Yes Virginia, it IS adjustable. This seems to be a little known fact, so I thought I'd post this here. Ah, the joys of carburetion and simple mechanical systems .

Just to bring everyone up to speed, the way the THERMAC works is this:

Both snorkels have a vacuum actuated "flap" in them. These flaps are closed at low temperatures and open at high temperatures. The driver's side snorkel injests hot air from the heat stove on the exhaust manifold when the flap is closed, and cold air from the snorkel when it is fully open. The passenger's side is completely blocked off with the flap closed, but begins to pull in cold air as the flap opens. This is designed to aid cold driveability, but a lot of times the flaps don't open early enough. You could disable the THERMAC completely, but it really does hurt driveability in cold weather.

I have mine set to open completely at 75-80* F. Made a big difference, Compared to the stock setting of about 120 degrees and the 150 or so mine came out of the box with. Simply disconnecting the hose made the car run terrible in winter weather until it warmed up, so that wasn't an option (and yes, I *did* block off the vacuum port on the carb).

At any rate, to adjust, use a pair of diagonal cutters and cut the blue plastic shield off the thermal vacuum switch (it's easy to find, look on the bottom for where the two hoses plug into the air cleaner, or just look for the blue plastic anti-tamper cover over the switch).

You can then a adjust it with a screwdriver. I don't remember which way is which; what I did (and suggest) was put a digital thermometer with a remote lead in the air cleaner right near the switch (just tape it down). Close the air cleaner up and remove a snorkel (I found the passenger's side easiest to observe). Mark down the temperature the flap starts to open and the temp it's fully opened at (if it EVER fully opens, mine didn't initially). Open the air cleaner and adjust the screw so you can see which direction is colder/hotter. If you turn the screw and flap opens farther, that's colder; if it closes more, that's hotter. Realize it's a vacuum motor and it takes a second to respond, so adjust slowly.

Shut the car off and remove the air cleaner assembly (it cools faster this way). Once completely cool, reinstall on the engine, close it up with the thermometer in there, and start immediately... and don't screw around the hot engine will heat the sensor up pretty quickly. Watch the thermometer until it gets about 5-10 deg below where you want it to be fully open. SLOWLY adjust the screw as it heats up to make sure it opens RIGHT on the correct temperature.

Incidently this is best done during the winter; the air cleaner will get to colder temperatures, faster and be harder to heat the sensor in general; and also you need to start with the air cleaner BELOW the temp you want it to open fully at. If it's 100 degrees in the shade you are probably wasting your time . Also cold temperatures will allow you to see how badly you've affected cold driveability. It's certainly possible to go to low, I did and had to raise it.

Open elements suck in nice hot engine air so cold driveability isn't as much of a problem... but even at 20mph the dual snorkel might be sucking on 20-30 degree air and the engine DOES notice that. Great for performance once the engine warms up but not so great for driveability until it does.

Wonder why I'm so big on driveability? Simple. If you don't care about driveability (IE your care is a drag strip warrior, not a street car), you shouldn't even be messing around with a dual snorkel. Get a nice big fat open element and be happy.


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"You live and you learn, or you don't live long"
"Good judgement is often the result of experience. Experience is, unfortunately, often the result of bad judgement."

'84 Black Camaro ZZ4 M5 (V-6 in a former life)- Blown 10-bolt
'99 Navy Blue Metallic Firehawk #120 M6(!CAGS, B&M Ripper, stock POS clutch)

 

Very cool , I like it. I didn't even know it was adjustable and I've been working on carbed SBCs for over 10 years. Thanks

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George P. Lara
1984 Z28 High Output
Proud Member of SCFB
V6 T5 For Sale

 

Hmmm... perhaps I should submit this as a tech article instead of as a FAQ submission? Since it's more of a tuning/tweaking issue than just a FAQ... hmmm...