ZCar Forum banner

Status
Not open for further replies.
1 - 7 of 7 Posts

·
Registered
Joined
·
1,152 Posts
Discussion Starter #1
My book doesnt tell how much fluid or what kind of fluid I need to be putting in the dampers on my SU carbs (flat top). It just shows a cutaway view. Where should the level be on the stick? This is a 73 240Z. Thanks.
 

·
Registered
Joined
·
71 Posts
> My book doesnt tell how much fluid or what
> kind of fluid I need to be putting in the
> dampers on my SU carbs (flat top).

There is a Government conspiracy to keep this information from the public :)

I have a Series 2 Jaguar manual (for bigger SU's) in front of me that says to use SAE 30 motor oil for the piston dampers in the Recommended Lubricants section, but doesn't mention its' use anywhere else, nor how full the damper oil should be - Refitting is the reverse of the removal procedure. Ha! My Z and 1600 (510) manuals are just as helpful. Anyway...

Datsun/Nissan recommended SAE 10 motor oil, but SAE 20 will work and some people recommend it for worn carbys. I personally think that the damper pistons never wear this much (but the main-piston to suction chamber seal does wear!). However, a FAQ I found somewhere says:

You can also modify your car's throttle response
characteristics slightly by adjusting the viscosity of
the oil in the dashpot damper. SUs are set up
so that a thicker oil will resist the piston's attempt
to rise in the dashpot for just long enough that the
engine's increased load (when the throttle is
opened) will pull more fuel across the bridge; this
enriches the mixture and temporarily bumps power
up to help the engine achieve higher speed more readily.

Still no mention of how high to fill :)

There is an SU FAQ for MG's at:

<A HREF=http://www.british-cars.org.uk/kimber/mgoc/emg/basic4.html>http://www.british-cars.org.uk/kimber/mgoc/emg/basic4.html</A>

that says:

Top up each of the piston dampers with carburettor
damper oil. Unscrew the damper cap, withdraw the
damper and top up the reservoir until the oil level is
1/2 in (12mm) above the top of the hollow piston rod.

my '72 dampers have two lines scored in each above the damper pistons that *may* be dipstick marks...

My personal opinion is that it doesn't really matter how full the damper oil is as long as the piston is submerged when the engine is off (thus it will always be submerged because the main piston/damper-resevoir rises). 1/2 above the damper piston sounds good.

Brendan.

PS. Hope my terminology wasn't too inconsistent. As each SU has a piston within a piston it gets hard sometimes :)
 

·
Registered
Joined
·
107,695 Posts
I'm no expert, but I put the oil up to the top of the dome. Used to use oil from MotorSport Auto until someone told me 3-in-1 was the same and I've been usng it ever since with no problems.

Mike
 

·
Registered
Joined
·
107,695 Posts
Thanks for the info. I'll see if using a thicker oil will eliminate the sluggish start offs I'm having. I dont know whats in there now, but it doesnt seem to have much consistency.
 

·
Registered
Joined
·
107,695 Posts
> My book doesnt tell how much fluid or what
> kind of fluid I need to be putting in the
> dampers on my SU carbs (flat top). It just
> shows a cutaway view. Where should the level
> be on the stick? This is a 73 240Z. Thanks.
I use marvell mystery oil in mine. There is a line on the stick but it is hard to get the oil right on this line. Pour the oil in, then insert the stick and pump it a couple of times. Then the oil will gurgle out of the top. This has worked for me.
 

·
Registered
Joined
·
134 Posts
Here is what the mauals for my series one E-Type, Haynes SU Manual, and my Z manual have to say. All agree that the level should be 13 mm (.5) below the hollow piston tube for dust proofed carbs (which the Z's have ) and 13mm above for non dustproofed (vent hole in the cap). Early datsun SU's have marks and should be filled to the upper mark. Flat tops use the 1/2 below rule.
They all agree that the correct oil is straight 20 weight.
 

·
Registered
Joined
·
107,695 Posts
Brendan -

That was a good description of the damper action and its effect on engine performance. I learned something else, too. With the dampers in place, remove the air cleaners and lift the vacuum pistons with a finger through the carb intake with the engine off. Feel the resistance for each piston, and if they are wildly different the engine won't run right upon acceleration. It's because one of the damper pistons (the little piston) is worn more than the other. The fix is to replace one of the damper pistons - usually the one where the vacuum piston rises with little resistance because the damper piston is not doing a good job. Before conducting this test, of course check the dashpots to see if the oil levels are correct and the same. - John

> There is a Government conspiracy to keep
> this information from the public :)

> I have a Series 2 Jaguar manual (for bigger
> SU's) in front of me that says to use SAE 30
> motor oil for the piston dampers in the
> Recommended Lubricants section, but doesn't
> mention its' use anywhere else, nor how full
> the damper oil should be - Refitting
> is the reverse of the removal
> procedure. Ha! My Z and 1600 (510)
> manuals are just as helpful. Anyway...

> Datsun/Nissan recommended SAE 10 motor oil,
> but SAE 20 will work and some people
> recommend it for worn carbys. I
> personally think that the damper pistons
> never wear this much (but the main-piston to
> suction chamber seal does wear!). However, a
> FAQ I found somewhere says:

> You can also modify your car's throttle
> response
> characteristics slightly by adjusting the
> viscosity of
> the oil in the dashpot damper. SUs are set
> up
> so that a thicker oil will resist the
> piston's attempt
> to rise in the dashpot for just long enough
> that the
> engine's increased load (when the throttle
> is
> opened) will pull more fuel across the
> bridge; this
> enriches the mixture and temporarily bumps
> power
> up to help the engine achieve higher speed
> more readily.

> Still no mention of how high to fill :)

> There is an SU FAQ for MG's at:

>
> <A HREF=http://www.british-cars.org.uk/kimber/mgoc/emg/basic4.html>http://www.british-cars.org.uk/kimber/mgoc/emg/basic4.html</A>
> that says:

> Top up each of the piston dampers with
> carburettor
> damper oil. Unscrew the damper cap, withdraw
> the
> damper and top up the reservoir until the
> oil level is
> 1/2 in (12mm) above the top of the hollow
> piston rod.

> my '72 dampers have two lines scored in each
> above the damper pistons that *may* be
> dipstick marks...

> My personal opinion is that it doesn't
> really matter how full the damper oil is as
> long as the piston is submerged when the
> engine is off (thus it will always be
> submerged because the main
> piston/damper-resevoir rises). 1/2
> above the damper piston sounds good.

> Brendan.

> PS. Hope my terminology wasn't too
> inconsistent. As each SU has a piston within
> a piston it gets hard sometimes :)
 
1 - 7 of 7 Posts
Status
Not open for further replies.
Top