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Mix nuts - again

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I'm having trouble understanding how to correctly set the mix on my roundtops. I knew for a fact my mix was too rich, so I attempted to lean things out, using the procedure described in the engine manual (reset idle nut and throttle pin positions, measure low and high rpm flow, etc.).

According to the procedure, you first have to "reset" the mix nuts, thereby achieving a certain clearance between the needle tip and body opening (2.2mm I think is what it is). Well without taking the carb apart, this is damn near impossible. The book says to tighten one turn, then loosen 2 1/2 turns, then tighten 1/2 turn to achieve the desired starting position. Well what if your mixture is too rich like mine? That means the nuts will be lower than they're supposed to be and that you won't achieve the desired starting position.

Next the book says to set the throttle pins to 600-700rpm, then slowly (1/8 turn at a time) tighten the mix nuts until the engine reaches the fastest and most stable speed. Ok, no problem, so I start tightening. Well, I keep tightening and tightening, and the engine slowly gains speed, but does not become unstable. I end up tightening the nuts completely and the engine is still stable. Will the car run like this? No. Toooooo lean.

So I back the nuts off a bit more (2 turns) to try to richen it back up, but now I'm guessing as to where the final position should be.

Would any of you SU nuts like to take a shot at describing the process of correctly setting the mix (daily driver setup, not race). I'm looking for better fuel economy. If you can describe it, a few "why's" along with the "how's" would really be helpful.

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Welcome to the wonderful world of SU carbs. Once you get them figured out, you will be happy with them...

As far as the mixture nuts on the bottom. First, make sure to turn them both in ALL the way closed. Then, turn them out 2 1/2 turns.

Make sure the valves are adjusted, the timing is correct and the carbs are synched. Then to adjust the mixture with the nuts...

...with the air cleaner off and the engine running at idle (and at temp), cover one of the carbs' intake horn with the palm of your hand (some guys will lift the piston all the way open instead - I've always just used my hand). This will cut off any fuel/air to that bank of cylinders. The engine should run on the other bank of three cylinders pretty smoothly. With your hand still in place, adjust the idle mixture nut on the OTHER carb. If the engine is loping, it is too lean so open it up a bit. If it is running smooth and fast, slowly close it down to where it just wants to die (starts loping). Adjust it back out about an 1/8 turn or so. Once that one is adjusted, remove your hand and rev the engine (at least 2500 rpm) to get things back to normal with both banks. Repeat on the other carb. This is a pretty easy and reliable way.

Good luck.

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Yes, you have to start with the mix nuts turned all the way up to get a starting point. Here are a couple more things to consider.

The fast idle adjusting screw, up by valve cover where the linkage takes a right angle and turns down to the carbs, may be turned in. You should set the engine idle with the fast idle screw turned all the way out, so it doesn't impact the idle at all. Then you can use the mix nuts to set the idle.

Next, you to check that the needles are installed properly. Take the domes off the carbs and pull out the pistons, not difficult but be careful so you don't spill the oil or bend the fuel needle. Then you can make sure that the needles are installed correctly, with the needle shoulder at the same depth as the flat base of the suction piston.

If that doesn't work than you probably need to start at the beginning and check the valves, timing, plugs, and reset the carb balance.

Hope it helps.

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Ah yes, now we're getting to the meat and potatoes :).

Interesting approach hmsports. Sounds quite reliable. I'm still in the bottom of the learning curve, so I have yet to learn how to make valve adjustments. I'm pretty sure the valves are good or close, since I have good performance and no tapping. I WILL learn to do this though. Do you use a flow meter to balance after setting the mix?

Royce, I'm aware of the function of the fast idle pin, and that this pin stays out at all times except when tuning carbs at fast idle (3000rpm is what I use). Good heads up though! One thing on the list to check is my needle position. Scott from ZTherapy shows how to do a simple and accurate needle alignment. For now it's just trying to get my mix right.

I gave it another crack today and - viola - things are right with the world again. I HAD previously taken the mix nuts all the way up, but hadn't brought them far enough back down. My car sounds better now than it sounded before, and it sounded damn good at the time. Idle is smoother and my backfiring even sounds a little different - more little pops and less big pops. Yeesir, all is good :).

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