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Damper piston movement

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A

aroostok

Jedi Hopeful
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I'm sure somewhere in the vast archives there is probably an answer to this but I'll try anyway.
Recently I've been noticing the B running roughly. I cleaned everything up electrically, put ATF in the SU dampers (HS4's), new air filters. To no avail. Today I took the cleaners off and noticed that when I throttled that the front carb damper piston was not moving as much as the rear, meaning the needle travel was not as great giving a leaner mixture in the #1 and #2 cylinders (I suppose). Any ideas why this might be? The needles are the self centering kind.
 
You might just need to remove the dashpots and clean the inside of them and the pistons. Make sure you use the same amount of oil in each piston the oil, weight of oil, amount will reflect the rise rate of the pistons.
 
Isn't that the objective of going through the carb balancing procedure? To ensure that the throttle plate is opening at the same time and the same amount on each carb, thereby sucking each damper piston up the same amount?
If they are not balanced correctly isn't that exactly what you would see, the dampers rising different amounts?
 
"other" factors such as Hap described can be affecting the piston rise. It only takes a small amount of 'schmaze' to interfere with piston movement in the chamber. Those piston/chamber clearances are very close.
 
When cleaning the vacuum chamber and the edges of the pistons, make sure that no abrasive is used. when reassembling them, they have to be absolutely dry, no oil is allowed on the vacuum chamber wall or the piston edges. Another possibility that will cause the piston to hang up is a jet that needs to be re-centered.
Cheers,
 
agreed, the piston clearances are ~very~ tight. make sure their clean on the inside. One other thing that could effect piston movement is spring weights. A stronger spring will limit the movement of the piston. So, that may be worth looking into (making sure the springs are equal strength).

I have two different weight springs for my HS6. The one that came with it and a heavier weght (yellow 8oz., if I'm not mistaken). The biggest difference between the two that I've noticed is for a given jet setting and engine speed the heavier spring is a lot richer. For example, at idle, the heavier spring required a jet setting of only 1.5 to 2 turns (about 9 to 12 flats) out from max leaness for ideal mixture. The weaker spring required an additional 6 to 9 flats out (a total of 15 to 21 flats/2.5 to 3.5 turns) to reach the same mixture.

I don't really want to get into the physics behind it all, but just know that at a given speed & jet setting the mixture will be richer if the piston is lower.

That's all besides the point. Make sure you're springs are equally weighted. Also, make sure your pistons are matched to the proper domes. Those pieces are matched sets. It would be bad to inadvertently put one or the other on the wrong carb.
 
Two springs of the same color & length can have different tensions. If they do, the lifts will be different at a given rpm/load.

The intended spring weights are given for different spring colors.

To match the springs, put them inside a close fitting clear plastic tube, put a weight on top that is close to the spec weight & measure the compressed length. For example, red springs are speced at 4.5 oz. at 2 5/8" length. Yellow springs are 8.0 oz. at 2 3/4" length & so on.

The weaker spring can be slightly stretched to match the stronger.

It's also important for the two sets of pistons & domes to have the same clearances. This can be checked by a piston drop test. Temporarily plug the vent/vacuum hole with clay or something. With the dome inverted, damper out, place the piston in it & measure the time the piston takes to fully drop. It should be several seconds. Any variations in piston to dome clearance will show up as different drop times. The drop time of the slower one can be adjusted by very carefully polishing the inside of the dome to increase clearance & reduce drop time. As Rob said, if two assemblies are interchanged the intended clearances can be very much off.

The absolute spring weights & drop times are not so critical, they just need to be matched. Old carbs or new can be far out of spec if someone has mismatched the parts.

Even with these items matched, a bent or dragging damper can screw up the works.
D
 
I just remembered another possible reason for unequal piston rise. The damper caps. If they're not equally vented, then one piston will rise more than the other. So, I'd check that both are equally vented... or at least that the vent hole in the caps are clear in both.
 
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