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It sounds like the previous owner installed the o2 sensor and then used one of those external fuel/air ratio meters I mentioned in my last post. Rather than permanently install the meter, a lot of people just put it on the dash when they want to tune, then take it off, leaving the sensor welded in the downpipe because it's easier than removing it.
You can try buying just the head of one of those meters and using it with the O2 sensor that's already installed -- I think you can find the meters on the Pierce Manifolds website among other places. If you do that, then what you'd do to select jets is just drive the car through its rev range under various loads (on the flat, going up hills, etc.) while having somebody watch the LEDs on the meter to see whether it stays in the normal orange or goes into the "rich" or "lean" range. You note where in the power curve it goes rich or lean and change the appropriate jets. It's a little more complicated with the DGxV series carbs, like the one you have, because they're progressive -- the engine runs most of the time on the smaller 32mm barrel, but as you approach full throttle the secondary 36mm barrel also opens up. So that's two different jets you might potentially need to change, depending on whether you go rich/lean while running on the primary barrel or on the secondary barrel.
If your car was rejetted for high altitudes, I'm concerned you might now be running too lean rather than too rich. If I understand this correctly, rejetting for high altitude involves going to SMALLER jets, because the air is less dense at high altitudes and so needs less fuel for the correct mix. If you're now back down near sea level, you've got more air density but still the same amount of fuel, so you'd be running lean. (One tipoff of lean running is "lean surge," a slight pulsation when you're cruising at constant speed on the highway -- it feels as if you're jiggling your foot on the gas pedal.)
Running slightly too rich is bad for power and emissions, but it won't particularly hurt the engine, at least not away. It's more of a problem if you're running lean, because that makes the engine run hotter and can burn valves. It's tricky on a V4 because the air distribution to the cylinders isn't particularly even -- three cylinders can be running normal or slightly lean but the fourth one might be very lean, and then that's the one that will give you problems. So, most people tend to jet their carbs just slightly toward the rich side of normal, to make sure none of the cylinders go too lean.
A really easy fix for you might just be to call Mile High, give them the VIN of the car, and see if they kept records on what its original carb setup was before they rejetted it; then you could go back to that as a starting point.
If you don't mind spending a bit of money and really want to be sure you don't mess up that nicely built engine, a good dyno shop should be able to do a pull and measure the fuel/air ratio throughout the range with their wideband meter (which has a more accurate sensor than the plain O2 sensor you have.) They should be able to look at this and tell you exactly where you're too rich or too lean, and by how much. If they're knowledgeable about Webers (the 32/36 DGxV is very widely used) they may even be able to tell you almost exactly what jets you need to put in.
Good luck, and let us know how it turns out -- a lot of us have to rely partly on "voodoo" in setting up carbs on a V4, and the more good information is out there, the better it is for everyone!
posted by 204.76.11...
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