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MG MGA - Dash pot oil

I know we have talked about the grade of SU dashpot oil many times. This morning, doing my routine weekly checkover, I inadvertantly put heavier grade oil in the pots than normal (I must mark my oil cans clearer!). I cleaned it all out and put my normal thin oil back in (we call it 3-in-1 over here), although I am aware that some people do go as high as grade 30.

I just wondered what the consequence is of too heavy a grade oil (greater than 30) in the pots. Does it just dampen the acceleration/retardation or does it have an effect on the fuel/air ratio?

Steve
Steve Gyles

The thicker the oil, the slower the rise of the piston. The slower the piston rise, the richer the mixture until the piston reaches equilibrium. This is how SUs and Zenith get by without an accelerator pump. Most carbs have little pump that squirts an extra bit of fuel in when you snap the throttle open, else the mixture would be momentarily too lean and the engine would stumble. SUs do this without an extra pump by making the pistons lag the throttle a little and thus cause the mixture to go rich for a bit.

HTH!
Rob Edwards

Steve,
Rob is correct.
I have a rich/lean gage installed and was able to dial in not only the correct needles, but also the dashpot viscosity.
With twenty-weight, I was indicating a lean condition on acceleration. This was resolved by switching to thirtyweight. Added benefit...When things warm up, the dashpot oil thins out a bit thereby leaning the mixture somewhat...cool
Doug
D Sjostrom

Rob and Doug

Thanks. Sounds logical to me.

Steve
Steve Gyles

Doug - what exactly is a rich/lean gage? Did you place a temp sensor in your manafold?
Skip

it is the Yank way of spelling Gauge.
Cecil Kimber

Skip, here is the unit I have in my car. It meters the fuel mixture through a bung welded in the exhaust pipe. It is very responsive and allows for precise tuning - at least close enough to minimize rolling-road time.

http://store.summitracing.com/default.asp?Ntt=edelbrock+fuel+gauge&searchinresults=false&Ntk=KeywordSearch&DDS=1&N=115&target=egnsearch.asp

Jeff Schlemmer

Sorry that link didn't fully work.
Type in "EDL-6593" as a part number.

Jeff Schlemmer

Jeff,
That's the one I used.
Appologies to Cecil, gauge is the correct spelling, although I regularly see either used.
Doug
D Sjostrom

Jeff

I have had a look at the link. Fascinating. Do you just attach the meter to your welded bung when you do a tune or is it a permanent display on your dashboard? What is the optimum fuel/air ratio for the MGA?

Steve
Steve Gyles

You can build the same thing yourself for about $30 plus a single wire Lamda sensor ($20 on ebay). Here's how:

http://www.niksula.cs.hut.fi/%7Emdobruck/siililand/mini/diy/alien/lambda/mixture.html

I am just starting, will build it into an old midget fuel gauge I have.

This stuff starts to get important with a supercharger you see....

When the anti-knock computer arrives, it will be a fully electronic engine management system in the A, with the old distributor carried in the boot "just in case",

dominic clancy

As I started this thread I hope you don't mind if I just go back to the thick oil issue for a moment. I recall now that when I last filled the pots (month or so ago) I put in 30 SAE as I had run out of my thin stuff - poor excuse and bad engineering practice.

During the recent cold weather I have noticed a strong lingering smell of oily petrol fumes in the garage, permeating through the interconnecting door to the kitchen. I have spent some time looking for leaks without luck.

Last weekend I put the real thin stuff back in as mentioned above. Still about zero centigrade outside, but hey presto, the car is back dancing round the lanes, very spritely on the throttle and guess what? No fumes! My logic suggests a connection somewhere.

Back in love with the car. All hate disappeared. Sorry Glenn, won't be putting any more in your thread. Still swear at the distributer though!

Steve
Steve Gyles

This thread was discussed between 11/12/2005 and 13/12/2005

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This thread is from the archive. The Live MG MGA BBS is active now.