As the weather gets warmer, idle mixture seems too rich. The throttle shafts seem snug. I know that air leak here can cause the carb pistons to rise and richen the mixture. If the pistons are sticking I guess that can cause a rich idle, but shouldn`t that cause other problems? I changed air filters. I can`t find the piston lifting pin on these HS6 carbs. Should the pistons be easy to move up and down by hand? How often have you seen the pistons stick?

I`ve been trying to leave the carbs alone, but I may have to tackle them! The car runs well, but the tailpipe always looks black. I`m beginning to wonder if the ignition coil could be weak. May look at that. Forgive my rambling "stream of consciousness."

Many thanks to everyone for the excellent tips. Yep, these are the original carbs and I think they have been "rebuilt" at least once. Not re-bushed, however. I think I would like to try cleaning the pistons and maybe replacing the needle and seat, with float check and measurement. I`m getting an occasional fuel leak at the jet on both carbs, so a complete rebuild/replacement may be in the future. Seems like some good shops around that will rebuild/exchange, but like everything else these days, it ain`t cheap.

I get such a kick out of this car, I don`t mind trying to make it the best it can be!

I've had lots of SUs and they all leaked, so the following is from years of experience fighting with them. I recently had mine rebushed, so if there's any indication of throttle wear, I recommend rebushing instead of forever messing with them.

While air leaks near the throttle shafts can cause a mixture that is too lean, the extent to which it causes problems is not consistent and varies depending on engine speed. Accordingly, when someone tries to adjust an SU with an air leak, they often get a mixture that correctly compensates for the air leak at idle, but is too rich at higher engine speeds.

You can't tell if you have a leak around the throttle shafts by looking or jiggling them, unless it's really, really bad. The best way I know to check for a throttle shaft leak is to buy yourself a can of Gumout. With the engine running at idle, spray Gumout around the throttle shafts. If there is an air leak, it will suck Gumout into your carbs and engine speed will drop appreciably. If there is no drop in speed, you have no air leak and your problem lies elsewhere.

Gumout is also good for cleaning the carbon deposits from the carb pistons and carb bodies so that the pistons move up and down freely. They should move quite easily by hand, with some dampening if you have filled the damper pots with the appropriate oil.

All the piston lifitng pins do is lift the piston a particular distance. (I forget exactly how high it is, but it's around 1/16 of an inch). If you can't find yours or they are gummed up, you can just use your finger or screwdriver to move the piston up slightly and acheive the same effect.

While a black tailpipe might be common, too much crap flying out of it is not. Get the car up to a decent operating temperature, and put your hand (or a piece of white paper) about 18 inches behind the tailpipe. If your hand or the paper get covered in black crap, you're definitely running too rich.

The best way to check whether you have the appropriate mixture is to pull your spark plugs and check for deposits. Pull all four plugs, put them back in, then drive at normal conditions for fifteen minutes or so, then immediately stop the engine and pull your plugs. If they are black and sooty, it's too rich; white crumbly deposits means its too lean. The right mixture should leave them clean and a nice, healthy tan color. You only need to pull the #1 and #4 cylinder plugs to check, as they will correspond to your front and back carbs, respectively.

Good luck!
--
Dennis

Having a black tailpipe is normal... If you don't find anything wrong or find a slightly better setting, I wouldn't bother with it unless you're getting smoke at idle, or if you're a perfectionist, =D.

Lifting the piston not only opens the jet but it also lets more air in.
What you are looking for is balance between the two.
Rich idle and normal running could be because of a float level too high
or a float valve not closing completely.
Unlikely but possible is that you have a needle that is way off for the
use of the carb.

Be sure and check for vacuum leaks at all possible places and especially
things like hoses, manifold gaskets, etc.

At one point I was getting erratic running and some clanking and found that
somehow all 4 of the mounting nuts on one carb were loose so the carb was
banging against its mounting flange on the manifold. Considering that,
it was running fairly well.....
--
George Downs Bartlesville, Heart of the USA!

Air leaks normally cause a lean condition by introducing air downstream of the metering device. It is extremely difficult to get reliable operation of any automobile engine until unintended induction air is stopped.

You can check piston operation by removing the air filters and lifting the piston directly. The leading HS6 piston on my '66 Amazon got stuck a couple of times in the winter due to condensation accumulating in the intermediate piece (the one that accepts the hose from the oil filler cap). The condensation would mix with oil and cause sludge to form on the carb piston -- yuk.

The dome can easily be removed for cleaning both the inside of the dome and the piston. Just be very careful not to damage the metering needle attached to the piston.

Joe M in WV