Carb Pistons different resistance on pull up

[Mikes Z car]

This seems rather off the wall to me but the two dampers on the SUs on my car are rather different in their resistance as I stick my finger in each carb and pull up on the piston. I had emptied the old oil from both carbs with my finger over the end of a drinking straw and then I put 30 wt in both dampers on the carbs (couldn't find 20 wt here). I was setting up the mixtures in the carbs VIA a 2003 beandip post and while getting good results (accidentally laid some rubber for the first time with this car today) I noticed that the one piston was much easier to pull up than the other. I swapped the dampers between the two carbs and the problem followed the dampers indicating the dampers are different. There is plenty of oil in both carb dampers as indicated by a smooth pull of the piston all the way to the top position. They seem to drop at the same speed.

Is there a way to know which damper is right or in this case would it be better just to get two new ones? If they are different I would think the slower damper would cause that carb to be richer leading to an imbalance. Anyone seen this problem before? Thanks for any ideas.

Mike

Edited September 9, 2012 by Mikes Z car

[olzed]

If it can lay rubber, and runs well I would not worry unduly.

[Mikes Z car]

That may well be a good way of looking at it. I just didn't want it to confuse me by causing some other operating anomaly that later on would leave me scratching my head.

My parents went to New Zealand a few times many years back. They really liked your country.

[240260280z]

The space between damper and well wall, and the space between piston and dome wall should be the same for both of your pistons and for both of your dampers. These are the critical tolerances that need attention when matching carbs. In your case, one damper is too small in diameter as you have proven by your experiment. (you can try to measure the two dampers to put a number on the difference).

For a solution, find a few more dampers and measure them or swap them until you get a close match.

As an interim solution, experiment with a thicker oil in the faster piston.

Edited September 9, 2012 by Blue

[Captain Obvious]

I had the exact same issue on my car and I tracked the problem down to a bad damper stalk.

First you need to figure out which is the good one... The good one is the one that is hard to lift. I know it's a subjective thing, but as a guideline, I would say "it should be hard enough to lift that it hurts a little and leaves a dent in your fingertip." The other one (the one that lifts easily) has a problem.

Next, you need to figure out what's wrong with the damper:

a) Measure the outside of the two dampers with a micrometer or a set of calipers. They should be the same (probably are).

Under magnification, look closely at the brass jiggly bits on the bad one. Look for gouges in any of the surfaces or goobers of crud stuck anywhere (doubtful).

c) Under magnification, make sure the arrangement and orientation of all the jiiggly bits is the same between the two (probably is).

d) Stand the stalks on the workbench knobs down and check that the stalks are perpendicular to the workbench surface. (This is what caused my problem.)

In my case, everything was clean and assembled correctly, but one of stalks was not perpendicular to the knob. This causes a problem because when you screw the stalk into the carb body, some of the the brass jiggly valve bits at the bottom of the stalk will not be in correct alignment and the check valve won't seal when lifting the piston.

I replaced the entire stalk and the problem went away. The insidious part to this is that everything looked fine at first glance and it wasn't until I really studied the system that I figured out what was going on.

[Mikes Z car]

Wow, Thank you Blue and Captain Obvious!

Great info. I do have a micrometer, I will compare the diameters of the two dampers. I will also check how perpendicular the stalks are to the knob. I didn't look at them closely because I figured I didn't know what I was looking for.

I am curious Captain Obvious, did you notice any performance issue with the stalk that was bent? The more difficult to push up piston will definitely dent my finger as I push it up. First time I tried it I thought it was stuck.

[240260280z]

I defer to CO in that it is most likely not the damper dia. but rather valve leaking.

For a solution just find spares and swap out.

[Mikes Z car]

Blue,

Thanks. I took the leaky valve apart, wiped it off and put it back together like the other one was and now it works okay for some reason. Maybe some dirt got stuck in it. I will recheck it at a later time to make sure it is still working.

Attached is a drawing of the valve as I was seeing the one I have.

[Captain Obvious]

I did not notice any performance difference at the time, but I was fighting several fires at once. One of the other fires was that I was running rich under all conditions and that would mask problems with the pistons rising too fast. If I had gotten the fuel ratio down to reasonable levels and left the original misaligned stalk in place, I might have bogged a little when I punched it, but I never ran it like that. I fixed the stalk first and then fixed the mixture later.

One thing I forgot to mention is that the stalks can tolerate some amount of misalignment before the valve fails to seal. But beyond that point, you run into a problem. Maybe you dislodged some dirt? Maybe a previous owner had it assembled incorrectly and you fixed it when you put it back together? Maybe you are right on the razor's edge of tolerable misalignment? In any event, keep an eye on it.

Out of curiosity, have you had any backfiring through the carbs leading up to this? Not only is that a sign of a lean mixture, but the shock of the backfire can damage the stalk assy. I believe that's what happened to mine.

BTW - I like your drawing of the valve bits. Missing a sealing chamfer on the underside of the top (fixed position) washer, but other than that, looks good.

[Bruce Palmer]

Don't overlook the need to maybe remove the pistons and domes and give them a good cleaning. Then switch 'em back forth and drop test them as a pair match 'em up one or the other 'til you get the best pairing. When reinstalling the domes keep liftin and dropping the pistons as your tighten the screws to make sure you aren't thightening a misalignment and binding up a needle in the nozzle. That should be done with the nozzles all the way up then move the nozzles to where you wnt them.

Running rich maybe bring the nozzles up a tick and see how that does.....

[Mikes Z car]

Captain Obvious,

If the valve sticks open again I will take Blue's advice and replace it. I did have some backfiring through the carb as they were running lean until this last weekend but it didn't happen a lot. Your noticing the missing chamfer on the bump stop washer is a good observation! I realized that chamfer was missing shortly after I posted the drawing.

Bruce,

Cleaining the pistons and domes seems a great idea, thanks. My front carb piston had been sticking but I freed it by moving the dome as I tightened the screws. Probably need to verify I didn't bend the needle and look at repositioning the nozzle as you suggest.

Thanks to everyone for the great information!

Mike

[olzed]

That may well be a good way of looking at it. I just didn't want it to confuse me by causing some other operating anomaly that later on would leave me scratching my head.

My parents went to New Zealand a few times many years back. They really liked your country.

A forum member from US and his lovely wife visited down here a few months back. Mike as well.