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MG TD TF 1500 - Brass pistons in Carbs

With all the work done on my TD recently I decided to pull the carbs apart and have a good look at them and ensure that they were balanced and working well. Taking the bell off I noticed that the pistons are brass (guess I never really gave it any thought previously) but they both have springs to push the piston back down.

The pistons drop with a clunk without the springs but I was wondering what I could expect if the springs were removed. I believe I read that the brass pistons were originally not fitted with springs so will fitting them change the way the car runs?

Brian Smith

Yes, it will run richer throughout the range. The idle mixture can be changed by adjustment, but the needle profile is then miscalibrated.
Piston load, by weight or spring, is a tuning variable. The ideal is that the piston is fully up at max power. If not, power is restricted somewhat, and sensitivity to tuning adjustments is decreased because only the part of the needle that moves past the jet top is in play. It is common that when the hp is increased the springs (or springs plus weight) are stiffened = loaded more heavily. If your engine is near stock, it will likely work better with the springs removed, after suitable adjustment. If it has been modified to where the piston tops out before max power, then it needs the springs. It is very undesirable for the piston to top out early, as it screws up the mixture right where it is most critical.

FR Millmore

I was always under the impression that the brass pistons did not come with springs. I have two sets (0ne for parts) and neither have springs. I thought that the aluminum pistons had to have the springs because of the light weight?

I assume a previous owner made the modification. The pistons are brass but both have springs installed????

Brian Smith

Bruce is correct as to how they came originally, but the variable is total load on the piston vs lift produced by airflow. It doesn't matter if it is springs or weight. Brian's carbs have had springs added, either wisely as a tuning aid, or stupidly because "SU pistons have springs".
FR Millmore

Bruce and FRM are right on- Shouldn't have springs with the brass pistons if you have the correct needle and jet size with farily stock motor. Springs would make it run too rich. George
George Butz

The engine is pretty much stock. Has been bored .030. It runs well but is running very rich no matter what I do. I have new needles and seats and all new gaskets and seals but still tends to run rich. I will remove the springs and give it a try that way. Thanks for the helpful comments.

Brian Smith

I had the same problem you are having in that I could not adjust the carbs enough in order to get a leaner mixture. I am at an elevation of 4500 ft. and my problem was solved by going to a different needle. The "lean" needles do not seem to be available through Moss but I got a set of "NV" needles and they have made a big difference in the tuning of the carbs. If you are at sea level, you might want to confirm that your jets are .090. If the jets are worn, this could be the cause of your rich mixture. I hope this gives you some ideas, and is some help in resolving your problem.

George Raham
TD 4224
G. L. Raham


I had the exact same problem for years. Carbs that ran rich no matter what I did, with the correct needles.

I have found that the floats do not all float evenly. i.e. even though they are all dry inside, and don't leak, some ride higher than others. This makes setting them by the rod under the float fork method impossible as the lower the float rides in the fuel, the higher the fuel level presented in the bridge of the carburetor.

Instead (and since you have your piston chambers off) look down the carbs with the floats adjusted and then raise or lower the jets to the level of the fuel in the bridge. With a dial caliper measure the depth of the jet from the bridge. This reading should be about .16 and a range of .12 to .20 is OK but I like to get them both the same. The higher the fuel level in the bridge the less suction it takes to get the fuel up into the airstream and into the engine. If it is too high, it is difficult to run lean enough. If your fuel in the bridge is high, bend the arm so the float will float lower. You can suction out the fuel between tries and allow the fuel pump to fill them up again.

Then, start with the jets about .070 below the bridge. If your needles are mounted in your pistons equally, moving the jet adjusting nuts an equal amount each time will result in keeping the jets at the same height as you adjust them. By measuring their depth below the bridge, you will be starting at the same point, and not dependent on irregularities in the jets or the carb bodies. Find the spot where the idle is strongest (you may have to adjust the idle down several times) and then do your lift tests of 1/32 piston lift from there. Clear the throttle frequently between moving the jet adjusting nuts.

Dave Braun

I drove the neighbours nuts yesterday setting and balancing the carbs and then blipping them to ensure that the new setting was good. They are now balanced for airflow and the float levels are equal and fall within the guidelines Dave has noted. The needles and seats are new "standard" needles, the lean ones were not available when I purchased them.

The springs are now removed and I will hopefully get an opportunity to test everything later today. We are also trying to get a reno done prior to #3 sons wedding using wedding cars sevenoaks in a couple of weeks as we will have a house full.

Brian Smith

When Jim Taylor rebuit my Carbs a few years ago, he noted that someone had put springs in my brass piston SU carbs. They are not supposed to be there in the early brass style and since he took them out mine have given me no trouble and run just fine.
TRM Maine

This thread was discussed between 25/06/2009 and 26/06/2009

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