These SU HS6 carbs were rebuilt 15 years ago with new shafts with delrin bushings that shouldn't wear out.There hasn't too much use for this is a summer car. They have stock KD needles and I have replaced the rear jet because it was leaking fuel. I did not replace the front jet and this is the carb that has problems adjusting the mixture. Brought the mixture to it's starting point at 12 flats out. When I lift the suction piston there is no difference in idle or rpm.
I could replace the front jet but only reason would be because it might be worn but that doesn't make sense to me. The carbs are clean inside & out.
Scratchin' me head about this problem.

15 years is a long time but have you had them running "OK"? Did something change? Why are you adjusting them now. Assuming the damper has oil in it how does it feel when you lift the damper with your finger. Does it appear to lift normally when you open the throttle? Does it lift differently to the rear one?

Derek, The carbs were rebuilt by a reputable person 15 years ago & they were used for a maybe 3 summers on my 1800, then placed on the garage shelf since I modified another engine with another set of refurbished SU HS6 carbs with SM needles and have been operating that setup from that time on.
So, I acquired this '67 144 with 67K last year with a BW35 that was removed and I switched it to a M40 manual. The back carb adjusts like it should with a 1/16" lift on the piston affects the tuning factor so that is dialed in and just fine. The front carb doesn't do anything bringing the damper piston all the way to the top without affecting anything at all. There is no binding either,,,it drops with a small clunk. I use ATF for dampening fluid. When it gets a little warmer outside I will take a look at the condition of the front jet and maybe replace it with a new one that I have in the drawer.

Check the dampers and the domes. If both dampers have holes in the caps, brass or plastic there shouldn't be a hole in the ledge just below the threads for the dampers in the chimney of the dome. Pistons and domes should also be kept as a pair as they were matched at the factory to maintain the correct piston rise with the same vacuum. Over time the parts might have been robbed or changed about.

I seem to remember a procedure for centering the jets. i rebuilt mine some 20 years ago, so I sort of forget..its probably like a nut that frees the jet up .. I wondered if maybe someone trying to adjust them could have simply loosened it and maybe not realized that the jet needed to be aligned to the needle, thus maybe making the needle rub on the edge of the Jet's orifice.

if the PCV hose is hooked up to barf fumes into the air cleaner the pistons might get dirty from the regurgitated smog. I should probably take mine apart to clean, but they work fine. If I recall right the 66 had a connection to the manifold and in 67 it wen to a thicker air cleaner with a PCV hose to
make the air cleaners dirty. very early ones may have had a ground tube or similar. I think older cars, like around 62 and earlier may barf that crap on the ground. motorcyclists won't love that.

I'd want to take a look at the parts involved. I'd remove the dome, piston and needle. Are their shiny streaks on the needle, or is it worn flat due to misalignment with the jet? Look straight down at the top of the jet. If its worn it'll look oval. Is the needle secure in the piston? Just to get a starting point you'll need to do some investigating. Thinking a problem through is good up to a point - I'm guilty of going past that point many times myself - but the SU is so simple and easily disassembled I'd get right to it --- after water spritzing just to be sure there are no vacuum leaks. - Dave

Dave, The KD is seated all the way down in the chamber. The KD doesn't appear to have any wear on it. I will take your advice and look inside the jet to see it's condition then the water spritzing.
I appreciate your help.

In case you didn't know, the starting point for adjusting the jet tube down 12 flats(2 full turns) is not when the adjusting nut is fully seated/turned in all the way.

If you screw the adjusting nut all the way up, you will raise the jet tube too far and the top of the jet tube will stick up and push the piston up.

The proper starting point is when the top of the jet tube is flush to the top of the brass sleeve the jet tube slides in and out of that's inside the throttle body and is visible when you lift the piston.

--
Eric
Hi Performance Automotive Service (formerly OVO or Old Volvos Only)
Torrance, CA 90502
hiperformanceautoservice.com or oldvolvosonly.com