TR2/3/3A SU pistons sticking in TR3

[Got_All_4]

Will I ever get the 3 back in commission? New problem that came out of the blue. For some season now the pistons in the SU's are sticking. If I pull the dampeners out I don't think it does it. I tried driving it that way but it still is missing.
This all started after some missing and hesitation was plaguing me. I cleaned the carb bodies and pistons to get off a bunch of carbon and varnish. Cleaned the sparking plugs put in my Pertonics I had for years. Re balance the carbs and used my color tune to adjust the mixture. All that and no effect. Runs and idles great for 1/2 mile then starts missing. Thought maybe floats. I had an extra float from a couple of years ago when I changed the rear float needle. Adjusted it to .44" and no change.

 

[vivdownunder]

A bit of a check list for sticking pistons -

If you had the jets out, did you "center" them on re-assembly ?. Basically that means refitting the jets with the needles in place as the centering device. The large top nut on the jet assembly has to be tightened really slowly, or the jet gets pulled to one side and binds on the needle.

Were the pistons returned to their respective chambers, as they are matched sets ?.

Could some grit have entered the interference fit between the piston and chamber ?.

Could the shaft of one of the damper valves be bent ?.

Are the coil return springs in the chambers ?. (I've found them on the bench after re-assembling SU's in a hurry).

Sorry if you know all this stuff, but at least it's a start,

Viv.

 

[TR3driver]

So how do you know it's the pistons sticking? Both of them? Any particular spot?

If it is just at one spot, one possibility is that the dome has gotten dented. They are soft and easily damaged if, for example, you drop one. If the damage is not too severe, you may be able to carefully polish away the high spot inside.

 

[Got_All_4]

As I was checking the balance and mixture by lifting the pistons and the rpm's should pick up slightly then fall back the pistons would only go up half way and then would not prop back down all the way. You could not push them up or pull them down at that point. That's when I took the dampener out and the pistons dropped back in place.

I know I did not drop them as I was very carful in cleaning them. After I thoroughly cleaned them I sprayed a dry Teflon lubricant on the inside of the dome and the edges of the pistons. As always I checked the fit of the pistons before re assembly.

The jets were not removed because the car was running so well. In fact after Color Tuning it a couple of years ago I did not want to touch anything because the car was running the best it had ever ran in the 35 years I've owned it. I did check the piston in the carb body with out the dome on it and it slides just fine and no marks on the needle.

I'm glad to know now that I'm not the only one who forgets to reassemble without the springs.

 

[Andrew Mace]

After I thoroughly cleaned them I sprayed a dry Teflon lubricant on the inside of the dome and the edges of the pistons.

Right or wrong, I've never lubricated the inside of the dome or the edges of the pistons on SU carburetors. I have to wonder if that Teflon lubricant is causing the sticking?

 

[Got_All_4]

I've done it in the past with out problems. Also used it on the jets when the choke was soo hard to pull out. Works on a micro leavl. But again with out the dampener in the pistons slides freely. No hint of a bind.

 

[Got_All_4]

I'm going out in a couple of minutes and recheck everything. I only have today to get this done. Lord help me!!

 

[3798j]

Sounds possible your damper rods somehow got bent and are allowing them to come in contact with the tube. This video may (or may not) help. Just another consideration: https://www.youtube.com/watch?v=IfU47Oqq9wA

 

[Got_All_4]

Thanks
I'll watch it. Can't see any bends though. I just removed the domes and took a white Scotch Brite pad and very lightly cleaned the surface. I did find on the left carb that the relief hole on the top of the dome was almost plugged. I cleaned it out with a 3/32 drill bit. Double checked everything and the pistons stuck again. With the dameners in of course. Went for a drive and same thing. Ran good for half mile that give it some gas or let up and miss after miss.

 

[Geo Hahn]

Were the pistons returned to their respective chambers, as they are matched sets ?...

I didn't see where you replied to this. If unsure you could swap the pistons front & back and see if they still stick.

 

[TR3driver]

As I was checking the balance and mixture by lifting the pistons and the rpm's should pick up slightly then fall back the pistons would only go up half way and then would not prop back down all the way. You could not push them up or pull them down at that point. That's when I took the dampener out and the pistons dropped back in place.

Ok, so that's something mechanical going on with the dampers. Seems really strange that it would affect both of them though. Have you tried cleaning out the inside of the shaft and using fresh oil?
<div class="ubbcode-block"><div class="ubbcode-header">Quote:]After I thoroughly cleaned them I sprayed a dry Teflon lubricant on the inside of the dome and the edges of the pistons.[/QUOTE]Since the piston is not actually supposed to touch the dome at all, adding lubricant should be unnecessary, and may lead to sticking.
<div class="ubbcode-block"><div class="ubbcode-header">Quote:]The jets were not removed because the car was running so well. [/QUOTE]Your description above makes it plain that jet centering is not the problem.
<div class="ubbcode-block"><div class="ubbcode-header">Quote:]I did check the piston in the carb body with out the dome on it and it slides just fine and no marks on the needle. [/QUOTE]
Just for clarity, the piston is supposed to be guided by the fit of the center shaft sliding in the precision bushing inside the dome. It should not be touching anything else, except the spring and where the damper rides inside the shaft.

 

[Got_All_4]

The problem ended being inside the dapener shaft. Looked at the brass tip a lot closer and noticed some scratches. Took some 1500 grit and cleaned them up and also inside the dapener shaft by wrapping it around a phillips screw driver and twisting and going in and out with it. Cleaned everything up, reassembled and they stayed free.
I agree Randall unknown why both were effective. Could have the excessive cold and damp weather we had here in northern Ohio caused corrosion? I'm sure there was oil in the pistons all winter long.
Thanks for the help guys!