MG-Cars.net

Welcome to our resource for MG Car Information.

Recommendations

Parts

MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG Midget and Sprite Technical - winter time dash pot oil... new ideas welcome

Because im driving the car almost daily now... im wondering what is a good harsh winter time dash pot oil weight

I use 20/50 and does great for summer but winter time at 10 degrees the oil viscouisty is like thick honey and just dont flow meaning the carb needle is barely funtioning and creats im guessing a rich condition

should i be looking at 10/30 or 10/40 so the carb runs a little more lean ... my plan is to also lean out the carbs a couple off flats for winter

ive been running the carbs a bit more rich over fhe summer to couter the harsh heat that tends to lean out the carbs when set at normal 2 full turns down (12 flats)

What is your thoughts ?

Btw... haging oil in the dash pots is huge ... its been running a little rough the last 2 weeks a d tried retopping the carbs off today... and a huge differsnce pozitively

prop
Prop and the Blackhole Midget

I think some owners manuals said to match it to the engine oil used for the temp, so lighter in the winter. Might consider ATF.

Also, though I never heard of it being used, full synthetic does not get near as thick in the cold as regular oil. I used to use it in my diesel cars in the winter to help them crank easier.

Charley
C R Huff

I used to use Redex upper cylinder lubricant in SU's, though in them days Redex was quite thick, not like the watery stuff you can buy now.

Rob
Rob aka MG Moneypit

Hi Charley,
ATF is around the viscosity scale of ‘3-IN-ONE Multi-Purpose Oil' which is SAE 17 or ‘3-in-ONE Motor Oil’ (blue label) which is equivalent to SAE 20 so you could experiment with whichever you have to hand.

John Twist recommends “90 weight gear oil” which is what I use all year round but I've no idea of the coldest temperature I've used it in as I tend to avoid using the midget in really bad weather, only because I live in a large town where there are many drivers of large vehicles that don't know how to drive them to prevent or when all the electronics are overwhelmed. This is often because of the oversized wheels and tyres fitted and drivers and vehicles not being adapted to the prevailing conditions. Worse often with the rear wheel driver of Mercs and BMW who don't fit winter tyres to compensate for the big wheels and tyres - phew, rant over.


Nigel Atkins

Per the owners handbook: "Carburetters Each damper reservoir must be topped up periodically with thin engine oil to Ref. A (page 70). Under no circumstances should heavy-bodied lubricant be used. Unscrew the damper cap, withdraw the damper, and top up the reservoir until the oil level is 1/2 in. (12mm.) below the top of the hollow piston rod. Push the damper assembly back into position and screw the cap firmly into the reservoir."

Keep in mind that the oil in the dashpot is allowing it to function like a shock absorber, thick oil will cause it to react very slowly. I have always used ATF or 'Marvel Mystery Oil' year round with excellent results. You should also periodically remove the dashpot, damper & spring, wipe them with a clean cloth dampened with alcohol or mineral spirits to clean them. I then spray a light coating of clear silicone lubricant on all the moving / sliding parts & reassemble. Be extremely careful not to bend or damaged the mixture needle. This is also a good time to ensure the needle is properly positioned & the screws are secure. There is an excellent book that was published by Speedsport Motorbooks title 'Tuning S. U. Carburetters (including needle charts)' which is out of production but can sometimes be found. I found a pdf copy on line and it is definitely worth searching for.
s1

Nigel,

have you just been 'Handbooked'?

Jeremy
Jeremy Ti

ssssshhh, I was hoping no one had noticed


. . . and I let the "owners" bit go!
Nigel Atkins

The way i fill dashpots with oil...is a bit differant...

i start the engne and warm up to temp then run the rpm up to 4000 rpm..and mark where the carb pistons hits the top at that range then turn off and over add oil into the dash pots reinstall the damper with an oil absorbant rag wrapped around the top of the dash pot and the lift the piston sevral times to the 4000 rpm carb pistion mark.

I rarely go past 4000 rpm so it would seem ive got a good balance of bounce


prop
Prop and the Blackhole Midget

Seems a bit elaborate, Prop. The oil level has no effect on how high the piston lifts. The oil is there to damp out any fluttering of the piston / choke assembly, so as long as the piston tube is filled as described then that is all that is needed. The 1/2" below the top of the piston tube is simply to allow for the length of the brass plunger when it is re-inserted. Any overfilling just drains into the carb and is very quickly burnt off in the engine.

That said, if your system works for you then that's fine, it certainly won't do any harm.

I keep the oil in one of those little liquid hand soap dispenser bottles with a plunger top. Recycled and washed out of course! Handy one handed action and a couple of controlled squirts is all it takes. ;-)
GuyW

Prop,

I think you are confused about putting oil in your carbs. I think that the piston is not always going to be at the same place at 4000 RPM. That is, I think the piston is responding to RPM AND throttle position. So, sitting in the drive way holding 4000 RPM won't be the same as out on the road at 4000 RPM with your foot on the floor.

My understanding of putting oil in the carbs is to fill the inner tube to no more full than will allow the plunger to be fully installed without overflow. Thus, the rule of thumb of about 1/2 inch below the top of the inner tube works out.

To the best of my knowledge, as long as there is enough oil to completely submerge the brass piston on the damper rod when the piston is at the bottom of the stroke, the actual level makes no difference.

The small exception to this argument is that the weight of the oil acts a bit like a stronger spring. When say weight I mean grams/pounds, not viscosity. I doubt your carbs are so finely tuned for the weight (grams/pounds) of the oil to matter to the tune.

Charley
C R Huff

Gday lads my understanding is that the dampers stop the pistons from rising too quickly under acceleration and leaning out the mixture and yes they will damp out any flutter.
Putting heavier oil in them slows the piston rise and maintains a richer mixture until you level off on the pedal. Thin oil disappears faster than thick oil I've found, thick oil will lengthen the time between top ups.
cheers
Rod
R W Bowers

That was always my thought that 20/50 weights more then 5/30

The reason i do a partial fill at 4000 rpm is because if you fill the tube to 1/2inch the excess oil shoots out the damper vent hole. When filled up ... so i know the 4000rpm is the top end i use so i know my carbs are not over or under filled.

probably messed up idea
Prop and the Blackhole Midget

I use PAS/ATF
Karl Bielby

I've seen where someone has put they use WD40, not one I'd rush to try.
Nigel Atkins

Nigel Ive seen that also ... i to would avoid that also.
Prop and the Blackhole Midget

Surely a softer spring would be a better solution than oil that light!?
Karl Bielby

The recommendation of SU in 1966 in their handbook is SAE 20 . The level of topping up depends on the construction: dust proofed or not.


Flip


Flip Brühl

This thread was discussed between 16/10/2016 and 21/10/2016

MG Midget and Sprite Technical index

This thread is from the archive. The Live MG Midget and Sprite Technical BBS is active now.