ausmini
https://www.ausmini.com/forums/

SU dashpot piston stopper
https://www.ausmini.com/forums/viewtopic.php?f=2&t=70761		 Page 1 of 2
Author:  simon k [ Tue May 17, 2011 12:44 pm ]
Post subject:  SU dashpot piston stopper
I've wondered in the past what the purpose of the little nylon stopper thing at the base of the SU dashpot piston is, and why it's always worn down to nothing.

Reading Des Hammill's book, I see that it's there to hold the piston up off the bridge by a certain amount - depending on the carb, about 2mm. So with the engine not running, the piston would be 2mm above the bridge. No carb I've ever seen had anywhere near that much clearance, and usually are sitting metal to metal.

I looked through SU Midel and Burlen's website, and also rang Midel to talk to their tech (he's on leave) to find out if rebuild kits contain this little jobbie - no pictures show it. The only reference to it I've ever seen is in Hammill's book

Here's a photo of a piston with the stopper removed - it's a blind hole, and has no other purpose.

Image

I'm about to make one and fit it into my carb, as I have a minor issue I think it'll help with, however, I ask a question

Has anyone ever bought a new carb that had this clearance, or reconditioned a carb themselves and fitted a new one?
Author:  simon k [ Tue May 17, 2011 12:51 pm ]
Post subject: 
hold the phone - looks like it's called a "striker pin"
Quote:
NEEDLE DETERMINATION

To find the dimensions of a needle that will match up to the optimum results obtained by varying the jet drop during tests, it is first of all necessary to plot out the test needle.

The needle diameters in thousandths of an inch are plotted on the horizontal axis and the needle length in 1/8 in. increments on the vertical axis, conforming with the method used to dimension needles in the needle charts. The piston heights corresponding to various speeds can then be super- imposed on the vertical axis.

All needles with the same profile over the first 1/8 in. of their length will require the same jet drop for idling, thus allowing an immediate assessment to be made of the effect on mixture of the remaining dimensions. If these idling dimensions -are not maintained accurate assessment of the effects of different needles cannot be made until the alteration of the jet drop necessary to give the same fuel discharge at idling has been established.

Image

In order to maintain a constant idling setting when using different needles every effort is made to maintain the shoulder and 1/8 in. dimension at standard figures, i.e. 0•089 in. and 0•085 in. for 0•090 in. jets, 0•099 in. and 0•095 in. for 0•100 in. jets and 0•124 in. and 0•1205 in. for 0•125 in. jets.

The position of jet drop below the bridge, and the piston lift above, are determined at idling conditions. Assuming that the idling dimensions will be retained this jet drop (shown in Fig. 26) must be added to piston lift at all readings to establish the actual distance of the metering diameter from the shoulder of the needle.




from here: http://www.sucarb.co.uk/technicaldetail.aspx?id=89
Author:  miniron [ Tue May 17, 2011 4:04 pm ]
Post subject: 
:)
From what I've read over the years, if the pin/striker etc is missing it allows the piston to close completely. If this occurs the engine won't idle properly and will hunt around all over the place. You need a minimal clearance betwen the piston and the jet bridge to get stable idling.

Edit: The pin is brass in early carbs and plastic on later ones.
Lots of good info in that link

RonR
Author:  Mick [ Tue May 17, 2011 4:20 pm ]
Post subject: 
I'd like to know how you go about making up a new one, the material you used etcetera. I was lucky and had a few twin SU pistons lying around to choose a good pair, but would like to fix the bad ones up.
Author:  simon k [ Tue May 17, 2011 9:40 pm ]
Post subject: 
miniron wrote:
If this occurs the engine won't idle properly and will hunt around all over the place. You need a minimal clearance betwen the piston and the jet bridge to get stable idling.


that sounds more than a little bit familiar!

mick wrote:
I'd like to know how you go about making up a new one, the material you used etcetera. I was lucky and had a few twin SU pistons lying around to choose a good pair, but would like to fix the bad ones up.


I was thinking I'd just machine a bit of nylon to size and press it in - it's about 4mm in the body, and 2mm protruding so it should stay in place. I'll do a few extras. What would be better? teflon, nylon, white delrin or black delrin?

Getting the old bit out was the hard part. I tried digging it out with a screwdriver but it was too hard, I ended up flicking on the soldering iron and melting through the guts of it, came out nice and easy then... in hindsight I could have screwed a self tapper into it, but that'd mean going down to the workshop
Author:  simon k [ Wed May 18, 2011 11:22 am ]
Post subject: 
machined up a little bit of delrin .010" oversize and bopped it into place, then trimmed the excess with a razor blade - works like a champion

I have 3 in my wallet for you Mick, make sure you get them off me on Sunday
Author:  Mick [ Wed May 18, 2011 11:23 am ]
Post subject: 
simon k wrote:
machined up a little bit of delrin .010" oversize and bopped it into place, then trimmed the excess with a razor blade - works like a champion

I have 3 in my wallet for you Mick, make sure you get them off me on Sunday


You can have my babies Simon. Thank you very much :)
Author:  winabbey [ Wed May 18, 2011 12:00 pm ]
Post subject: 
Mick wrote:
You can have my babies Simon. Thank you very much :)

I'm also after some babies. If you get any more please keep me in mind. I'm happy to pay packing and postage.
Author:  winabbey [ Wed May 18, 2011 12:02 pm ]
Post subject: 
I was thinking of putting an emoticon in my last post but felt that made it too easy for readers to see it was meant in jest. Better for people to squirm a bit before working it out for themselves. :)
Author:  Mick [ Wed May 18, 2011 12:29 pm ]
Post subject: 
Oh, I wasn't sure I would have enough bubble wrap around here anyway.. :P
Author:  BBY755 [ Mon Jun 27, 2011 7:02 pm ]
Post subject: 
Anybody tried them and found a difference?

Rebuilt my carbies and pulled the existing ones out with a screw and set them back in sticking out 2.2mm. Couldn't start the car so, in my checklist, pulled them out completely so that it was as before (nothing sticking out)

Now my car runs, I will put them in and set them at 2.2mm and see. I will let you what difference I get if no-one has tried yet....

Keep tuned.....
Author:  gafmo [ Mon Jun 27, 2011 7:27 pm ]
Post subject: 
miniron wrote:
:)
From what I've read over the years, if the pin/striker etc is missing it allows the piston to close completely. If this occurs the engine won't idle properly and will hunt around all over the place. You need a minimal clearance betwen the piston and the jet bridge to get stable idling.
RonR


I'm trying to work out what is wrong with Marcia at the moment and originally I thought it was the carb. Since then I have checked everything and still gotten no where. I'v borrowed a second piston and dash pot and that hasn't worked eather. I'v just checked my original (has scratches on the inside of the Dash Pot) and as with the otherone it has bugger all or this sticking out..hmmm
Still don't think it will fix my problem thou
Author:  adamstuart [ Mon Jun 27, 2011 8:17 pm ]
Post subject: 
Did you check the butterfly and shaft?

Mine on the daily is worn and it idle's funny as a result.
Author:  gafmo [ Mon Jun 27, 2011 8:28 pm ]
Post subject: 
Me.....No mine is fine. Additionally we've never used a Choke on eather car, not even hooked up..and no slop in shaft
Author:  TheMiniMan [ Mon Jun 27, 2011 8:44 pm ]
Post subject: 
gafmo wrote:
Me.....No mine is fine. Additionally we've never used a Choke on eather car, not even hooked up..and no slop in shaft


they`re too rich then :-)
Page 1 of 2 All times are UTC + 10 hours
Powered by phpBB® Forum Software © phpBB Group
http://www.phpbb.com/