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SU carb dampers

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Old Feb 20th, 2018, 14:36   #1
arcturus
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Default SU carb dampers

Up until recently I have been using engine oil in the dampers and then i changed to ATF. now i notice under high revving that the oil is spurting out of the breather hole. So, should I stick to ATF or go back to engine oil?
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Old Feb 20th, 2018, 14:54   #2
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Probably a combination of too much and too thick of an oil. As the piston rises, the oil isn't able to bypass the piston damper and is then forced out of the cap. I think I'm using a light machine oil in my Volvo SUs - something like a sewing machine oil. Filled to about an inch below the top.
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Old Feb 20th, 2018, 16:08   #3
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Quote:
Originally Posted by blueosprey90 View Post
Probably a combination of too much and too thick of an oil. As the piston rises, the oil isn't able to bypass the piston damper and is then forced out of the cap. I think I'm using a light machine oil in my Volvo SUs - something like a sewing machine oil. Filled to about an inch below the top.

No No No No No No



SAE20 is the correct oil for SU's and it needs to be that thick to function correctly. AFT is too thin (but correct for strombergs or in conditions below zero.

Running a thinner oil will cause leaning out under acceleration and this can lead to VSR. As the carbs wear it can be better to go slightly thicker again with the oil
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Old Feb 20th, 2018, 16:17   #4
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Well at least the ATF will have made it all nice and clean (!)
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Old Feb 20th, 2018, 18:33   #5
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Arturus;

Damping Oil squirting from the vent simply means there's a bit too much...eliminate by holding a rag over the hole, and with Air Cleaners removed, manually lifting the Dashpots to their maximum height once or twice...all extra oil with shoot into the rag...done...reinstall AFs.

CS; I don't feel as strongly about not using ATF...because our practical viscosity tests long ago showed it was quite close to 20W, and as it is used with wet clutches of the automatic gearbox, we know it has very good temperature operating range, so that's what we have used (with good result) ever since. ...and I can't really agree about SUs "wearing"...the Damper Valve(2) is 1. ... always submerged in Damping oil(1), and therefore 2. ...makes NO contact with ID of tube..., and 3. ...a film of Damping oil also serves to separate OD from ID of Dometube (at (3))...so there is no contact there...and where there is no contact, there can be no wear...thanks to the brilliant design...it's true!

(...unless things were completely dry...and that doesn't happen with even the most neglectful owner, because even when completely neglected, there will always remain a small amount of oil in the bottom of tube, but IF it were to get completely dry, then Dashpots would soon thereafter freeze in place, and engine would not run, or run so badly, even the most neglecting owner would soon realize something was very wrong...and hopefully check it out!) It does emphasize the point that Damping oil is important!


See also: http://www.sw-em.com/su_carbs.htm#Da...20of%20Dashpot

I Love SUs!
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Old Feb 20th, 2018, 20:41   #6
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Criminies! Although I have not been a "neglectful owner" with the newish PV 444, I have been running the SU H6's on the MGA race car without oil and without piston dampers for a few years - maybe 3,000 miles. Guess I should at least add back some oil to the extent that it serves to lubricate the dashpot.
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