Carburetor piston not falling freely

[SMGoose]

Finally got the new gearbox all mounted up, but then I couldn't get the fill plug off...of course. So I decided to move on to the carburetors.

I attempted to center the jets but the best sound I could get from the falling piston was a dull thud. After a few tries, I cleaned up the piston and suction chamber pretty well and then removed the jet completely along with the damper, but still, just a thud. Here's a video of what it's doing:

The other carburetor is a little better but not much. Could it just be the piston spring is worn out? Maybe a new needle? What do you guys think?

 

[TR3driver]

The video won't play for me, says it's "private".

A dull thud is probably OK, as long as you can hear it land. But I just tried an old carb from the bone pile with no spring at all, and it still makes what I would call a click.

If you remove the dome, does the piston slide freely all the way out (careful not to drop it)?

 

[glemon]

When you center the jets move the mixture adjustment up to full lean, then when you back it off a couple turns for a ballpark starting adjustment you should have a little extra lay between needle and jet. this may help if you haven't tried it this way already. I assume you have dual carbs, the pistons and domes are matched sets, if you mix them you often get friction. If none of this works first make sure the dome is screwed down good, don't force it, but if screwed down tighter on one side than the other will get off kilter and hang up. Lastly, you can try rotating the dome, as I recall that is possible with most SUs, sometimes it works better in one position than the other.

 

[Member 10617]

If your needle is crooked or bent, you would have a problem. But you can check this pretty easily.

Centering the jet is a frustrating process and takes a number of tries before you get it right. But eventually you get it. Moss sells a tool that helps.

I doubt that the sping is the problem, but it's possible. There is a variety of springs available, so be sure you are using the one prescribed for your car.

Sometimes the suction chambers of the carbs are switched by accident, and the pistons don't fall properly, as glemon has pointed out . The pistons are matched with the suction chambers and operate properly only when they are in the proper suction chambers. Take a look at John Twist's University Motors video on Youtube on matching the SU suction chambers and pistons. This is a problem I found with my carbs.


Also, when I rebuilt and reassembled my carbs, I put a little engine oil on the rod that goes from the piston into the suction chamber, thinking that this would help it fall properly. Actually, the result was just the opposite. It fell too slowly with a clunk and not a click. I finally tried cleaning the oil off and putting a very little sewing machine oil on the rod, and it worked well.

 

[SMGoose]

Thanks, guys. I had the video set to "private" but it should play for you all now. With the dome off, the piston does move freely and there is a defined "click". It seems like the dome itself is creating a suction. I'll check out that John Twist video.

One thing to note: the domes have three mounting holes, not four like the base. As a result of the layout, the domes only go on one way (i.e. can't be rotated). I did try centering the jets with the mixture nut fully up, but as you can see in the video (now that you can actually watch it) the jet is completely out and it still doesn't fall like I believe it should.

 

[Member 10617]

Watching the video and listening to the noise, I think you are almost there, but I suspect the jet is still not quite centered. I wrestled with that noise (instead a desired click) for a long time before getting it right.

Also, the John Twist video may be of help.

 

[glemon]

Now that I have seen the video doesn't look too bad, without any oil in the dashpot it should be moving a little more freely, but it doesn't seem to be really hanging up on anything. I would try loosening up the screws on the dashpot or dome a little and see if it makes any difference. If something is just a little tight sometimes a very slight change in the dashpot alignment can make a big difference. I think they would probably work fine as is, but if it were my car have to admit I would futz with them until their movement was a little more free.

 

[twigworker]

Remove the chambers and pistons and remove the damper plungers.

Clean everything really well and turn each assembly upside down. Fit the pistons into their chambers and give them a hearty spin as you release them to fall. Repeat this several times. There should be no sticking or hesitation. If you encounter same, switch pistons as it is possible that the PO swapped them and didn't tell anyone.

If you find that there is some level of hesitation work the piston up and down over the period of hesitation without spinning it until you produce a shiny spot on the inside of the bell. You aren't supposed to do this, but IMHO it is acceptable to smooth that spot LIGHTLY with some 2000 grit paper. Some folks will say that this is a no-no and I agree to a certain extent, but the reality is that these carbs or OLD and there are going to be OTHER minor glitches that are similar but don't keep the engine from running decently. Wash the piston and bell down in hot soapy water and dry them before continuing by repeating the process.

BTW: There should be a very small protrusion on the face of each piston that stands a couple of thou proud. Depending on the carb model they can be of brass or plastic. This is there to keep the piston face from COMPLETELY shutting off the air flow over the bridge.

Once the pistons move well, remove the needles and install the pairs back onto the carb bodies.

Now see what you have in the way of "drop". If you still have some hesitation, try changing the torque on the bell securing screws.

The piston will probably drop smoothly now and land smartly against the bridge.

Now roll each needle across a bit of glass to see if one is bent. If one is, you obliviously should replace it with one with the correct station profile.

Once you are satisfied that the needles themselves are okay, install them on to the pistons and repeat the drop tests.

If hesitation remains, drop the jet by about a quarter inch and retest. This will get the jet low enough to rule it in or rule it out.

Raise the jet to exactly flush with the bridge and then adjust it down to about 0.065". The butt end of a vernier caliper is ideal for this. This will be a good starting point for your mix adjustment, assuming that the float levels are identical and to spec. If they aren't make them so.

Finally, take the bells back off and install identical damper springs. If necessary, and it is often advisable, replace them with new ones.

Use an oil squirt can to fill the wells of the pistons to their tops and install the bells over them again.

Last step is to install the damper plungers. Push them down slowly to avoid splash back of the oil.

The rest has to do with precise linkage adjustments, checking for vacuum leaks, possible throttle shaft and bushing wear, etc.

Jack

 

[JDub]

Finally got the new gearbox all mounted up, but then I couldn't get the fill plug off...of course. So I decided to move on to the carburetors.

I attempted to center the jets but the best sound I could get from the falling piston was a dull thud. After a few tries, I cleaned up the piston and suction chamber pretty well and then removed the jet completely along with the damper, but still, just a thud. Here's a video of what it's doing:

The other carburetor is a little better but not much. Could it just be the piston spring is worn out? Maybe a new needle? What do you guys think?

Check out this guys presentation on the spring and damper effect - a weak spring more likely to lead lean, since piston moving up to quickly sucking up air (if I'm understanding him right). Cheap enough part to replace.

About at the 11:10 mark, but whole worth watching.