TR2/3/3A SU Pistons - TR3

[martx-5]

I was messing around in the garage with the carbs, and noticed that the pistons weren't quite rising in unison. I found this video by John Twist on how to see if you have the right piston in the correct bell. I did the test, and sure enough, somewhere along the way they got switched. They now both drop in unison. However, I thought that the pistons should take about 5-8 seconds to drop. When I do it like John does it in the video, they drop in about two seconds. If I screw on the dampers (no oil or spring), they take a full eight seconds to drop. It doesn't look like John has has those dampers on. Am I missing something here?? Is that drilled hole he's talking about the one that runs up through the web boss at the top of the bell into where the damper screws into?? I see that boss on the MG carb he's working with.

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[Marvin Gruber]

You probably have your carbs sealed off a little more air tight than John. If it takes 5-8 seconds for them to drop, I'd say your good. You just don't want them dropping instantly.

Marv

 

[TR3driver]

When I do it like John does it in the video, they drop in about two seconds. If I screw on the dampers (no oil or spring), they take a full eight seconds to drop.

Sounds to me like your carbs have the internal vent hole from the damper area to the vacuum chamber area. That hole needs to be blocked somehow for this test. I think that's what John mentions at the very beginning, about making the test difficult and you have to hold your finger over the hole. Having the damper cap in place effectively blocks the hole.

Apparently he is using chambers that have a different vent method for the damper area. If I recall correctly, early SUs had a simple hole through the damper cap; then the internal hole into the chamber (through that 'brace' on the side of the damper tower); then even later ones (like the MGB ones that I would guess John showed) went back to a more-or-less hidden hole through the side of the damper cap.

 

[martx-5]

Thanks Randall...that's explains what I'm seeing.

 

[Tinkerman]

Maybe a dumb question here, but I did not do this test before assembly and I'm wondering if I can do it with the needles in place?

Thanks, Tinkerman

 

[Moseso]

Sure, you can. But -- you notice, in the video, that he doesn't let the pistons fall all the way out of the chambers. You REALLY don't want to let the pistons fall out of the chambers, if the needles are in them.

 

[Tinkerman]

Well I did the test with the needles in and they both took about 3 seconds to drop.
very consistent but, is that too fast?

Tinkerman

 

[TR3driver]

I don't have the book handy, but ISTR the spec says 5-7 seconds. 3 seems pretty fast to me. Did you try inserting the damper or otherwise blocking the vent hole in the top ?

 

[martx-5]

Mine dropped in about two seconds without the dampers in. With them in, it took about eight seconds. Make sure all the oil is out of the piston when using the damper to plug up the top. I also did mine with the needles in position. You can see how John has a finger under the bell so the piston can't fall out.

 

[Tinkerman]

With the dampers in they took about 7.5 seconds to drop, so I figure they are good to go.
This is another real good example, of the value of the forum!
Thanks! Tinkerman

 

[TR3driver]

from https://www.jcna.com/library/tech/tech0006.html
<div class="ubbcode-block"><div class="ubbcode-header">Quote:]
XII. VACUUM DROP TESTS -SU CARBS

SU carburetor suction chambers and pistons are furnished as matched assemblies from the factory. There is a controlled clearance and thus controlled air leakage between the piston and suction chamber bore. A convenient means of checking this is a vacuum drop test. The proper damper should be in place. Chamber and piston should be clean and dry .Check piston for any spots of drag or interference over full travel. Spray piston rod lightly with WD-40. Turn assembly upside down with piston against top of chamber. Plug holes in the bottom of piston with windshield caulking (commonly referred to as Dum-Dum). Measure the time for the chamber to slide down and fall off the piston. Specified times are as follows:

1 1/4" and smaller carb = 3-5 seconds
1 1/2" and 1 3/4" carb = 5-7 seconds
2" carb = 7-10 seconds

If vacuum drop time is too fast, carb will tend to run rich. If drop time is too slow carb will tend to run lean. If both carbs are too fast there is nothing you can do to correct. Chances are someone has sanded inside of chambers to clean them which is a no-no. If they are too slow you can polish the chambers or pistons or both. very lightly and recheck frequently for compliance. If one carburetor is fast and one is slow there is a good chance pistons have been interchanged. Try switching them. I recently had a pair of 1 1/2" TF carbs where the front was 5 seconds and the rear was 8 In seconds. Switching pistons gave me 7 seconds on the front and 6 1/2 seconds on the rear -just lovely. It is ideal if both are alike and right in the middle of specification. It doesn't happen very often. Minor disparities from specified drop times can be accommodated by tuning adjustments. If I had a 1 1/2" carb with a drop time of 4 seconds I wouldn't fret about it. If it was 1 second or 1/2 second (which I've seen) I'd hunt for some different pans. It should be noted that if you do interchange pistons. it will be necessary to re-center both jet assemblies. Ideally this type checking should be done at the time of a major rebuild. [/QUOTE]