99Hawk120

Interesting little beastie I ran into the other day. You know those parts on the diagrams that are always marked with a "*" which means "not found on all models"? Well, I ran into a carb from a 1977 Oldsmobile 350 that has all those damn things, and I'm trying to figure out what exactly they do.

First is this thing labeled "aneroid assembly". Where every other qjet I've seen has a plastic "spacer" cup and/or a mixture control solenoid, this one has a funky dome shaped thing. As near as I can tell it seems to be some method of compensating the carburetor for higher altitude. The diagram shows a rod on the bottom of the assembly that (I'm guessing) goes into a fuel passage somewhere; it appears as though atmospheric pressure acts on the assembly and causes it to raise/low the rod allowing more/less fuel. Is that about right?

The other part that I can't figure out what it's for is the secondary power piston. What exactly is its purpose? Does it add more fuel to the main system or what? When does it kick in?

Interestingly enough, this carb had significantly richer main metering rods and slightly leaner jets than a similar carb pulled off a 1979 Caddy 425 motor.

RB83L69

Sounds like you have one of the special carbs supplied for high-altitude usage.

You are correct about the aneroid; it is basically a bellows pre-filled with a speific number of molecules of some gas, which causes the whole thing to expand or contract according to ambient pressure. A crude but surpisingly effective method of compensating for air density.

I've never had to deal with a secondary power piston, but my guess is that it does something similar... probably does less enrichment at lower ambient pressure.

An otherwise identical carb calibrated for a smaller motor will always have richer calibration in the main system than it would for a larger motor. If you consider what is happening from the point of view of the carb, if you compare the air flow requirements of a 500 inch motor to those of a 300 inch motor; the 500 while loafing along down the highway (cruise, time to be lean) will use as much air as the smaller motor wide open and screaming (full power, enrichment time). Under any given operational circumstances, the smaller motor will produce a smaller "signal" at the venturis, and so will nee dlarger jets to flow the correct amount of fuel. The idle circuit is the reverse of that: the larger motor at a given vacuum and idle speed will be flowing 60% more air than the smaller one, so the idle circuit (which feeds gasoline proportional to vacuum not flow) will have to feed 60% more fuel too. This is where it's hardet to get a CC Q-Jet to properly match the needs of a larger motor.

99Hawk120

Originally posted by RB83L69
Sounds like you have one of the special carbs supplied for high-altitude usage.

Whee! I found a "special" carb!

You are correct about the aneroid; it is basically a bellows pre-filled with a speific number of molecules of some gas, which causes the whole thing to expand or contract according to ambient pressure. A crude but surpisingly effective method of compensating for air density.

Neat. I'll leave that system alone then. Doesn't sound like it will cause any issues.

I've never had to deal with a secondary power piston, but my guess is that it does something similar... probably does less enrichment at lower ambient pressure.

I don't know. The secondary power piston is spring loaded exactly like the main power piston, except it has a single, side-hung needle. The only thing I can think of is that it's set at a different vacuum cut-in (different spring) so that it kicks in at a different time than the main power system. Almost like a "two stage" power system. Fascinating