Carb Pistons different resistance on pull up

[Mikes Z car]

I THINK I figured out what was really happening with my carb piston being too easy to push up, I am a little slow to connect the dots sometimes. The formerly leaky valve has some melting on the cap, it looks like a PO dropped it on the exhaust when the exhaust was hot. The melting caused a part of the plastic cap to dribble down below the flat face on the bottom of the stalk cap that is supposed to mate with the flat corresponding face on the very top of the carb and apparently is supposed to make a somewhat air tight seal. This protrusion prevented the stalk cap from screwing down completely to make an airtight seal with the top of the carb. I had noticed the protrusion but not its effect before but only broke off the protrusion this weekend as an afterthought not realizing that fixed the problem. I tried an experiment tonight and unscrewed the carb damper a few turns and sure enough the piston is easy to push up if there is no airtight seal. Sorry I ran everyone around and then found out about the melted cap! At least some good info came out of this troubleshooting I think.

Mike

Here is an improved drawing. It shows the 3 chamfered areas. They are slightly exaggerated to make them easier to see.

[Captain Obvious]

You don't need an air seal above that valve to make that work. It works simply on the hydraulics of the damper oil on both sides of the valve. Atmospheric air above that fluid matters not.

Need proof? What happens when you drop the valve stalk into the piston? Does it drop quickly into place, or do you have to force it down into place before you get to the point where you can screw the cap in? If it's working correctly, you should have to force it down if you don't feel like waiting for it to drop on it's own.

Without laying my own eyes on it, I would suspect that you are on the razor's edge of acceptable misalignment of the valve and that melted plastic goober was causing a tiny bit more of misalignment of the stalk, pushing you over the edge.

I like the picture improvements. Makes it easy to see how it works when it's closed. The bottom cylinder pushes up on the washer above it which in turn contacts the fixed washer at the top. The beveled edges center themselves together and all the gaps close. The fluid then has to force it's way through the gaps which makes it harder to move.

[Mikes Z car]

The damper stalks drop relatively quickly into the top of the carb though the pistons resist being pulled up once the dampers are in place.

Thanks on the drawing. I am a big fan of drawings.

[Duffman]

Mike, if you have the right level of damper oil, you should get resistance in the last 1/2", or so, of putting the valve stock in. If you have any excess, it will burn off in the carb while running.

[Mikes Z car]

duffman,

That is what it does, I don't get much resistance at first but after the damper goes a little ways in to the carb the resistance is there. When I had a 71 240Z back in the late 70s I didn't have the support of this web site and I used 3 in one oil for the dampers. I had to pull the choke every time to pull away from a stop to avoid a stall due to running lean because of damper oil that was too light weight. Over 30 years later I finally found out why.

[Duffman]

I use 3 n 1 oil, which is about 20 wt, works fine for me.

[Mikes Z car]

The carbs on the 240Z I used to have may have been out of adjustment since I didn't know how to adjust them. 3 in one might have worked fine if I had had the mixtures set right.

[Captain Obvious]

I don't get much resistance at first but after the damper goes a little ways in to the carb the resistance is there.

Right. That's because the valve won't do anything until it's submerged. The point is, however, that having a good air seal above the valve is a red herring.

Here's a way that you can feel the effects of valve misalignment:

Fill the damper tube to it's proper level and then insert the valve stalk and push straight down on the screw knob until the threads contact the top of the body dome. Once the valve is submerged, you should feel the resistance caused by the properly closed valve (just like you described above).

Then pull the damper completely out and repeat the same procedure, only this time jiggle the damper stalk side-to-side as you as you push it down. You should find that the amount of force required to push the damper "home" is less. The reason is that when you shift the damper stalk from one side to the other you misalign the valve's sealing washers and it cannot properly seal with that much misalignment.

I'm a big fan of pictures too, and I'm sorry that I don't have any to help describe that process. Hope the text makes sense.

[Mikes Z car]

That is a good explanation, I will try that.

[Mikes Z car]

Captain Obvious,

I think I found the red herring as you described it. The damper with the melted cap has variable resistance in how hard it is to push the piston up when the damper is screwed on or slightly unscrewed. The piston is easier to push up as you move the damper from side to side with the damper unscrewed with your test but I also notice when it is properly screwed in I sometimes still get variance in the upward force needed to push the piston up. It is consistently easier to push the piston up on the carb with the damper with the melted cap than the other carb even when the melted damper is in its operating properly mood. I could try taking the valve apart and sanding it or spinning the valve seats together with an abrasive but I might be better off getting another one as I don't know how precision the valve seats have to be as they come together.

Mike

[Captain Obvious]

Yup. I bet when your cap melted, your shaft shifted a little. I'm trying to come up with some sort of test to check for alignment quality, but I'm not coming up with anything simple that wouldn't require special fixturing.

You can't roll the stalks on a table and look for wobble because the thumbscrews aren't round...

How about pulling one of your domes off the top, screwing in the stalks one at a time, and then flipping the domes open end up and measuring the distance between the jiggly bits and the inside of the domes at a couple different locations? It's crude, but if it might be accurate enough to see a difference between the good damper and the questionable one.

Of course, all of that is academic... You need a new damper stalk.

I don't have anything for round tops, but I've got spares from flat tops... If you do the legwork to figure out if they're the same or not, I'd be happy to send you one if they are the same.

[Mikes Z car]

Captain Obvious,

Thanks for the offer for the damper stalk, turns out they are different though I got lucky and found a round top carb damper locally I didn't know at first was available.

I was able to put the brass cylinder end of the bad damper in a drill chuck (took a few tries, the chuck just barely could grab the end of the damper stalk) and managed to spin it to check for bends, none found.

I installed the new damper stalk today and now both damper stalks pull up smoothly with the same force as I gather they are supposed to. Nice feeling to see that after chasing this around with everyone's help. The car runs great now though I also synchronized the carbs, adjusted the mixture nuts under the carbs using the pull up opposite piston procedure and set the idle speed with help from the forums here all of which no doubt helped.

Mike