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MG MGB Technical - carb oil
i have a 72 b with the std 1 1/2" SUs . what are peoples opinions on the best oil for them - SU oil or std engine oil? i would experiment but not sure how to tell whether it needs a thinner or thicker oil... any ideas? |
phil |
Phil, "Marvel Mystery Oil" works really well. Next time you change out your oil, re-fill your carbs with it and put the rest in the crank case. Works really well for vales, bearings and rings. |
Victor Gardino |
Phil- I've never tried Marvel Mystery Oil, but the Factory Service Manual says to use the same oil that's in your crankcase: 20W/50. Works fine for me. |
Steve S. |
Not something to fret over. Just use the same oil you pour into the engine...in my case, Castrol GTX 20w50 (never a shortage of this lying around in my garage). I think the original recommendation might have been straight 30w (the oil of the day), but 20w50 should be fine. You'll probably get all sorts of interesting opinions, but engine oil is the only thing that was evidently ever recommended in the manuals (I'd love to create something "specially formulated" for a niche market where the entire market no longer had the advantage of original literature...a real money maker, huh). Of course if you want to spend the extra money, you could buy the special oil from Moss...doesn't really matter as the dampers don't take that much oil anyhow (the tiny $8.00 container will last forever). Regardless, I'd shy away from anything really thin (e.g., ATF fluid) or exotic snake oils ('cause you just never know). Engine oil...simple, elegant, as nature (or was it the manufacturer) intended. |
Barry Kindig |
OK Steve...I must be getting long winded...that's twice you've beat me to the punch ;-) |
Barry Kindig |
Phil. The "orginal" oil recommended for SU carbs was about 20 weight back in the days the Brits did not use multi viscosity oils. After they did, the recommended you use what you use in your engine. For most of us, this is 20W-50. Marvel's is a much thinner oil, similar in flow to automatic transmission fluid, which is also used sometimes. The lighter oil will not dampen the rise of the pistons quite as much and may provide a slightly weaker mixture on acceleration. To the best of my knowledge, no one has ever set up a test bed engine and tried the various weights of oils to see what benefits may be gained or lost by using different weights of oil. Might provide some interesting insights on another form of carb tuning. Les |
Les Bengtson |
Phil, I use 20W50 in my 70 B and it does fine. |
Gary Mills |
this winter I've tried SAE60 (from the local Harley Dealer). the plugs look fine, greyish tan..and the acceleration seems improved. rn |
RN Lipow |
My brother used 3 in 1 in his early-mid 60s B but that was back in about 1980. I use sae40, same as the engine. I'm in FL and have no problems. Carb and engine seem happy. |
kids1 |
The amount of fuel getting through the carb is related to the needle size, the spring strength and the thickness of the oil in the damper. The thinner the oil, the less the damping and the faster the pistons rise. This could give you more acceleration, but it could also make the mixture go weak momentarily. To prove it, try running with no oil and the engine will hesitate on acceleration. I supose you should therefore err on the thick side. If you have had the car set-up or tuned on a rolling road, then you should use the same oil as was used then. Neil |
Neil |
There's a guy with a Datsun Z website who played with different weight damping oils in his Hitachi/SU carburetors. Using an air/fuel monitor, he affirmed that SAE 20 is the proper oil to use. What a surprise! I guess those old SU engineers really knew what they were talking about. BTW, 3-in-1 in the blue can is marked SAE 20. Needless to say, it's less messy despensing oil from this can than the quart bottle. |
Art DiLello |
Just read in a book this morning that the SUs used to use SAE 20. The book suggests getting it from a motor cycle shop. Apparently they use it in the front fork dampers on some bikes? Simon |
S Jansen |
RN- You're using an engine with a 1950cc Big Bore kit, so the low-frequency pressure fluctuations in your intake tract are more powerful. As a result, the increased damping effect acheived by the heavier 60W oil is necessary for your your SUs to work well. I wouldn't recommend the stuff for a stock engine, though. |
Steve S. |
If you can help it, stay away from parafin based oils - they have a tendency to build up sludge in the carbs. Castrol and Pennzoil I believe are the 2 main manufacturers. Jeff. |
Jeff Schlemmer |
Regarding SAE 20 - so doesn't that bring us back to Castrol 20w-50? If I remember right, a multigrade oil has the viscosity (flow characteristics) of the lower number but can withstand the bearing pressure of the higher number. Since (resistance to) oil flow is what we're after in our dashpots, then in that application, SAE 20w-50 would act as SAE 20. BTW, agreed that it's a pain filling dashpots from a quart bottle of oil. I use a pump oil can - $3.95 from the hardware store. |
Matt Kulka |
Matt makes a good point about the viscosity characteristics of 20w-50 0il, bringing things back to OE SU specs. As for sludge build up from Parrafin based oils like Castrol or Pennzoil, I have yet to see a significant sludge accumulation on mine, on two cars, after almost 4yrs (since new) with no major cleaning other than a simple wipe down with a dry rag when they have been apart for some reason (like cleaning the K&N air filters). I don't doubt Jeff's comment, but all I can figure is that the sludge build up that should be coming is either counteracted by the additive package in the Castrol GTX I use, or the conditions have not been right to foster this sort of build up, making it a moot point, at least in my case. So, empirically, perhaps that is not as big a factor as is the viscosity?? BTW, for anyone interested, I believe the Z-Car SU sites Art mentions are at: http://www.jetlink.net/~okayfine/sutech.html and http://www.ztherapy.com/ , the latter offering a good video via Moss. FWIW |
Bob Muenchausen |
The factory recommendation is to use engine oil, the viscosity of which is dependant on local climatic conditions. It just so happens that 20W/50 is for the middle, temperate range, thinner and thicker grades should be used at the extremes. |
Paul Hunt |
Phil, The piston spring controls the height of the piston for a given inlet vacuum and hence the working position of the needle and hence the amount of fuel getting through. The oil in the damper controls the speed at which the piston can rise or fall. If you put oil in which is too thin then the piston rises and falls very quickly when you open the throttle. Conversely with oil too thick, the piston rises and falls too slowly. In the 'too thin' case, throttle response may be improved. However, the carb may not be able to provide the extra fuel quick enough in which case there is a hesitation as the mixture goes lean momentarily. In the 'too thick' case, then throttle response may be dulled and - if the oil is really thick - I suppose that the engine speed will not drop as quickly (a similar effect to that you get with carbs with those little valves in the throttle discs). I agree with Paul - use the recommendation. Unless you have had the carbs tuned on a rolling road in which case you should use the same oil as you had in the dampers then. Try talking to Burlen Fuel Systems www.burlen.co.uk - they are the people that make SU's nowadays and have always been very helpful to me. I have also found a link: http://www.gordon-glasgow.org/RSR_SU.asp. Although it is about Nissans, it has a good explanation of how the carbs work. Neil |
Neil |
Just to help things along here, when I rebuilt my HIF4s, I ordered new springs since mine were not the same length (one stretched, was replaced or shrank I suppose). Gordon at theBhive in the Carolinas asked for the color of the spring. These leads me to one point..with various springs (different strengths or lenths) available, won't the weight of oil depend on the spring you use? A stronger spring and lighter oil may or may not be the same as a weaker or older spring and heavier oil. Maybe the right combination of spring and oil would give optimum initial throttle response and mixture. Maybe. |
kids1 |
I prefer to use orange-scented dishwashing liquid. It has just the right viscocity for my climate and the aroma is delightful when opening the bonnet (the trunk, for you yanks). |
Margret Thatcher |
Well there you go again Maggy. It's hood, not bonnet to us yanks. |
Ron Reagan |
SU damper oil is the thing, readily available, cheap and one bottle lasts for ages. All the manuals share one thing in common, they're sponsored by an oil company to flog their range of products. None of them make specific, branded carb damper oil, hence........... Having said that, my dad used to use 3-in-1 cycle oil. It worked OK. I've used 20/50, 15/40 and 10/40 in the past, they work OK. Apparently, Mrs Thatcher has used washing up liquid, I assume it worked OK? I now await somebody holding forth on the merits of ATF vs. raw crude for this purpose............... |
Tim Cuthill |
used ATF last year, it disappeared after 400-500 miles. i guess it evaporated. dino fluid seems to stick around rn |
RN Lipow |
Neil, Good write up but I think the damper only works when the piston is moving up. The damper valve opens to the oil flow when the piston moves down and even if the piston was slow coming down the air flow is controlled by the throttle plates not the pistons. |
Leland Bradley |
Ahh another of the great SU mysteries, what damper oil to use. Well to start with, anything is better than nothing, so keep them topped up with your selected grade. I've tried allsorts : 3-in-1, hydraulic jack oil, motorcycle fork oil, synthetic engine oil, 20/50 Castrol, they all work. My V8 seems to like the fork oil at SAE10, in my 1380 Mini I use a mix of fork oil and synthetic engine oil, so somewhere about SAE15. Yes, there are a number of springs to choose from, but your modified motor was set up on a dyno wasn't it ?? So your dyno operator will have sorted that out for you....... In general a lighter oil will allow the piston to rise faster, when required, but it won't provide so much damping. Obvious really, but don't be fooled, the SU is more complicated than you think. I thought I understood it but now I'm not so sure..... For example, from steady speed you nail the throttle : While at part throttle there is some vacuum in the inlet, butterfly is semi-open, piston is steady. Now you've opened the butterfly, what happens ?? Initially the butterfly was the restriction to airflow, now it's fully open and the piston itself is the restriction. At the same time the vacuum in the bell increases and the piston rises against the "weight" of the spring and damper. If the piston rises too fast, more needle will be exposed but the restriction to airflow will be minimal because the piston is out of the way, so velocity of air across the jet bridge will go down, and the mixture goes lean ?? Heavier oil or spring will prevent the piston rising so fast, the velocity of air across the jet bridge will be greater, pulling more fuel from the jet, keeping the mixture richer ?? Like I said, I'm not so sure anymore, I'm sure the SU is a constant vacuum device, but not sure what that actually means........ Any scientists/carb experts want to explain ?? Still think the SU is a nice simple carb ?? Phil. |
Phil Hill |
I never thought carbs were easy, but I can adjust them all day and sleep well, try that with my wife and I won't be able to sleep until things are right. One more reason to love an MG! |
kids1 |
In addition as the piston rises the fuel jet gets larger. So does the mixture go lean or rich? Depends on oil, springs, needles, rpm, cam, etc. |
Leland Bradley |
Leland, The pistons are damped to some extent on the way down, otherwise they would thunder down rather than the "soft metallic click" mentioned in the SU manual. There isn't as much damping, though - push them up and watch them fall if you want to check. You are right about the throttle discs controlling airflow. However, the throttles are never completely closed; there is hence a short period when the piston is coming down when a rich mixture is inevitable for a moment. I think that was the point of those throttle discs with the overrun valves - to make sure that extra, unwanted fuel was burnt. There are a million and one different operating scenarios. I do still think the SU is a simple device because it works so well in many different applications no matter how ham-fisted the maintainer. You can almost always get an engine running with SU's roughly the right size and work on the tuning from there. They are also quite economical. The best bit from a UK point of view is that we've already got at least two of them each. You can spend a lot of money on refurbishing them and experimenting with needles, springs and oil - but you can spend a lot more on getting yourself a Weber which will use more petrol for a what gain in performance? I like SU's! Neil |
Neil |
This thread was discussed between 26/03/2002 and 05/04/2002
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