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MG MGB Technical - SU carburetor chamber and piston fluid

Folks,
Another question from the novices.
What lubricant do we use for the chamber and piston assembly on our SU carbs?
I have been told to use transmission fluid and have read use engine oil, also read use light oil.
Moss lists S.U. Carburetor dashpot oil while Victoria British lists Damper oil-carb.
Thanks,
Bruce
Attaching photo of my son Shaun in the car.

B Barbour

Bruce - I use 20 weight oil, others use ATF and some even use 20W 50. all seem to have good results with their choice. Cheers - Dave
David DuBois

I've been mostly using ATF since Volvo told us to c1967. Or engine oil 10W30 as per the book. They are essentially the same at the usual carb temps. "Under no circumstances should a heavy bodied lubricant be used." = 20W50 is usually too heavy, especially in cold weather. I have an ATF squirt can, and my basic oil can has a mixture of whatever oils are left in cans, which might include a fair amount of 20W50. The parts purveyors are simply ripping you off with their "special" oils.
Better fix that wiper before they get tangled up and make a mess!

FRM
Fletcher R Millmore

Must be 40 years ago, I read an article in Car Mechancics, where they tested different oils, ( cannot remember the tests ) but 3 in 1 oil worked best.
c cummins

Can't say as I've read the article, but I've used 3 in 1 oil for over 40 years, and in some of those years I was driving MG's as my daily driver in the winter months of Ontario. Never had a complaint or problem with it, and continue using it today.
JR Ross

Would it be a good guess that oil that works best depends upon how rich or lean your needle and jet adjustment is? That is, if you are set to the lean side you get better results with thicker oil, and if set to the rich side you get better results with thin oil.

Charley
C R Huff

Fletcher, your reply was = engine oil 10W30 as per the book. "Under no circumstances should a heavy bodied lubricant be used." = 20W50 is usually too heavy. Now in my book it says 10w/30 oil is only recommended for temperatures of -15 to -5 degrees C, and 20w/50 is recommened for all temperatures above -10 degrees C, I only ever use 20w/50 in my dashpots and have never had a problem. A.T
andy tilney

Bob-
I always use a 20W/50 oil that is relatively uneffected by temperature change, such as Mobil 1 synthetic oil. It works fine no matter how hot or how cold the weather is.
Stephen Strange

Andy -
You are almost correct for engine (my OE manual says 5C to -12C), but for carbs etc it says "all conditions" and recommends Castrolite, which was 10W30.
However, the effect of these damper oils is pretty subtle usually, and a wide range generally is OK if not optimum. And there is considerable overlap on viscosity ratings, such that the numbers are generally just a guide, especially if you are not near the ends of the temp and viscosity ranges. It also varies a lot from oil to oil (brand).
It is worth considering that since the engine requirement for richer mixture on accel varies with temp, as does the viscosity of the oil and the resultant damping effect, it might even be worth considering using a single weight SAE10 or SAE20. Both 3-1 & ATF are probably in this range.
If the thing coughs on acceleration and everything else is known good, then it is worth trying a heavier oil; if it doesn't cough but accel is lazy, try lighter. Too heavy oil can cause noticeable fuel consumption increases if you do a lot of driving with ON/OFF throttle conditions, and a general lack of "sparkle" in response. Some individual cars seem to be more sensitive than others, but ATF is good 95% of the time.

FRM
Fletcher R Millmore

I think non standard cams tend to have more violent pulses in the carb than milder cams. The heavier oil helps to damp this problem under acceleration and prevent piston flutter under hard acceleration, easily visible under rolling road conditions. The lighter oil tends to squirt out the top too. I feel, for the B series engine, 20/50 works best in UK conditions. High altitude and different ambient temps may vary this. Gentle acceleration needs no oil, the oil provides acceleration enrichment under hard use.

What did Peter Fonda say in Wild Hogs? Ride hard or stay home....or maybe it should have been drive hard or ........ :)

Peter
P Burgess

The damping effect of the oil only comes in AFTER the free play of the damper piston has been taken up - so another way to vary the amount and timing of the acceleration enrichment is to use a damper with more or less free play.
Chris at Octarine Services

Chris-
What are the Burlen part numbers for those dampers? Which ones provide another way to vary the amount and timing of the acceleration enrichment?
Stephen Strange

Same as Andy. Only ever used 20W/50 (or its replacements such as 15W/50 then 15W/40) and never had any cause to think it needed anything else. 3-in-1 is quite a bit thinner than any of those. Can't agree with the implication that only hard use requires the damper, anything other than minute increases in throttle will cause a stumble without any damping, unscrew the damper cap and try it. Different cars do have different free-play in the damper piston on its spindle, my HSs are quite a bit more than my HIFs.
PaulH Solihull

I always just use the same Castrol GTX I use in the engine. Have some in a squirt can (also used to wipe down my lathe and tools) and I top them up from that.
Simon Jansen

I've used 20w50 for over 40 years. I'm using 90w now because I have a modified SU HIF 44 that was supplied with the supercharger that Moss sells. RAY
rjm RAY

Hi Paul

I posted a response yesterday but it doesnt seem to show up, anyone else noticing this with the threads lately ?

Did you say sometime back you tend to set your cruising mixtures on the weaker side ? If so this might explain why you feel even slight throttle openings cause the stumble. The effect of stumble only seems to appear below around 3000 revs and under fairly s[irited driving....ie nipping out in traffic at a turn.

Peter
P Burgess

Gentleman. I have used Castrol 20W-50 oil in four MGBs, here in Arizona, for the last 10+ years with no problems. Previously, I used 20 weight oil in my two MGAs in the northern portion of the US. If Florida, I used 30 weight oil in my A-H Sprite. None of these vehicles demonstrated any problems related to the oil in the carb dampers. Might I suggest that this discussion is of rather little importance? It seems that the people who run ATF, I am informed that this as an SAE 10 weight oil, do not seem to have any better results that those who run 20 weight, 30 weight, or 20W-50 weight. At least in the area that I live in. I have operated these cars from sea level to in excess of 6,000 feet elevation. The oil used has never been a significant problem. Might be a problem if one is using some excessively heavy, or light, oil when the needles are marginal for the operating conditions. But, within my experience, the oil used has not mattered a great deal if the needles are, mostly, suited for the area of operations.

Les
Les Bengtson

Peter - nope, but I think someone did. I set mine for the brief rise in revs then fall back as the piston is raised, which AFAIK is the correct point for these engines. Later engines are set slightly weaker just short of the point at which they fall instead of rise and fall. But these are very small differences.
PaulH Solihull

Everyone,
Thank you for your input.
Sorry I / we haven't responded sooner but had to get my eldest off to collage. Also had to replace the computer in this time frame.
Bruce and Shaun
B Barbour

BMC always specified 20w/50 engne oil, I seem to remember SU specifying 20w. I use engine oil mixed with a bit of Red X (upper cylinder lubricant).
Allan Reeling

To throw a wrench into the works...John Twist recommends 90wt oil!!!!
Bob Dougherty

Everyone,
Thanks for all the input on the subject.
We elected to follow the manual and used 10W30.
Bruce
B Barbour

90wt gear oil is very similar viscosity to 30 weight engine oil, they are just measured on different scales. Why the carb damper needs the shear resistance of gear oil is another question ...
PaulH Solihull

The SU HIF44 carb, that Moss includes with their supercharger kit, requires 90W oil. It slows down the time it takes the damper piston to rise when under suction. RAY
rjm RAY

This thread was discussed between 11/08/2010 and 10/09/2010

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