vacuum gauges and proper crankcase venting

[whagan]

I have been restoring a 67 USA Duetto that was occupied by mice in my fathers barn for over 30 years (they enjoyed the use of the combustion chambers as toilets).

I've been a bit mystified about function of the small tubes on the #1 cylinder intake manifold and the one coming out of the crankcase vent tube. I assumed they were connected together as some kind of PVC type setup but plugged them when I realized that it might be the reason why #1 runs lean. It's just a well filtered air leak in the manifold. What are these for, and how can a hole in ONLY #1 intake not cause uneven mixtures?

My other question is there a way to hook up my mercury vacuum column gauge that I use for the motorcycle to sync the Webers? The only port is the one mentioned above on the #1 intake manifold.
Thanks!
Wayne

[FreeLSD]

What's carb models do you have? Place a pic if possible.

[seani]

The small tube is part of the crankcase vent system for the original air filter housing. When I fitted rampipes to my car I plugged this tube and used a longer hose from the cam cover to vent under the car like they used to.

[whagan]

I understand how the larger hose works as a crank case vent, it's the small one that makes no sense to me, (see attached pic from other post) the small one runs from the intake plenum/crank vent hose (post air filter, pre carb) down to the #1 intake after the carb, so it acts like a big vacuum leak only to #1 i.e. lean #1 cylinder running?
Thanks,
Wayne

[papajam]

Yes, it would appear that the small hose would create a vacuum leak. But the hose is not a source of filtered air. The hose routes crankcase blowby gases to #1 cylinder at idle. These gases have little to no oxygen.

[whagan]

The lack of O2 makes sense, although I'm still suspicious of it's effect on #1 because with mine installed I have to richen up #1 more than usual.
Thanks!