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SU carb dampers

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Old Feb 20th, 2018, 13:36   #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, 13:54   #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, 15:08   #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, 15:17   #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, 17:33   #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, 19:41   #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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Old Feb 20th, 2018, 20:54   #7
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bo90;

"SU H6's on the MGA race car without oil and without piston dampers for a few years"
...I might understand running them without Damping Valve on a racer (and would ask you to elaborate on throttle response when configured like that!...does it cough [lean stumble] when you quickly go to Wide Open Throttle [WOT]?, or do you have it adjusted so rich that it doesn't even notice? ), but I can't see running mechanical assemblies (your carbs!) without lubrication! Are you not aware that they need lube???

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Old Feb 21st, 2018, 13:22   #8
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Forum;

...and while we're on the subject...see: http://www.sw-em.com/Setting_the_SU_...d_Straight.htm

Cheers
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Old Feb 21st, 2018, 15:46   #9
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" Are you not aware that they need lube???"

I am now!


I think they run pretty well. No discernible coughing, sputtering, hesitation. I can't remember what needle I run, maybe KW.

I do run them fairly lean with the jets set about .08" below the bridge, tweaked maybe one or two flats depending on how it's running given temperature and/or humidity. I have EGT sensors for both the front and rear carbs and try to keep EGT readings at about 1,350 degrees F. I'll go richer or leaner and for better balance depending on EGT temps. I mostly run in the 3,500 to 6,500 rpm range.

Here's a link to a video from last fall (Lime Rock Park) and you can hear how it sounds. Sadly, I broke the crankshaft later that same weekend and I'm not sure if I will have anything better than a parade lap engine (if that!) for the upcoming season. (I do have a spare Volvo B16 engine, but it hasn't occurred to me yet to try to drop that one in!)

https://youtu.be/q45MAx-rfRE
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Old Feb 21st, 2018, 22:20   #10
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bo90;

I know Lime Rock well...your MGA does sound OK and seems not to be suffering from lack of accelerator pump effect, so I wouldn't change anything without good reason...but I'd still fill Damping Oil Reservoirs with oil for purposes of lubing...

I can't help you with the broken MGA crankshaft (and I'll spare you my thoughts about it, given we are on a British Forum, and I'd like to keep my friends here...), but if I was going to race a (any!) car, I'd make it a priority to know every bolt on (at least in the engine bay and suspension!) it by its first name, and certainly all points which require lubrication...but that's just the engineer in me talking!

May I suggest you check your manuals for equally important lubrication points on the distributor, similar to the many found on the Volvo/Bosch unit:



Lube thy Steeds!
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