MG-Cars.info

Welcome to our Site for MG, Triumph and Austin-Healey 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 TD TF 1500 - SU H4's and dashpot oil

I'm getting further along on the engine and have come to a point that I need some sage advice. I have both of the SU tuning manuals but can't find the info regarding the dashpot oil.. how much and exactly where does it go. I bought these carbs off of eBay as fresh rebuilds. I changed the needles and jets to stock MK II'e as a staring point. Using the Moss jet bearing centering tool I then installed the jets the pistons. The pistons fall with a definite thud, I then turned the jet sdjustment nut down 12 flats per the initial setting proceedures. But nowhere does it say anything about the amount of oil and where.

Regards

Bill Chasser jr
TD4834
W. A. Chasser Jr

I use 90 at gear oil in mine. It goes in the reservoir under the brass octagon nut on the top of the carb bell. fill it up till it reaches the small shoulder you see in there. It's not much though so be carefully to add just a little.

Alex
Alex Waugh

I use SAE20 motorcycle fork oil. I don't think you want anything more viscous than that.
Dave H
Dave Hill

I bought SUs own dash pot oil in a small plastic bottle with a drip type spout. I thought since they manufactured the carbs, they surly should know what oil to put in them! Bought it from Moss 10 years ago for the MGB and have plenty for the TF, looks like it's about a 20 weight oil. PJ
Paul S Jennings

Bill

This subject has been the subject of much discussion, rightly or wrongly use Automatic Transmission Fluid, works fine here in Aus.

Graeme
G Evans

Bill -

The attached image shows the correct level of damper oil in an SU H carburetor. It is from the SU Workshop Manual, c 1995 Burlen Fuel Systems Ltd.

Text in image:
"Finally top up the piston damper with thin engine oil of grade S.A.E. 20 until the level is just below the top of the hollow piston rod."

I use 3-in-One SAE 20 "Motor Oil" in the blue and white container.
http://www.3inone.com/products/motor-oil/

> Thicker oil = richer mixture on acceleration (piston rises more slowly).
> Thinner oil = leaner mixture on acceleration (piston rises faster)

Lonnie
TF7211

image = Damper Oil Diagram

LM Cook

from the posts over the years...and the few here.. It does not seem all that important as to what you use. Regards, tom
tm peterson

Lonnie beat me to the post but I'll show this photo I captured.

It should be about 1/2" below the top of the hollow piston rod, NOT the threads in the top of the suction chamber.

To give credit where credit is due, I found the picture via Google since I couldn't find mine on my computer.

http://britishclassicmotors.com/3301.html

I used 20 WT oil bought by the quart at a metric motorcycle / snowmobile shop for $8.

Frank Cronin

The easiest way to add oil in a controlled fashion is to push the piston all the way up until the hollow piston rod is close to the top of the dashpot. You can then add oil without putting in too much.

I use Marvel Mystery Oil in the racecar, but ATF in the street cars. A lot of racers use no oil, but I like using the very thin MMO.
David Littlefield

Thanks everyone for your answers, pic and link. I was doing it right. This is my first taste of SU's. I used the SU oil but when I pushed up on the pistons it was very slow to respond. My thinking was that this would cause the carbs to be very slow to accelerate if the pistons are vacuum sensitive.

After talking to my dad this morning and demonstrating what I was seeing with my spares he told me that the pistons are supposed to be slow to respond to increase the vacuum in the venturi. When the butterfly is opened, more fuel is pulled from the bowl to enrichen the mixture and thus improve the engine response under load, which made sense after I put more thought into it. I was thinking that they had to open simultaineously for more air movement but then that would decrease the fuel draw and create a momentary lean condition and cause coughing in the carbs. My dad has a wealth of information on these cars that sadly I will never be able to glean from him given the time he has left. I am thankful for the moments of lucidity and calm conversation that we are able to have.

Cheers

Bill Chasser jr
TD4834
W. A. Chasser Jr

Cherish your dads presence Bill! When my dad passed it was a tremendous loss to us. Him and I were very close. I wish I had half the knowledge he took with him. I miss him a lot. PJ
Paul S Jennings

Thanks Paul

Regards

Bill Chasser jr
TD4834
W. A. Chasser Jr

These SU carburetors are actually pretty cool when you see how they work. Simple but elegant and very functional.
Christopher Couper

I grew up around Holley and AFB four barrels so it's been a little hard wrapping my head around these. I've read the books over and over but I'm a visual learner and until I can manipulate something I don't understand it.
W. A. Chasser Jr

SU carbs are dead simple especially the ones on the T- Series cars. Essentially 1 moving part when operating, and self compensating with RPM. Once set, you should not have to adjust them until the throttle shaft wears.
...CR
C.R. Tyrell

This thread was discussed on 27/04/2015

MG TD TF 1500 index

This thread is from the archives. Join the live MG TD TF 1500 BBS now