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Dashpot oil question

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M

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I've been told by many to use 30W engine oil in my dashpots. It works fine during the very hot summer days here in Virginia, but it seems a bit too "thick" for winter. Is there something else that works better in cold weather (Yes, it does get cold in Virginia in the winter!).
 
I use the Moss dashpot oil in my SUs with success. The car is well-tuned and starts the same way, hot or cold weather. Less choke in warm weather, of course.

I've heard of using everything from light machine oil to molasses in these things, with varying degrees of success :crazyeyes:. If your carbs are in good nick, the 30 weight should be just fine.

The condition of your carbs is the key. :yesnod:

Perhaps just stick with the manual's recommendations, with a tad lighter oil for winter, heavier in summer.
 
If you use synthetic you should have less of a difference between winter and summer thickness.

John
 
The carbs have just been entirely rebuilt, so I assume (hope) they are in good shape.

I haven't been using synthetic for the dashpots, but others have recommended it highly. I'll give it a try. (30W synthetic?)
 
It's interesting that viscosity is proving to be a problem. The "original" synthetic oils were developed decades ago for aircraft turbines, as they had similar issues. They need an oil that would not burn at full power in the 120 degree Mojave desert, but would not thicken to a jell when they powered back at -65 degrees at altitude.

The old prop plane bush pilots in Canada had to drain their sumps as soon as they landed in winter, or the mineral oil would jell and they would have to wait till spring to turn over the engine. Even then, the next day they would build a fire under the engine and place the oil on the fire to thin it out. Once warm, they poured it in and fired the engine up.

But I digress...if you have been using 30W, I'd probably start with 5W-30.

John
 
Just curious, Ed, what makes you think it is "too thick" ?

I actually seek out straight-weight oil, because the engine seems to need a bit more enrichment on acceleration when the air is cold. But I prefer straight 20 weight, as 30 weight seems heavier than necessary in the summer, to me.
 
Randall,

I should have said "heavy." I mention this because my mechanic friend was looking at my carburetors yesterday, which I rebuilt over the past week, and he lifted the pistons. He said they seemed very stiff and I might want to try a somewhat lighter weight oil in the dashpots. That's the only reason I raised the question.

I have put in 5W-30, but could switch to 20 weight if you think that would be better.

I'm just trying to "fine tune" the performance of the carburetors, which are working splendidly otherwise, thanks to the Moss rebuild kit and the Moss/SU DVD on rebuilding the SU carburetor. As before my work, however, they seem to require a fairly rich setting -- perhaps it's a function of today's fuel -- and the shop that rebuilt the engine put in RH needles.
 
CinneaghTR said:
vagt6 said:
I use the Moss dashpot oil in my SUs with success.
Click to expand...

Isn't the SU dashpot oil, which I also use, 20 weight? I have that written down somewhere...
Click to expand...

Jeremy, I'd like to know what weight the Moss oil is, too. The Moss catalog doesn't indicate the weight of the oil. Just says it's "specially formulated".

Anyone know?
 
You can go with the oem specs, as per Burlen Fuel Systems
--------------------

We sell two different types of oil for this purpose which are still made to the original SU factory specifications.

The first is SU Damper Oil, which is a straight 20 grade and comes in a handy 125ml bottle. This oil is for use where the carburetter is fitted with a damper inside the piston and can be used on all SU, Zenith and CD carburetters.

SU Damper Oil (125ml), Part No: BDR125S


The second type is SU Dashpot Oil, which is for use in earlier, pre-war, SU carburetters where no damper is used. In the case of carburetters fitted with dampers the oil should be filled to a level just below the top of the hollow piston rod, so that when the damper is fitted it does not displace the oil any higher than the top of the piston.

SU Dashpot Oil (125ml), Part No: BDP125S
--------------
or you can treat oil as a tuning variable, and set it with the viscosity that works best for you. I tend to use as thin an oil as I possibly can, letting the throttle response be as quick as possible without bogging. I usually find the Dexron - sewing machine oil range to be fine.
 
foxtrapper said:
You can go with the oem specs, as per Burlen Fuel Systems
--------------------
The first is SU Damper Oil, which is a straight 20 grade and comes in a handy 125ml bottle. This oil is for use where the carburetter is fitted with a damper inside the piston and can be used on all SU, Zenith and CD carburetters.

SU Damper Oil (125ml), Part No: BDR125S
Click to expand...

Yeah, that's it. 20 weight. I still have not found where I found that info, but that is what I was thinking.

Regardless, I pretty much bought the "damper" oil to get the handy-dandy bottle since I knew I could refill it on my own.
 
Old small cans of 3-in-1 oil indicted specifically that it was SAE 20. But the new cans say nothing about the weight of the oil. I assume it is still 20 weight.
 
There are 2 different containers of 3-in-1 oil on the market: Black & White and Blue & White. The Black & White doesn't say the weight, however the Blue & White says "SAE 20" right on the front.

Art
 
:iagree:
Unfortunately, the local stores I checked did not carry the Blue & White; and I believe the black & white is rather thinner than 20 weight.

But MMC still sells straight 20 weight motor oil.

However, I still don't think any of this is germane. We've already seen that your mechanic doesn't quite understand these cars, so I would simply ignore his observation that the oil is "too heavy" and instead look at performance. The same thing goes for the initial jet setting (12 flats or whatever it is); it is simply a starting point, not a reference point. Get the mixture right, and then it is right.

Oh, one other point, "rich" needles are NOT rich across the board, in fact they are somewhat leaner at idle. The "rich" part is in the mid-range, where they will produce slightly better performance, at the cost of much worse fuel consumption. I would suggest going back to the stock needles and working to get everything just right in stock trim, before starting to make modifications.

Especially modifications because your mechanic "thinks" the factory didn't know what they were doing.
 
Randall,

Many thanks... very helpful.

As for dashpot oil, I have tried 5W 30 and things seemed to go well. Just for the heck of it, I will try 20 weight (which is what I started with about a year ago before I was advised to switch to 30 weight).

I do start with 12 flats down, following SU recommendations, but find that I need to take each carb down a few more flats for better idle and what seems like better performance.

At this point I am baffled, however, because here is what I am experiencing: idle is good (800-900 rpms, quite smooth), but when I accelerate from stop the car stumbles a bit, and when going downhill in second, there is a hint of backfiring. Could it still be too lean? The carbs are balanced. I have used the lift the piston procedure to get the air/fuel mixture "technically" correct (doesn't rise, doesn't fall). Timing is as correct as I can get it (newly rebuilt distributor). But something is still not working right.

On the road, in third and fourth, 50-60 mph, it sails along beautifully.

I have purchased stock standard needles and have been meaning to substitute them for the RH needles that the shop put in about a year ago. I have rebuilt everything else on the carbs... but have stuck with the RH needles for now. I'll swap them for the standard needles tomorrow.

My goal, as you suggest, is to make it as close to "stock" as possible, although many tell me that I am chasing an illusion.
 
As usual, Randall is dead right. Nevertheless, even as a non-Triumph chap I'm going to poke my nose in here.
If you're getting backfiring, by which I take it you don't really mean backfiring but small explosions in the exhaust on the overrun, that's caused by unburnt mixture, maybe with a little bit of additional air if you have s slightly leaky exhaust system. That points to a rich mixture rather than a lean or weak mixture, but is also a characteristic of CD carbs anyway..

I like the piston-lift method too, being careful not to lift too far which messes up the whole plot. However, I believe you have reached the ideal when, on lifting the piston, the revs momentarily increase then settle down again. If the revs increase and stay up, your initial setting is too rich, if the revs drop and the engine is "hunting", you're too weak. A tiny bit too rich is ideal.

With the idle OK, stumbling on initial acceleration can be due to too weak a spring, dashpot oil too light, or wrong needle. With your symptoms, I'd richen the mixture a couple of flats and try it again. If that doesn't work, put the mixture back and try a heavier weight oil. Then try both!

There are no universal truths, unfortunately. When the cars were new, no unworn components in fuel system or ignition, the factory could make settings confident that they'd be pretty much right. Which leads me to another question - how's your ignition timing and advance? I always keep in the back of my mind the old garage-man's saying that 90% of carburetter problems are ignition. Maybe a couple more degrees of advance might help?
 
I always heard it was fish oil.
 
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