MGB & GT Forum

What type of oil/fluid goes in the carbs of an early '74 MGB? When should it be checked, hot /cold? How high should it be? Thanks in advance to anyone who replies!! Mike

20/50 engine oil. to the line about 1/2" below the top. hot/cold - does not matter
G

You are looking down the wrong hole, but getting to know your carbs is a good thing

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72 engine, hif carbs, black od, rest of car stock

All the maintenance intervals,clearences,gaps, and fluids are here
http://www.geomatique-liege.be/MGJP/DocumentsPDF/MGB_Workshop_Manual.pdf
read it over,learn the hows and whys
work you way through the process.

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LNDRVR4X4.COM
Home of Project "INCARN8'


1957 Series 1 Land Rover electric VEHICLE CONVERSION

FIXITUPCHAP.COM
FIXITUPCHAP INCORPORATED

RD3 Radar/ Electronic Warfare Technician
VIETNAM 1969-1972

FWIW...
My car came with a Weber carb... and it just didn't seem right. So I installed a set of HS-4s and dialed in the Fast Road cam. I spent the next several months chasing an ever present "pinking." I changed everything I could (one step at a time) without success.

When I installed the SUs I used the same engine oil as in the sump, just as the factory said. I fiddled with the needles, changed the timing, dabbled with rich/lean until I couldn't see straight. Still the annoying pre-ignition persisted. On further reading I tried different fluids in the carb dampers... mineral oil, brake fluid... you name it I tried it. I was desperate. John Twist suggests using 90# gear oil. I tried it and the pinking has stopped and I have used it ever since. Just my $.02

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1978 B Tourer
Black label OD
de-smogged & lowered
Kent 715 Fast Road cam
Petronics & HS-4



Edited 1 time(s). Last edit at 2016-04-29 06:07 PM by kencampbell.

90W is what Moss recommends using in the SU HIF44 carburetor that comes with their supercharger kit. I been using it for 14 years with no problems whatsoever. RAY

There is also some confusion over the amount of oil, some say 1/2" below top of tube, other say 1/2" above.

Over filling is not a problem, as excess oil is sucked through the engine and burnt. It also lubricates the hollow rod.

Full publication here

http://mgaguru.com/mgtech/books/pdf/Tuning_SU_Carburetors.pdf

Herb

I use 90W in my HS4's.

I'm not sure that the viscosity of the oil really makes much difference, to be honest. As long as there is oil in there - the damper will damp.

I did change the damper oil seasonally when I daily drove an MGB-GT (and a Mini) in the Canadian winter. 3-in-1 oil or ATF in winter, 20-50 or 90W in summer.I don't drive the MG in the frigid winter season any more, so no need to change now.

Tim. viscosity DOES make a difference. It affects how the piston rises and hence the car's response to throttle changes. Thin oil promotes hesitation to throttle opening, thick oil the reverse. Fuel economy is higher with thin oil but not sure by how much.

Herb. The line i referred to is the line above the one which is highlighted in your image. just visible below the thread. The image you have is different to the one in both my workshop manuals which refer to the top line.

Your image is of course correct as any oil over your line just flows into the carb. I assume that BL decided to use the top line because it's the only reference point that is clearly visible to the average motorist and as you say, excess just lubes.

G

Graham I believe that you have it a bit backwards or I misunderstand you. The oil slows the rising of the piston upon acceleration. So a thicker oil slows it more. If the piston is rising more slowly then the air is restricted and there's a stronger vacuum in the venturi, that sucks in more fuel out of the jet. No oil (or very thin oil) results in the piston slamming open giving too much air and a lean condition under acceleration. At least that's the way I understand it. I'm not an expert by any means.
I'm convinced that you don't really need to look at the oil level. Take the plunger out and push it back in. If there is resistance than you have enough. I fill mine at oil changes with 20w 50 though it's never low enough to cause a problem. Experiment with different oils if you want. The differences are subtle.
I'm rather skeptical that a different weight of oil would be enough of a change to cause or remedy pinging.

Too much oil will just flow/spurt out the hole in the damper top as you tighten it down. I'm not commenting on the viscosity as I can't recall what I'm using - might even be a two stroke mixer. I do recall 3 in 1 machine oil was too light.

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Mk1: CKD 11/66 first registered 8/5/67; owned since 3/77. 18GB +40 balanced. Peter Burgess BVFR head. Piper 285. 123. FidanzaFW. 4synch c/r box. Lots more as I did a nut and bolt rebuild; finished 2015. Tartan Red.

GT: December '78. VW Golf guards, flush fit front and rear valances. Torana XU1 vents, frenched indicators & Mk1 rear lights. 'Worked' Rover V8 with Monsoon ECU for EFI. GM4L60E, Lokar tiptronic & Quick4 controller. Vintage Air A/C. FC IFS. CCE 4 link rear. Salisbury with Quaife. Jaguar Storm.

I understand that viscosity of the oil in the dampers does make a difference. I've tried many different types of oil over the years, and except for the very cold weather running I noted above, I have always found it hard to feel or detect any difference in the way the car operates. I guess my comment should have been clearer; it's just an observation, really. Many people seem to get obsessed with small details like this and I'm not sure the anguish over their S.U.-carb-damper-piston-oil is warranted. Just my opinion.

Here's another little tidbit for those new to the wonderful world of S.U. There is really no point in continually topping up the oil in the dashpots. Once the internal tube is full of oil, it can't really go anywhere. Continuously refilling it just means you're over-filling and allowing extra oil to spill over and eventually find its way to the combustion chamber. Just check the level every once in a while (how's that for a strict maintenance schedule?); if there's oil in there, and there probably will be, just leave it alone.

edit: I see that others have already made this last tidbit of info clear - missed those comments (reading is hard!). Feel free to disregard - (this actually applies to any of my posts).



Edited 2 time(s). Last edit at 2016-04-30 05:44 PM by 33EJB.

Barrie

I'm with you on this. On those occasions where I might have put in too much oil in the SUs, the excess oil has squirted out of the hole in the dampers and dripped down the carb body and can then be seen on the black heat shield. Years ago, I used whatever oil I put into the engine but for something like 30+ years, I've used "SU Damper Oil" as recommended by SU / Burlen.

As further info, SU / Burlen offer two oils (Damper Oil or Dashpot Oil) depending on the type of carbs installed. Here's a reference to the two oils:

http://sucarb.co.uk/technical-carburetter-maintenance-settings-chamber-oil

Bob

The oil does get consumed, the action of attempted compressed by the upward movement of the piston against the damper produces atomisation which passes by the damper and is then subject to the vacuum of the engine and subsequent burning, there is no other mechanism for oil to be lost from what is a blind reservoir.

I also agree with those who maintain that so long as resistance is felt when inserting the dampers the actual level of the oil is largely unimportant, BL did nobody any favours when they blithely wrote 'top up the dashpots' !

Like David I find it difficult to understand how the viscosity of the oil could or would affect pinking and having experimented with different oils myself ranging from 3 in 1 to EP90 I too found it hard to detect any meaningful difference between them.

re post #9. I meant to write
"Thin oil promotes hesitation to throttle response
sorry for confusion.
G