the new Zenith carb. It drained fuel but the car would not run and I could not get the carb set. As soon as I plugged the hole the car ran fine.

I still worry about the possibility of extra fuel being dumped passed the pistons and into the sump, should I be worried?

Regards,
Andrew

 

[Ken Ellis]

Andrew,
Well, a (brand-new?) Weber should be pretty well-behaved. I have a used-but-fine Weber, with no manifold hole, and no evidence of fuel-contaminated oil. I know I'm not real worried about mine, and it doesn't seem you should be worried about yours.

Especially in the tuning phase, complete sealing of the intake path would seem important. Later, if you want to reinstate the drain, try for a ball valve that is held closed by engine vacuum, but falls open and drains when the car is off.

The drain isn't going to flow fuel when the car is on, anyway, because everything will get sucked into the combustion chamber. It's only when the engine is off that fuel could pool... and then be drawn in at next start.

You could likely "tune around" a full-time-open drain, but I don't believe it's actually functioning as a drain when the engine is running.

 

[tony perrett]

But since he replaced the original drain pipe with brake line, that hole is much larger and WOULD be a problem, no?

Tom

Possibly - I should have said the hole in the original drain tube. The brake pipe could be flattened at the end to leave a hole just big enough for the petrol to drain away.