• Hi Guest!
    You can help ensure that British Car Forum (BCF) continues to provide a great place to engage in the British car hobby! If you find BCF a beneficial community, please consider supporting our efforts with a subscription.
    There are some perks with a member upgrade!
    **Upgrade Now**
    (PS: Subscribers don't see this gawd-aweful banner
Tips
Tips

Filling SU Dampers?

Share this page

B

blkcorvair

Jedi Knight
Offline
I apologize in advance if this was answered once before, I am very Green with these SU Carbs and although I seem to have it set pretty good for now something is bothering me.

Everything I read manuals web page etc says to fill the damper with oil. Easy right! But how much oil goes in? I fill it to the bottom of the cap threads, but as soon as I lift the piston/pin with my finger much squeezes out the top hole. Is that right? Or maybe I should just keep my finger outta there?
 
There is a small difference in the ID of the dampner. Fill it to where the transistion is.

You will be good to go.

Different 'OIL' aka '710' will give you different results.

Good luck-Steve
 
Displacement my man /bcforum/images/%%GRAEMLIN_URL%%/smile.gif , it's a moving part, the air piston that is, so as it travels upward it going to have to displace oil if there's too much in there, and out of the dampner rod cap vent hole is where it will puke, I would fill the tube no more than 1/3 full, I beleive you said you are going to drive this car on ther street as well, so use something pretty light ATF, on the race track, whe don't use any oil, except for a squirt of WD40, the amount of oil and it weight controls the rise rate of the air piston, lighter to no oil menas a quicker rise rate, faster acceleration, on the street however you should run light oil, so the car accelerate smoothly, on the track you want those puppys rising as fast as you can get them. Hope this helps
 
Hap, I hate to quarrel with one who ssems so well qualified, but I think I must.
First, any overfill of oil in the dashpot should be removed when you screw the damper back in, provided the damper's the right size, of course. Even when the piston is as low as it can get. As the piston rises and falls, oil flows though the damper - it's not pushed out of the top. At least, it ought not to be.
The way your own engines are tuned for the track, it may well be that you want your pistons to rise quickly, but I think that's very misleading as a generalisation. The rise and fall of the piston is to control mixture strength. The SU is a constant-depression carb, and unlike an Amal or other slide-throttle carburetter it doesn't regulate the intake volume. That's done by the butterfly that sits downstream of the piston.
I have known people with too light a spring and little or no damper oil whose pistons rose too fast and therefore they lost depression over the jet with consequent weak-mixture stumbling. So they go to richer and richer needles and find themselves in a cycle of compounding trial and error caused by an initial misunderstanding of the way the carb is meant to work in the first place.

Why do you "want those puppys rising as fast as you can get them"? And if you do, why should anyone else?
 
I've wrestled with this a bit and I think I have to agree with Roger. I can't see why you would want your piston to rise any faster than intended. My best guess is that it may give some sort of "accelerator pump" effect, but even if that is the case, I would have to wonder if the mixture ever levels back out.

Let's remember that the A/F ratio is a RATIO. Stoichiometry is based on chemistry and in a chemical reaction (here taking place as an exothermic combustion episode) the reaction must be balanced or else you end up with unintended "left-overs" which, in this case are unspent fuels and noxious emissions. In other words, the ratio is off. This is the idea behind O2 sensors and fuel injection. The O2 sensor "sniffs" the tailpipe emissions and when it senses an unbalanced combustion, it tweaks the A/F ratio until the chemical reaction, and therefore O2 comes back into balance.

Since the piston is controlled by things like air volume and vacuum, it should be a constant whether a street car or race car. In other words, X throttle position and X RPM will always have the same vacuum and air volume. Once you have established this baseline constant, you can then begin to dial-in your ratio with needles and jets. That is why there are hundreds, perhaps thousands of needle/jet combos, each with 13-16 different stations with specific A/F characteristics.

I know I would like to add more and I'm sure I've forgotten a thing or two that I intended to include here, but I'm already late for work as it is. I'm looking forward to everyone's input on this, particularly as I don't have the race experience with tuning an SU.

JACK
 
I've always run my Spridget racecars with no oil and just a light spring in them like Hap does. The 948, once on the track never gets below 4000rpm and seems to run/accelerate the best without oil (I tried oil once and it didn't feel right on acceleration). But then it's usually wide open throttle most of the time.

On the street Spridgets and the MGTD, we always use a lighter weight oil.
 
Scott!

<u>Run Away!</u>

QUICKLY!

You've unleashed the most disputed question in all of LBC-dom.

<span style='font-size: 8pt'> Didn't you get the memo? This question is NOT ALLOWED!</span> /bcforum/images/%%GRAEMLIN_URL%%/jester.gif

I was at an Austin Healey meet a few years ago (at Princeton University, no less) and this question almost started a fist fight!

For the record, no oil in my carb....but then again, I'm not running an SU or ZS. /bcforum/images/%%GRAEMLIN_URL%%/laugh.gif
 
on the race carbs, no only no oil, but no spring either, and that's nothing new, it's what Tabor, Griffin, Bachman, Haynes,Huffaker, myself, etc., etc. have been doing for decades. Now I do recommend oil in street carbs, just so they don't stumble uner dead stop acceleration, but on a race car that's not really a issue. And Roger it's ok to disagree with me, you won't be the first or the last /bcforum/images/%%GRAEMLIN_URL%%/smile.gif
 
Yes, it's important to recognize that we are talking about two completely different uses. Street or track, the rules are different for each - totally different worlds.

If I were using a car strictly on the track I would certainly take the advice of "Hap, Tabor, Griffin, Bachman, Haynes, Huffaker, etc." - in fact I would treat their knowledge as valuable as gold.

However for a street car, a "driver", I would go with the general wisdom which is to use a light oil like ATF or MMO.
Personally I use MMO, always have.
 
Yes!
I've used it for about 25 years in SU's - in my MGs & my Volvos. It seems to work fine and it comes in small, medium & large bottles. The small bottle will last a long time if you are only using it for your carb dampers.
 
Been useing 3 in 1. Seems fine to me.
 
Don't want to get in any trouble... but I wonder how whale oil would work? It used to be great for sewing machines.

I guess any light oil works fine.
 
RickB said:
Don't want to get in any trouble... but I wonder how whale oil would work? It used to be great for sewing machines.
Click to expand...

It might make them run a little fat /bcforum/images/%%GRAEMLIN_URL%%/smile.gif
 
Oh sure we are ok the eco environment groups with our low carbon foot print well sort of 30 mpg is Pretty good . Now you will have PETA protesting SU at the track! I can see them now all lined up at the fence in there little whale costume’s holding up there signs Screaming SAVE A WHALE RUN A WEBER! /bcforum/images/%%GRAEMLIN_URL%%/laugh.gif
 
I know little about the American racing scene, sorry, so the names don't mean a lot. Nor do I know much about the regulations under which cars are prepared. And, without being rude, saying you do it because a lot of other people do it doesn't convince me! Saying you do it because you've found it works, without really knowing why, I do find a powerful argument though. I'd just like to understand why.

Let me say this. Early SUs, at least those with pistons and not leather bellows, had heavy pistons, with no springs or dampers. To them this debate is irrelevant.

I assume we're talking here about later SUs with light alloy pistons, usually with springs and dampers? If so, and you're running with no spring or damper, then from a certain point in the depression range you're running it as a fixed choke carb, surely? And that would seem to me to indicate that the carb you're running is too small, since you're running it a near-permanent maximum choke opening. If you're constrained by regulation to run carbs of a certain maximum size, I start to understand.

Happy Holidays!
 
Roger said:
If you're constrained by regulation to run carbs of a certain maximum size, I start to understand.

Happy Holidays!
Click to expand...

BINGO!! The rules dictate the carbs we are forced to run, and generally we run at, or very close to, wide open throttle.
For instance, I can only run 1 1/4" SU carbs on the 948, and 1 1/2" on the 1275. The 1500 is restricted to a single 1 1/2", as the dual HS4 setup was never offered for the US market.
No sidedraft Weber, Dellorto, or Mikuni's allowed.
Jeff
 
Right: Ban the SU & save the whales!

I'm getting some T-Shirts made up right away.

MMO has worked for me, it's supposed to be a good top end lube as well.
 
Back
Top