But its all my fault. I pushed it into this bad habit.

Here's how:

For the winter, I'm in the run it up to temp in the garage camp, so before I put the car away for the winter, I changed to fresh plugs, which reminded me that the front three always look fouled.

I did some reading around here and there and decided that against all the advice on how to adjust everything and in what order, that I would simply lean out the front carb using the mixture knob on the bottom. Thats where I hit a snag. I could not find a definitive description of what people mean when they write up or down, in or out, and clockwise or counter-clockwise. In other words, a writeup might say to turn the knob clockwise to go leaner, but it doesn't say clockwise looking from above or below, which are two different directions.

So I tried 2.5 turns down from seating the knob up as close to the carb as possible (which to me is toward lean), and same from the bottom (which should be rich), and now, no matter how I adjust them, I get really thick black smoke where before, the exhaust was clean.

I had found this neat video for SU tuning on an MG, http://www.youtube.com/watch?v=ASeMfXfjNpw and he explains at least what to listen for when adjusting the mixture, which is if its rich, the engine will speed up when you lift the piston, lean, it will stall, and just right, it will raise a bit then settle back down.

Using that bit of info, I can tune the front carb to proper mixture by turning the knob all the way down (which to me means full rich, BTW, so I would think that turning it up from there would lean things out, but it doesnt appear to) but the back carb always seems rich now.

Everything was fine with the exception of some fouled plugs in the front 3 cylinders before I started messing around, and now no matter where I set the mixture knobs, even full up or down seems to make no difference. Based on what little knowledge I have of how the needle is metering fuel, that doesn't seem possible, and yet I have managed to knock things out of whack.

Any suggestions?

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HLS3013940

Hi Joe ~ great to hear from you.

"ClockwiZe" refers to the view from the bottom of the SU, according to the manuals. So if you're looking down at the carb, you would be viewing it as CCW.

That means you turn the part of the adjusting nut which is closest to the engine, toward the radiator. Just picture it like a big woodscrew in the bottom of the carb, and you'll be fine. To tighten it, you turn it clockwise as viewed from beneath.

Instructions read, "Screw up (clockwise) each of the carburetor mixture adjusting nuts until they seat, then unscrew them equally btwn 1/2 and 1 turn".

Top off the reservoir of both carbs while you're at it. That doesn't affect mixture, but just reminds you to do it.

Check the smoke. If your problem is a rich mixture like you suspect, it will be black and the inside of the pipe might be sooty (finger wipe). If it's blue, that's another problem altogether (oil burning in the cylinders, which is usually valve guide seals or rings).

Before adjusting anything, make sure your choke isn't sticking on one or both carbs. That will cause you to adjust until you grow grey, without changing anything! Also, if you have a seasonal damper on your air cleaner ("breadbox") inlet, make sure it's open. Many winter drivers close them for better cold weather performance, then forget to open them when it gets warm again.

Frank

check your float level first, make sure the float hasn't started to leak. If you have a flow meter, balance the flow, too. These are the prerequisite steps to any proper SU adjustments. A refusal to lean out no matter what you do is a very strong indicator of a float problem or a stuck-down choke.

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71 on a rotisserie, two more in pieces in my shed...and a 69 1600 waiting for me to get to it

Ahhh, right, Mark. I never even considered a possible sunken float.

it can be caused by a sunken float or by a bad float needle or seat, or by improper adjustment, or by excess fuel pressure. Often times, it is simply a problem of dirt in the needle and seat keeping the needle from sealing right, a problem which arises when cars are kept in storage for a long time and varnish is starting to build up in the float chambers.

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71 on a rotisserie, two more in pieces in my shed...and a 69 1600 waiting for me to get to it

Right ~ I once caught a tiny bit of rubber hose in my needle seat, which did the same thing.

Thanks for all the suggestions. And Frank, thanks for explaining CW and CCW to me. I also thought the proper adjustment was 2 - 2.5 turns, not .5 - 1.

So, now that I know the proper base adjustment and which direction, I reset the idle mixture and it made no difference. However, I _did_ notice that the nozzle was sticking in the choke out position. I pushed both nozzles back in to immediate effect, with literally little clumps of black carbon being blown out the tailpipe. After this excess richness was blown out, I got clean exhaust, even while revving, instead of the acrid black sooty mess I was getting before.

Problem is now, that the nozzles are sticking in the down position every time after using the choke, and I don't know why. It looks like there is a plate thats attached to the nozzle which may not be properly attached. As I push the nozzles back in place, they click into position and cant be manually pulled down, but if I use the choke lever, yes they stick.

So it appears that what happened to me was while adjusting the idle mixture nuts, I somehow either loosened the connecting plate, or maybe it was loose already and by adjusting the idle nuts after 40 years, the gum that was holding everything together has worn away.

I havent had a chance to work further than this, but I figured I'd update the status here.

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HLS3013940

If you have no idea what the carbs look like on the inside, I suggest a simple rebuild. MSA has the kits, and I will be doing mine soon. I did the backyard tune last year.....3 turns down on the floats, adjust individual carb idle using a hose to the ear and once you have the same pitch from each carb, then set the idle and you might have to turn the choke srews down a little. They won't be even turns. That there tells me they are not equal and a rebuild is required, plus the occasional adjustment. Also there is that area of the throttle shaft....the bushing gets worn and sucks air.....and you knows what else I'll find when I take them apart. The good thing is we have 2 carbs, so only take one apart at a time, so the other can possibly save you if you forgot were something goes. FYI, if you ever rebuilt a Holley or Rochester carb, this should be a walk in the park.