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Dashpot oil disappearing!

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vagt6

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The dashpot oil in both of my MKIII Midget's SU HS2s seems to leak out rather quickly. It's gone every time I check it, about once per week or just about every time I drive the car.

However, after reading SU diagrams I noted that I do <span style="text-decoration: underline">not</span> have "damper gaskets" on either carb, or at least I can't see any. Also, I'm using 3-in-1 oil for dashpot oil.

Would either of these things cause a dashpot oil leakage problem? I've ordered the damper gaskets from Moss, along with the Moss SU damper oil (4 oz. for $12, ouch!). I'll install the damper gaskets ASAP.

<span style="font-weight: bold">Question</span>: do the damper gaskets simply fit between the plastic damper cap, and the neck of the suction chamber?

I wonder if the absence of the damper gasket is the source of my oil leakage problemo???

Viewing the exploded SU diagrams, I don't see any other gaskets that hold in the damper oil. Where would the damper oil go if and when it leaks????

Damper oil gremlins (well, it is almost Halloween! :devilgrin:).
 
How full are you filling them?
 
Just like the manuals say: 1/2" above the top of the hollow piston rod.
 
vagt6 said:
Just like the manuals say: 1/2" above the top of the hollow piston rod.
Click to expand...

NOT the top of the carb body itself. Look down inside. The moving piston down inside. A common mistake. Just want to be clear. (don't ask me how I know)

I also use 3 in 1 oil.
 
I think I've got the fill level right: NOT to the top of the suction chamber neck, but 1/2" above the hollow piston rod (that's surrounded by the piston spring).

So, the fill level is observed at the bottom of the suction chamber neck, looking inside the carb with the damper rod removed. It should be 1/2" above the bottom of the hollow tube at the bottom of the chamber neck.

That's where I fill it. :yesnod:

So, where's the oil going? There's no gasket that holds it in, is there?
 
They only hold a teaspoonful or so. If the oil is real light, tiny droplets are consumed through the vacuum in the chamber, but the hole itself is a blind hole with no chance for leaks. Topping up should only be required once or twice a year. Maybe try a heavier oil.
 
Sounds good, Trevor. I'll give some 20 weight motor oil a shot.

Thanks!
 
I have 30wt motor oil in mine, working great and not disappearing. I have had it vanish while using lighter oils.
 
If when you remove the piston (damper) from the dash pot, you know, unscrew from the top and pull it up you can feel the suction for the first quarter inch or so all is well. Thats all the oil it takes.

Only way oil can get out is if the Cap and Damper Moss 370-765 or 370-750 depending upon your carbs is bad and I sort think not.
 
I top mine off quite regularly, maybe every couple of weeks, and I'm not leaking any oil, but it is going somewhere.

I should only be doing this a couple times a year?

Can any explain in 20 words or less why we need to fill these with oil? Whats the purpose? I would like to know why I'm doing this every 2 weeks !!!
 
walshja said:
Can any explain in 20 words or less why we need to fill these with oil? Whats the purpose? I would like to know why I'm doing this every 2 weeks !!!
Click to expand...

The oil provides resistance for the piston, which moves up and down, depending on the amount of air required for proper combustion mix.

Well, that's 23 words....
 
Tried about everything from ATF and 3-in-1 to straight 40W... 30W is what was settled on. Seems to be fine, winter or summer.
 
walshja said:
Can any explain in 20 words or less why we need to fill these with oil? Whats the purpose? I would like to know why I'm doing this every 2 weeks !!!
Click to expand...

Dashpot oil slows (dampens) the piston's movement so you get a bit richer mixture on acceleration like an accelerator pump. (20 exactly)
 
The oil in the dampers restricts (therefore meters) the piston lift to keep it from getting too MUCH fuel on rapid loud pedal application. Without it the piston would slam to the top and dump fuel down the throats.
 
I understood that since the piston's rise is restricted by the oil, the flow through/over the saddle/threshold is greater therefore a bit RICHER, not sure where I picked that up or if it's even correct.

I guess it is.
 
As a footnote to the postings above, the text below is copied from a carb tuning document I have at work:

<span style="font-style: italic">"NOTE:
On dust-proofed carburettors, identified by a transverse hole drilled in the neck of the suction chamber and no vent hole in the damper cap, the oil level should be 1/2" (13mm) <span style="font-weight: bold">below</span> the top of the hollow piston rod."</span>

I have never filled any carb to the point that the oil has been above the hollow tube. I just don't see the point. Once the oil is above the tube, it's only a matter of time until it finds its way down between the mating parts and into the engine. I've always filled to just below the top of the hollow tube with the understanding that any excess would be sucked up by the engine vacuum.
 
DrEntropy said:
The oil in the dampers restricts (therefore meters) the piston lift to keep it from getting too MUCH fuel on rapid loud pedal application. Without it the piston would slam to the top and dump fuel down the throats.
Click to expand...

Sorry, Doc, but that's a popular misconception!
Certainly the dampers reduce the speed of lift of the pistons, but the reason for that is that if they fly up too quickly, depression across the choke is lost and far from having a mixture that's too rich, you have one that's too lean. Fuel won't be dumped down the throats, it'll barely struggle its way out of the jets, even though the taper of the needles means the apertures are enlarged. Just the opposite of what you wrote, in fact. Ivandyke got it right in his post.

Now, Mark, I suspect the reason you're losing oil is simply because you're overfilling. If you fill above the level of the hollow shaft of the piston, then when you insert the piston it will displace oil from inside the shaft to add to that which is already above the level of the shaft. Then that surplus oil will seep down between the shaft and the dashpot cover and be sucked into the inlet manifold and burnt.

Think of it this way. The only useful oil in the dashpot is inside the hollow shaft, and in which the damper is immersed. It's what makes the damper work! Any oil outside of the shaft is surplus.

I suggest you fill to within 1/2" of the top of the hollow shaft, and not over it. It doesn't really matter much if you add more, for as you have discovered, the surplus disappears pretty rapidly!
 
man, I can't seem to get a grasp of carbs and all their components. Wish there was a class I could take on this.

using this picture, where should the oil level be?
https://www.mgbexperience.com/article/images/su-hs4-section.gif

and I am guessing it is better to have more oil, then being low? since any extra oil will disappear?

thanks for all the, close to 20 words, descriptions !
 
The oil level is shown in that picture. #7


m
 
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