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MG MGA - Keep Your Carbs In Tune

Many of you know that I monitor fuel consumption closely on my spreadsheet. My last 3 refuels have indicated 23.62mpg, 21.71 and 21.72. My normal travel to get these figues is an 8.5 mile journey to work and back - both cold starts.

Two weeks ago my car started misfiring at idle, was poor on initial acceleration, but settled down when at highish RPM cruise (above 2500RPM).

Investigation showed up a sticky piston in the front carb that I quickly sorted. Latest fuel top-up (same driving routine) showed an instant rise to 27mpg.

Just goes to show how important it is to keep the carbs tuned. The dianosis of my problem was helped by the SU Carb tool: http://www.mgaroadster.co.uk/su_carburettor_tool.htm . It is such a quick indicator of sticking and out of balance pistons that it is now part of my weekly checkover, especially with fuel prices as they are.

Steve
Steve Gyles

Steve, Were you able to determine what made the piston stick? Was it dirt in the chamber, a needle not centered in the jet or what?
George Raham
TD 4224
G. L. Raham

Steve,

As an addition to George's question, do your damper plunger caps have the hole drilled in them, or are they the dust-proof type with internal drillings only?

Mick
M F Anderson

George

Looked like a general build up of oil/grime over a period of time. I just had to remove the pot and give it and the piston a good wipe over with a cloth. I suspect that the front piston had been slowing down gradually over a number of weeks and was not fully closing, resulting in richness of fuel at low rpms. As it got worse it caused the eventual misfire. All in all this created the greater than normal fuel consumption. That's my theory anyway.

Mick

Yes, my pots require the damper caps to be drilled. Would the rocker box connection have anything to do with just the front piston being affected? The rear pot was fine.

Cheers

Steve
Steve Gyles

Just an addition thought. Could overfilling the dampers cause overspill, with oil (and dirt through the cap hole) migrating to the piston and pot wall? If so, I will plead guilty on that account.

Steve
Steve Gyles

Steve,

Yes, I believe that that the breather pipe from the valve cover could cause the problem. While the car is travelling at any speed the draught tube at the side of the engine draws fresh air from inside the filter element and into the valve cover. However, when stopped, with the engine running, hot oily fumes rise up in the engine and pass into the front filter. These then pass into the SU piston chamber as shown by the large arrow in the attached diagram.

I don't believe that overfilling the damper tubes would cause the problem. To what level do you fill your dampers?


Mick


M F Anderson

Mick

To the top of the piston tube and probably more. Light was bad and I could see fully what I was doing! I believe it should only be filled to about 1/2" from the top?

Steve
Steve Gyles

Steve, Thanks for your reply and the solution to the problem. Sometimes the answer will help others in similiar conditions. Regarding the oil level in the tube, I have never filled it up but usually put in about 10 drops from a squeeze dispenser. I find that is all I need to get the desired effect in the chamber. I would like to hear from others who have a different idea concerning this subject of oil level in the piston tube. Do you use light or heavy oil? I have used an automatic transmision fluid for a number of years. What have you got in the pot?
George Raham
TD 4224
G. L. Raham

Graham

Burlen, the SU owners in the UK, say 20 grade oil: http://www.sucarb.co.uk/ProductDetail.aspx?id=26524

Steve
Steve Gyles

Sorry, meant George. Eyes fuzzed on a mixture of first name and surname!

Steve
Steve Gyles

George, I have not topped off my carb dampers in over 7 years. I check them a couple of times a season. As long as the damper meets some resistance before it is seated and tightened, it will do its job. Filling it with more oil will not make it work any better.

Chuck
C Schaefer

Chuck, Your comments are exactly are what I have experienced over the years. While it will not do any harm to fill the tube up near the top, there is no advantage to do so. I found the automatic transmission fluid did the job, and as long as I didn't spill it, I never had to add any over the years. If I remember, I changed from the heavier oil to the lighter oil as I felt the heavy oil created too much resistance.
George Raham
TD 4224
G. L. Raham

This thread was discussed between 30/08/2008 and 01/09/2008

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