Adjusting Fuel Mixture on E-type Series 3 V12

When changing the spark plugs on my 1971 E-type V12, I noticed that the left “B” bank plugs were all black and sooty, whereas the right “A” bank plugs were that nice light brown. Concluding that the left side was running rich, I attempted to lean out the two left carbs. With the engine idling I lifted the carb cylinder a little, which caused the idle speed to increase and stay high. This verified overly rich. My research on the forum indicated that I should turn the brass slotted screw on the bottom of the carb counter-clockwise looking from the top of the carb. Well, the screw wouldn’t budge. The screw did not have the locking nuts on its shaft, which then indicated that I needed to get a special Zenith Stromberg allen wrench tool to make the adjustment in the top of the carb. Mine is a Model 175 CS. Does this sound correct so far, and how about using a regular allen wrench? Any further advice?

There is no adjustment from the bottom of the CD 175, only from the top. You’ll need the adjusting tool as it locks the shaft in place while turning the screw. A simple Allen wrench will not work.

I agree with Mollyoliver, you’ll tear the diaphragm if you don’t hold the damper still when you try to turn the allen screw with the special tool.

I’ll also add that the adjustment range is relatively minimal with these carbs. An overly rich condition may indicate the float needs adjustment as well.

…or that the choke on that side is not fully turned off.

Thanks, guys! I’m starting with the choke, then float, finally carb mixture. Then, if need be, my trusty mechanic . . .

Well, it’s beginning to get a little weird! So, I’m checking the choke on the left engine bank, looking to see that the choke plate is fully opening when the choke is off, only to discover there is no choke plate! Instead, apparently the choke operates a valve or needle which lets more fuel in from the float chamber to enrich the mixture. It’s gonna be difficult to check this without removing the carb, which I’m not ready to do just yet. My mixture tool will be here Monday, so I will wait until then to adjust the mixture and hope that the choke and float are working properly. That would make for a very Merry Christmas!

Merry Christmas and/or Happy Holidays, All!!

Time to get that manual out and read about the choke on your carbs. It is an “interesting” design. The choke cable operates a disk that rotates on the side of the carb. As the disk rotates is allows a progressively larger amount of fuel to bypass the throttle plate directly into the intake manifold. A maladjusted cable on that side could mean the choke is never fully off. It is also possible to misassemble he choke disk assembly 180 degrees off, so it is on when it should be off, and off when it should be on.

Just make sure the tab operating the choke is not in contact with the choke disc when the choke control is in the OFF position. And that right and left carb banks are the same. Assuming your CD 175s were reassembled properly that’s all you have to do.