Dashpot oil disappearing!

[dklawson]

Also note that the oil in the picture (item #7) is below the top of the center tube, not above it.

 

[Hairyone]

Mine did disappear but that's because I have a HIF44 in a frogeye & have cut about 15mm off the top of the tube in order to shut the bonnet. I've solved the problem with using a small piece of rubber tube about 4mm long & 6mm diameter, slit it along it's length & slipped it onto the rod (making sure the rod moves freely) & wedged this into the top of the tube.
haven't had to add oil for quite a while now.

 

[vagt6]

Well, it seems the advice here was correct! Filled the damper with 20 weight engine oil (Valvoline 20W-50 VR1 Racing oil-what's in the engine) and it's still there 24 hours later. Drove about 40 miles yesterday, car sat overnight.

Oil didn't leak out. Man, I really like these easy problem fixes.

Regarding the damper oil fill level, I'm going to stick with the manufacturer's recommendation to fill 1/2" above the hollow tube at the bottom of the neck. The SU website recommends this, and also the MG and Haynes manuals. That's good enough for me FWIW.

Thanks to you all, this has been a very educational and useful thread (again).

 

[DrEntropy]

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.

I'll stand down. I can see where the throat vacuum wouldn't be strong enough to draw fuel up the needle to sufficiently atomize it if the piston were drawn to the top of the chamber suddenly. It'd be more like a fuel stream at the bottom of the bridge. Like a "drool". :shocked:

 

[davester]

I wonder if the '1/2" above' recommendation is so that a little oil seeps down and lubricates the piston, meaning that the oil is meant to disappear.

 

[Roger]

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 the diagram. Needs no further explanation!

Mark, the SU website tells you to fill within 1/2" from the top. Now, being an Englishman, I can firmly state that in British English that means 1/2" below the top, not above.

 

[vagt6]

The MG Midget manual AND the Haynes manual both indicate the following, clearly stating to fill ABOVE the top of the hollow piston rod located near the bottom of the neck inside the suction chamber:

<span style="font-style: italic">1) Top up the piston damper with the recommended engine oil until the level is 1/2 in. above the top of the hollow piston rod.</span>

The photo supplied in both of the above manuals clearly shows the oil level filled to ABOVE the hollow piston rod.

Above, not below. However, please feel free to use whatever method that makes your LBC the happiest.

 

[SilentUnicorn]

Here is what i do FWIW- I fill up the dashpots to the top, wrap a rag around the chamber and screw in the damper- with rag still in place slowly raise the piston and let the excess squirt out the vent hole. Call it good.


mark

 

[jlaird]

I got real news. If you fill the dash pot to the top of the piston damper then insert the piston, oil will flow over the top into the carb.

Try it, 1/2 inch from the top of the piston damper is correct, I don't care what the book says.

Fill piston damper to top, insert piston. Remove piston, measure the oil level remaining. And it sure isn't 1/2 inch over the top.

Enough said.

 

[DrEntropy]

In 1969 the choice between a GT-6 and an MGB was decided by my wallet... I purchased a Bentley Press "Official Workshop Manual" when I'd found the two-year-old MG of my (affordable) dreams. One of the first things I found was that filling the SU chambers to the level in the book would result in oil "disappearing" to the level about the displacement of the damper and rod in the piston tube, somewhere around 1/2" below the tube end. After "amping" over it for a few months I simply would make sure the damper was IN oil. If it got overfilled, well, the slide got some lubricant. NEVER did I have an SU with any evidence of premature wear at the slide/chamber, whether filled to book illustration or where it ended up after a month's worth of driving. Overfilling it results in some oil squitterin' out th' hole in th' cap, running down the chamber body.
...mebbe that was to lubricate all th' linkage?!? :smirk: