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Thread: How long should an SU damper top up last?

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    Obi Wan
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    How long should an SU damper top up last?

    Seems to me I topped up the dampers on the SU's about a week ago. Now the level is down substantially. I did use "3 in 1" oil as some people recommend. If I switched to SAE 20 would there be less of a leak down issue or is there something else going on with the carbs? How long, on average ,should the damper level stay at it's correct level before it starts to go down??

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    Yoda Geo Hahn's Avatar
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    Re: How long should an SU damper top up last?

    On a TR3 there is nowhere for that oil to go & oil, if it evaporates at all, does so very slowly. So what is going on?

    I suspect you are overfilling the reservoir. No harm in that, the excess is just being sent thru the intake and out the exhaust.

    You might re-read the fill instructions though once you notice the level has gone down you essentially have the right level.

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    Yoda CJD's Avatar
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    Re: How long should an SU damper top up last?

    If anyone dis-assembled the damper valve, it could be put together wrong and is shoving the oil out. The symptom of that is that the piston has little or no resistance when being lifted...and of course excessive loss of oil.
    John

    1955 TR2

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    Re: How long should an SU damper top up last?

    Use only 20W nondetergent. There is a book available free on the web called "practical hints" that gives recommended frequency. The oil does evaporate.
    Bob

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    Jedi Trainee gbtr6's Avatar
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    Re: How long should an SU damper top up last?

    I have been using ATF. That seems to work well and it is less viscous than motor oils. Any feedback?

    Perry
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    Yoda Geo Hahn's Avatar
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    Re: How long should an SU damper top up last?

    Varies based on many factors including ambient temps. It was possibly the temperature factor that resulted in the rule of thumb to use the same weight oil in the carbs that you use in the crankcase.

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    Yoda Marvin Gruber's Avatar
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    Re: How long should an SU damper top up last?

    I've always used engine oil.
    Marv

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    Re: How long should an SU damper top up last?

    Some have the view that you should use something thin, so that the pistons react quickly. But what that does is cause a lean mixture on acceleration. A thicker oil will cause a temporarily richer mixture on acceleration, which is usually what you want. The reason I think people suggest a thinner oil is that they think opening up the needle faster will cause a richer mixture, but it also reduces the air speed over the jet. Using a thicker oil will keep the piston down longer, so while the jet opening will be smaller, the increases air speed over the jet will actually pull more fuel.
    Darrell Walker
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    Obi Wan
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    Re: How long should an SU damper top up last?

    Quote Originally Posted by Darrell_Walker View Post
    Some have the view that you should use something thin, so that the pistons react quickly. But what that does is cause a lean mixture on acceleration. A thicker oil will cause a temporarily richer mixture on acceleration, which is usually what you want. The reason I think people suggest a thinner oil is that they think opening up the needle faster will cause a richer mixture, but it also reduces the air speed over the jet. Using a thicker oil will keep the piston down longer, so while the jet opening will be smaller, the increases air speed over the jet will actually pull more fuel.
    So you would use 20SAE or ?

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    Luke Skywalker Roger's Avatar
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    Re: How long should an SU damper top up last?

    You can spend many happy hours experimenting with different viscosities and springs, until you get it how you like it. There aren't any hard and fast rules.
    I find Marvel Mystery works well in the dashpots of the Strombergs on my Europa Twin Cam, arrived at by experiment. If it stumbles at low revs, try a higher viscosity, if it's sluggish higher up the rev range, try something thinner.
    Roger
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    Re: How long should an SU damper top up last?

    The laws of combustion are extremely hard and fast. The ability to interpret them is what alludes most.
    John

    1955 TR2

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    Luke Skywalker Roger's Avatar
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    Re: How long should an SU damper top up last?

    Quote Originally Posted by CJD View Post
    The laws of combustion are extremely hard and fast. The ability to interpret them is what alludes most.
    Since you alluded to my post by quoting it directly, I gather that the correct word eluded you. ��

    What I mean is that there's no hard and fast rule about viscosity of dashpot oil. Springs, temperature, engine tune, and lots of variables that today's cars sort out with computers mean we compromise as best we can.
    Roger
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    Re: How long should an SU damper top up last?

    I've been using Marv's mystery oil for some time with no ill effects. When I run out of that
    will probably go to ATF fluid. Mainly because that's what I got laying around in the garage.
    Harry

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    Re: How long should an SU damper top up last?

    Anything will work..anything from air to water to STP, but some things work better than others when it comes to affecting acceleration and you'll never know the difference unless you try different viscosities
    Having ZS carbs with the O-ring seal, I use a blend that contains a seal preservative..but if I had SU's I'd use something equivalent to a straight 30 wt motor oil.
    DRIVE 'EM IF YOU GOT 'EM

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    Yoda Geo Hahn's Avatar
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    Re: How long should an SU damper top up last?

    Quote Originally Posted by Roger View Post
    ...I find Marvel Mystery works well in the dashpots of the Strombergs...
    Plus it has that delightful Wint-O-Green smell.

    Quote Originally Posted by Roger View Post
    What I mean is that there's no hard and fast rule about viscosity of dashpot oil...
    Indeed, I know of one application (Weller engine in an AC Ace) where the SUs use no oil at all. Look like other SUs but just a brass cap - no damper under it.

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    Obi Wan
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    Re: How long should an SU damper top up last?

    Have always used 30W oil in all SU's & Strombergs for over 60 years with not one problem.

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    Luke Skywalker Roger's Avatar
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    Re: How long should an SU damper top up last?

    Quote Originally Posted by Geo Hahn View Post
    Plus it has that delightful Wint-O-Green smell.



    Indeed, I know of one application (Weller engine in an AC Ace) where the SUs use no oil at all. Look like other SUs but just a brass cap - no damper under it.
    Common pre-war practice. Heavy pistons, no spring or damper. Gravity was your friend!
    I had 2 x 1" like that on my Austin Seven special.
    Roger
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    '72 Lotus Europa TC
    '05 Lotus Elise

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    Yoda CJD's Avatar
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    Re: How long should an SU damper top up last?

    Quote Originally Posted by Roger View Post
    Since you alluded to my post by quoting it directly, I gather that the correct word eluded you. ��

    What I mean is that there's no hard and fast rule about viscosity of dashpot oil. Springs, temperature, engine tune, and lots of variables that today's cars sort out with computers mean we compromise as best we can.
    Little alluded me. I started working on my first engine in 1st grade...a cox .049 glow engine. That started my love of anything engine related for the rest of my life. I have found that either One understands carbs and mixtures and how to tune them almost intuitively,...or they never will. I truly meant no offense in my remark. In fact, you have had the most accurate response to Karl's questions so far on the thread. Rather, I was really expressing the frustration I get from these "what oil do I use" threads. As you pointed out...there is no correct answer for every engine. You must understand what the oil does, and then you can find the correct mixture for YOUR particular engine through tuning. Some will get it...most will not
    Last edited by CJD; 08-04-2016 at 02:45 PM.
    John

    1955 TR2

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    Jedi Warrior luke44's Avatar
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    Re: How long should an SU damper top up last?

    For what it's worth, the standard race car trick on my old Spitfire was to remove the plunger altogether for much faster throttle response. I don't think it's really critical what you use, but stay with something lightweight. On my streetcars I use 3-in 1 sewing machine oil.

    50da0c66-f19c-4275-82c2-5f03317c5796_400.jpg


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    Luke Skywalker Roger's Avatar
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    Re: How long should an SU damper top up last?

    Quote Originally Posted by CJD View Post
    Little alluded me.
    Still think you mean eluded, not alluded. But hey ho :-)
    Roger
    Ancient Briton
    '67 Lotus Elan S3
    '72 Lotus Europa TC
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