I am caring for a 74 S3 and have not had much time over the last few years to stay on top of things. This is my first post on the renovated forum. I recently started to check all fluids and have found one of the four Strombergs is without oil - really dry (piston damper).
The manual suggests refilling with 20w oil (apparently the same as the crankcase).
Questions: Where do these carbs typically leak oil and is the oil entering combustion or dripping? Is 20W still preferred and how much does it typically take to refill the carb?
My guess is that this carb as been oil free for years because I cant recall the last time I checked it. The car is running fine despite the issue here.
The oil is leaking into the carb and eventually burned IIRC. Mine did this when I first bought it but stopped after I replaced the ‘o’ rings around the needle, it’s been so long ago that I did this repair the memory is somewhat foggy but it was fairly simple.
There is some controversy about the best oil but 20W seems to be the preferred oil. 3 in 1 Machine oil, ATF, 20w50 motor oil have all been suggested. It takes about 10-15 cc max/carb. I have used all of these except 3 in 1 machine oil but I believe it’s a 20 w oil as well, bottom line, there are a lot of inexpensive options so I wouldn’t buy SU carb oil unless your nickel is burning a hole in your pocket.
Cheers,
LLynn
PS: I think I’ll go double check my manual on the seal but quite sure that the 74 had the adjustable needles with the 'o’ring.
With 34years of e type ownership I have always used ATF and seems to work great. On my twin carb Rover V8 I have started using Singer Sewing Machine Oil and it also works fine (comes in a nice little squirt bottle for storage in the tool box).
Careful! Standard 3-in-1 oil may or may not be SAE 20, but they offer an electric motor oil in a blue tin that is specified as SAE 20, and that is what I use. You buy one of these and it lasts a lifetime.
I would not use 20W50, given that Jaguar specifies SAE 20, and SAE 20W50 will definitely be quite a bit thicker than SAE 20 at operating temperature, which means richer in acceleration – all this oil is about.
And now for something completely different here’s John Twists take on the proper oil (min 1.51)
Cheers,
LLynn
PS: I think I’ll go drain out my 20w50 and put the ATF back in.
PPS: not sure if this applies to ZS carbs but seems like it might
PPPS: John Twist is very highly regarded in the MG community, kinda like a combination of Ray Livingston and Joe Curto
Note that the actual viscosity of SAE 90 gear oil is about the same as SAE 40 motor oil; the “90” makes it seem much thicker, but no. Wonder why he is using gear oil rather than motor oil.
Doesn’t change my mind. Jaguar knew about SAE 40 motor oil when they chose SAE 20…
Not advocating John Twists approach but the rational is that carbs are well worn by this time and using a heavier weight oil compensates for the wear and brings the dampening back closer to what the manufacture had in mind.
Cheers,
Lynn
The oil can be used as a tuning device. On Webers, you can control how much fuel is injected with the throttle movement by changing jets. On Holleys, it is controlled by a cam on the throttle. On the SU and ZS carbs, this is controlled by the dampening effect of the oil. The thicker the oil, the more fuel is mixed with the air. If you use a thin oil, a leaner mixture will happen when the throttle is opened. This has no effect on cruising, only sudden opening of the throttle. If you can get by with a thinner oil without backfiring, then use the thinner oil and you will also get a quicker increase in engine speed. The dampening effect really slows down the dampener from opening so as to create a higher vacuum at the jet letting it suck in more fuel. The slower it opens, the more fuel is ingested. No reason to use more fuel than necessary so try the thinner oil. If the car runs well on it, go for it. On race cars, the dampener effect is done away with by the needles being richer at the off idle position. Overall, I really doubt that you will notice any difference on a stock engine driven on the street unless you put something like 90W.
I recently switched oil in my Stromberg carbs from ATF to 75-90 synthetic gear oil. I notice the difference primarily when the engine is thoroughly warmed up. No more issue with misfires or lean operation under hard acceleration.
I presume the heavier oil resists thinning out when hot, giving slower piston response. I could actually feel a difference ATF vs gear lube in the pistons when I manually lifted them on a hot engine.
Car does seem to take a few minutes longer to warm up, but its a nice trade off.
A week ago one piston on my SIII got stuck in the up position. Spewed fuel exiting through the air filter. The cause I found thx to Dick Wells advise and of course going through his detailed explanation was the damper cylinder was the problem not the piston itself. After removing I filled the damper cyl with carb cleaner, let sit over-night, used a specific nylon circular brush to push through several times, and all was well. The cause may be a number of things, possibly to do with that damn ethanol fuel. Hope this helps anyone experiencing this problem.Todd
They are adjustable, just not as easily as the later Strombergs. There is a pinch screw. Loosen it and you can move the needle housing up and down. Not very precise but trial and error will eventually work.
Mea culpa. I should have said “fixed” instead of “non-adjustable”. I think that’s how the reference material refers to them.
The point I was trying to make relevant to the thread topic was that I had success managing my A/F ratio under acceleration using heavy oil, as John Twist had suggested. Not advocating or recommending it for anyone else, its just some observational / experimental results to add to the pile.
My 72 S3 has the original Stromberg carbs. Can the needle be adjusted with the 3mm Allen key inserted from above with piston damper removed or is it done differently?
There is an inexpensive tool made for adjusting it, it has an allen wrench in the center of a tube that is ‘keyed’ to hold the piston from turning (which would tear the rubber diaphragm if not so engaged).
Cheers,
LLynn
