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Carb Pistons on SU H6

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af3683

af3683

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Both of my TR3B SU H6 carburetor pistons will fall with a distinct "click" when the dampers are removed. However, when I put the dampers in the pistons drop with a slow thud not a "click." The dampers do not appear to be bend. I also tried the dampers from an old spare carb and got the same results. Could I have too much dashpot oil (Moss) in the dashpot? There is resistence at about 1/4 inch or so. Any help would be appreciated. Thanks.

Art
 
Is this with or without oil? The purpose of the damper, when the piston has oil in it, is to provide resistance to the movement of the piston. If empty, then you might want to check that the jet/needle is centered
 
Yes, the dashpots have the Moss dashpot oil in them. The pistons and needles seem to operate fine without the dampers installed. I know the dampers and oil are supposed to provide resistance. But I'm not getting the so-called "click" when they are in the oil-filled dashpots. Could it be the return springs?
 
The dampers have a check valve built into them, so they should only provide resistance when the piston is rising, not falling.

Not sure what the problem might be, but it certainly wouldn't hurt to check the that the springs are present. I would also check the fit between the pistons and domes, as it may be that the dampers are putting a bit of side force on the piston, enough to cause it to rub the dome and so fall slower.

Hmm, wonder if the air bleed into the damper area might be plugged or missing? ISTR that very early carbs had a hole drilled in the damper cap, instead of the hole through the dome. And if they were nearly full of oil, the limited air would provide more of a slowing effect.

Normally, too much oil is not a problem because the excess runs out through the air bleed as the piston rises.

I have no experience with any Moss "damper oil"; the book says to use motor oil. SAE 20 seems to work best for me. If the Moss oil is very thick when cold, that might slow the piston fall.

Bottom line, though, I'm not sure it matters as long as they fall promptly without the dampers. That tells you the jet is adequately centered, and the piston is not binding against the dome.
 
Thank you Randall. There are springs in the dash and they appear to be fairly new. I examined the dampers again and they look fine and straight to me. I'm not sure where to look for the air bleed. The Moss dashpot oil is #220-225 although Moss doesn't state what weight it is.

Also, my front H6 carb has a 3" spring that goes from one of the dashpot screws to the top of the short link rod assembly. I'm not sure what it is supposed to be doing or if it even belongs there. I don't see it in the TR3 manual or even the Moss catalog. Is anyone familar with it?

Thank you so much.

Art
 
Art, that spring you describe isn't original - nothing was ever mounted off the dashpot screws on TR3's, other than perhaps a carby ID tag.

Over the years TR's have been fitted with all sorts of additional return springs to get the engine back to idle speed. Most have run from the bellcrank to the adjuster arm on the generator, but yours sounds like a different take on the same idea.

In many cases the idle problem has been due to dry ball joints on the two linkage rods.

Viv.
 
It may be a good time to order new return springs so you know what you got. They arne't expensive. And I would use Joe Curto as the source.
 
It's not unusual though to find that the original return spring anchor point is broken. It was just a flap of sheet metal in the body seam.

Excess wear in the linkage balls can also be a problem, as can wear in the throttle shaft/carb body. Most recently, I discovered that the RF motor mount had sagged until the throttle rod was dragging on the bellcrank mounting bolt.

If you look at the top of the dome, there is a kind of support for the tower where the damper screws in. If you remove the damper, you should be able to see a hole drilled through that "support", which is the air bleed. It should communicate with the inside of the dome.
 
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