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MG MGA - Carb oil reservoir question

The carbs for the 1600 engine have been rebuilt with new parts (from a Moss rebuild kit). With SU tool kit "flags" in the oil reservoir, the pistons rise and fall in unison equally when taken from idle, "blipped", etc. So no apparent problems.

But I have found that it takes 1/3 to 50% more force to raise the front piston (finger reaching thru the mouth of the carb) than it does the rear piston.

I probably shouldn't be concerned since in operation they are rising and falling equally. But I would like to understand how this can be. The oil damper pistons are exactly alike (diameter and length) and in good shape. There is only about .005 difference between the front and rear inside diameters of the piston oil reservoirs, the front being .005 smaller at .365 (rear is .370). I'm using 20w50 sythetic oil.

Is there an explanation for the difference in force required to raise the pistons manually? Would .005 make that much difference? And if so should I hone the front one out to equal the rear ID at .370?

I should I just forget this and move on.

JM Morris

As your dampers rise and fall at the same rate you probably do not have a problem, but there is one check that you should do. This is unlikely to be your problem, but it is worth a quick check.

Many SU carbs have had dampers replaced with the wrong type. Referring to the image attached it should be noted that the area above the hollow piston rod must be vented. otherwise pressure will build up on the upward movement of the piston and vacuum will occur with downward movement. This will restrict normal piston travel. Venting may be done in two ways. The cap may be drilled to allow venting to atmosphere or the web on the chamber neck may be drilled to allow internal venting back into the suction chamber. You must have one or the other, but not both. If you have a solid cap and no internal drilling there is no vent and pressure/vacuum conditions will occur as mentioned. If the cap is drilled and the web is also drilled. then there is a direct air leak into the suction chamber. If the chamber neck has no web then it cannot be drilled internally and must have a vented cap. If the chamber neck has a web it may or may not be drilled, depending on the type of cap.


M F Anderson

Thanks, Mick. The caps on mine are not drilled. I recall running carb cleaner thru one of the web vents but I can't recall if it was the front or rear. Obviously, I wasn't giving enough thought to this part of the carbs.

That's well worth checking further for any gunk or obstructions.

Thanks again!
JM Morris

Another check worth doing is to test the drop of the piston.
Remove the cap and the attached plunger, take off the air cleaners,lift the piston and let it drop.
It should land with an audible "clunk".
You might be able to lift the piston with the lifting pin and maybe get the drop sound (saves taking the air cleaners off).
If it does not fall freely the suction chamber (bellhousing) has probably been refitted to different locating screw holes. When removing the suction chamber you should always mark the suction chamber and the body to ensure replacement with the matching screw holes.
The front and rear suction chambers could also be put back on the wrong body.

M F Anderson


The air cleaners are still off. I've done the drop test (using the aluminum tubes from the SU tool kit) and got the "clunk", although the rear carb had to have the jet bearing re-centered using quite a bit of force before I could get a clunk there (done prior to all of this).

The suction chamber(s) can only be fitted one way due to the staggered arrangement of the screw holes. Also, I have discovered a tiny "one" (1) stamped in the top of the front suction chamber, and a tiny "two" (2) at the same location on the rear suction chamber. I assume from the factory? So I think I'm OK there, unless the pistons have been swapped. (I wonder if they might also be stamped somewhere--I'll take a looksee).

Additional thoughts welcome!
JM Morris

Re-center the jet again, sometimes this has to be done several times to "get it right." Once "right" it shouldn't need adjustment for a looong time.

Update: Both carb web vents have been checked and they are clear. No # stamps were found on the pistons.

Above, I mentioned that there was a .005 difference between the front and back ID of the oil reservoirs. However, I have discovered there is another "step down" diameter in the oil reservoir (about 1 1/2 to 2 inches down. But I don't have a measuring tool that will reach that deep so I'm gonna have to let it go. Mystery unsolved.
JM Morris


When you put the rear filter box back on, make sure it is the right way round. It is quite easy to fit upside down and this will blank off the ventilation hole in the carburettor to filter flange face that goes to the top side of the piston - common error. Second item down on my Odds and Sods page:

Steve Gyles

Anyone know how often I should have to top up my SU damper oil? I seem to have to top mine up every 400 -500 miles. Cant figure out where it goes.
The car runs just fine.

Colyn Firth

Mine never seem to need topping up. I have 19,000 miles since the rebuild and the only time I have added oil is when I removed the carbs and lost the oil as I tipped them over. Where can the oil go? I only fill with enough so that you can feel the plunger enter the oil about 1/2 inch before the threaded caps touch the top of the threads.
Ed Bell

Hi, Steve, and thanks. No, the carb vent holes are not covered. I've had the air filters on and off twice and have watched that carefully.

I'm guessing that the diameter of the concentric, "step down" ID in the oil reservoir is probably critical, and that there is a variation between the front and back carb, the front being smaller, requiring more force to lift the piston past the oil plunger.

The oil plunger ODs are both .336. As noted above, the ID of the upper section of the oil reservoir is .365 on the front carb and .370 on the rear carb. The concentric step down "landing" looks to be about .010 wide, times 2 sides = about .020. Subtract from .365 = .345, less the OD of the oil plunger (.336) = only .009 for an oil bypass area. (The rear bypass area would be .014, if the .005 variation holds).

So if the upper variation of .005 is duplicated in the lower section, that much variation would be huge in regard to the area that the oil has to bypass the plunger during piston lift. But I don't have the measuring instruments to prove it.

I was hoping that someone else had already been down this road and could confirm or deny whether or not this is even important.

After hours of tuning, balancing, with all new internal parts, using Colortune, reading plugs after
hard runs, etc., I note that the front 2 cylinders' plugs indicate a slightly richer condition. And I'm thinking it might be due to the slower movement of the front piston.

However, I would certainly agree that there are probably too many variables to tie it down to this. But I WOULD like to be able to rule it out, if it's insignificant.

Maybe it's time to just ride! :)

JM Morris

Thanks for the reminder about the rear air filter assembly. I installed K and N a few years back. Always thought I had it right.. But after reading your post:

" When you put the rear filter box back on, make sure it is the right way round. It is quite easy to fit upside down and this will blank off the ventilation hole in the carburettor to filter flange face that goes to the top side of the piston - common error. "

And the fact the rear carb ran rich. I check it out yesterday and found I also did it wrong. If the weather holds out, I will give it test run today. Should have a little more get up and go.

thanks again.


Ray Ammeter


I have done it and, speaking with Bob West, any number of his customers have done the same. Always worth the occasional reminder on this gotcha.

Steve Gyles

This thread was discussed between 18/05/2010 and 24/05/2010

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