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MG Midget and Sprite Technical - SU damper that unscrews itself?

Anyone ever seen this before?

The front carburetor damper keeps unscrewing itself from the top of the dashpot. The first time it happened I thought to myself, "dang, what an idiot, I must have left that off or something." But, now it has happened enough times to begin to convince me that it is doing it by itself while I am driving. And, yes, I did make sure it was good and tight, and it still happened again.

!


Norm "haunted SU" Kerr

Norm Kerr

Hi Norm,
if it's a plastic cap could it have squashed or damaged threads

I know I once cross-threaded a plastic dashpot cap by rushing and had to unscrew it and carefully screw it back in again so that the plastic thread realigned in the metal cover, did no harm at all but did mean by rushing I'd wasted more time than if I'd took it steady

is your dashpot oil level perhaps higher than it needs to be which might be helping a damaged cap to rise

is there free movement of the damper (bit)

if there's a hole in the cap I found when I used to habitually overfill them they'd just coat an area of the underside of the bonnet

I'm using John Twist recommended "90 weight gear oil" in my dampers, in case you've not seen me put that before :)

and the 90 oil makes the operation of the dampers very smooth and the oil has found its own happy level so I don't feel I have to top up
Nigel Atkins

I checked.
Threads are fine, hole is clear, dampers move smoothly and drop together, 20W fluid as per the manual.

It is the strangest thing.


Norm
Norm Kerr

Norm
I had it happenning with my B at one stage. I suspect it was caused by worn threads. Ultimately I fitted a new damper and that solved it, but a very effective temporary fix was achieved with very careful application of Loctite.

Cleaned both male & female threads with brake cleaner (without getting any of the cleaner into the dashpot oil) and then smeared a small drop of loctite onto the thread of the plastic cap, taking care to have no surplus get wehre it would be distinctly unhelpful.

It worked well, solved the immediate problem but predictably had the side effect that polygrips were needed if the damper had to be unscrewed.
Paul Walbran

There's obviously a little troll living down inside the damper tube.

The piston is pressing on his poor little head so he is furtively unscrewing it to give him a little pressure relief, hoping you might not notice.

Ah shame!




Anyway to more likey (slightly more likely) answers the thread cut in the dashpot body might be at the highest acceptable size tolerance and the moulded on cap thread may be a little worn and allowing the piston to vibrate loose.

How about putting a rough cut across the plastic thread so the roughness stops it vibrating loose.

A small slot from a file should do it, leave the edges unsmoothed.
Bill1

Ooh, I like the troll explanation best, it could explain so many of the things that happen at times ....
Paul Walbran

What do you call a whole group of trolls ? A family ? A herd ? A pack ?
Andy Phillips (frankenfrog)

What do you call a whole group of trolls ? A family ? A herd ? A pack ?


A disease

What about some ptfe tape wrapped around the cap threads

Prop
Prop and the Blackhole Midget

mine used to come unscrewed i found the fiber washer was too thick not letting sufficient thread to screw in.
mark 1500 Lights on at the end of the tunnel

Northern Hemisphere: you probably have the Southern Hemisphere version by mistake which tend to unscrew themselves if used north of the equator. Try offering a swap with Paul.

(As the dashpot lifts and drops, the oil rotates in the same way as water running down a sink plughole. As you know, the direction of rotation changes between north and south so the wrong cap will tend to be unscrewed by the rotating oil. I'm sure Prop agrees with this theory.)

I'm sure I read that in the handbook.
G Williams

What do you call a whole group of trolls

An internet message board purrups...

;)

I hear that at the equator and within a narrow region aound it that the oil always disappears from the damper tube and the troll dies.




Ah bless
Bill1

Norm,

Like Mark I think it might have to do with the washer. It usually seems to me that the washers are a bit soft so that mashing the washer when you tighten it acts a bit like a lock washer. What happens if you swap the front and rear caps? Does coming undone go with the cap or the carb?

Charley
C R Huff

As a tempory fix, try some PTFE tape rapped around the thread. Worth a try if nothing else.
P Ottewell

I dont have a fiber washer on mine.... perhaps the troll ate it ???

Prop
Prop and the Blackhole Midget

Prop, you gotta have fibre for breakfast

:)
Bill1

Thanks guys, I will try it without the washer and see if that fixes it.

If not, then I'll try the cut thread idea!


Norm

Norm Kerr

Norm, That happened to my car several years ago. i fit a different damper and haven't had any problems with it since.
P Burke

Prop, you gotta have fibre for breakfast 

:)


Perhaps thats my problem...haha

Less pizza, more cardboard ....mmmm

Prop
Prop and the Blackhole Midget

A very common fault on SU carbs with both plastic and metal damper tops. Use a small dab of loctite on the threads or some ptfe tape (not too much or it will damage the threads even more)and the problem should be cured.
Nick Nakorn

I have the opposite problem on my Hif44. The damper is always so tight that I need to use mole-grips to undo it. I suspect previous cross threading, but how to re thread the plastic bit or should I just buy a new damper cap/complete damper?
G Lazarus

This thread was discussed between 19/05/2014 and 20/05/2014

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