After I signed off last night, I thought of three other places
where the Metzger Midget could be having problems. If memory
serves, the '70 Midgets have
1) some kind of crankcase ventilation system leading into
the carbs -- little rubber hoses going into brass tubes
set into the throttle body;
2) a vacuum takeoff for the distributor, running from one of the
carbs to the diaphragm housing atop the distributor;
3) the vacuum diaphragm itself.
Check these places, especially the connectors, for leaks; if you
really want to be sure, buy a set of rubber caps of various internal
diameters and plug all the holes in your carbs except the one
that you stick the Unisyn over. It could very well be that the
rubber hoses either vibrate loose or expand under heat, letting
more air in and thereby raising your idle speed. On the
distributor hose, check both ends of the hose. The way to test
the vacuum diaphragm (the advance mechanism on the distributor)
is to put a vacuum pump on it, give it just a couple of squeezes
to get a small pressure differential, and then hold it and watch
the gauge. If the dial won't hold a vacuum of 10 or 15 psi for
any time at all, you've got a leak in the vacuum advance mechanism.
Note that if the carbs are anything close to stoichiometric,
a vacuum leak from one of these plugs will cause the car to stall
because it goes far, far too lean. But consider that your 1275
should be returning about 30 mpg. You're probably running an 8:1
or so air-fuel mixture -- should be 14.7:1 for economy and emissions,
no more than 12.5 or so to 1 for maximum power.
We'll fix your car yet, even at a distance!
--Scott
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