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T-Series TD SU speed

alexm

Freshman Member
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My pal Bob has a 1953 MG-TD, and we were syncing the carbs today and setting the mixture. But, we noticed that one of the SUs was "faster" than the other. That is, its dashpot reacted quickly when we blipped the throttle. We swapped vacuum bells, left to right, and that made no difference. We swapped needles and that made no difference. When we swapped the carb pistons the problem moved to the other carb, indicating that maybe there's piston wear and a vacuum leak on one piston. I've never seen this before. BTW, we tried this with both oil and no oil in the dashpots. Also, on this 1953 TD, there's no carb spings in there. Is that normal for that year? Any insight would be helpful.
 
If linkage adjustment and everything else has been ruled out, the only other thing I know to try would be make sure the oil levels are close to being the same volume in each one, I've saw that vary piston/slide speed before.
 
The "standard drop timing test" in the SU shop manual is to remove the suction chamber, damper assembly, & piston, thoroughly clean with solvent. Temporarily PLUG the piston vacuum hole with clay or some such. Assemble the piston, damper, & chamber dry. Invert the assembly, push the housing fully up on the piston, & measure the time for the chamber to drop off the piston. Don't let it hit the floor. Standard specification is 5 to 7 seconds for the chamber to drop. A slightly dented chamber wall could cause binding & longer drop times, as would a bent/binding damper rod assembly. Air leakage around the damper cap could cause faster drop timing.

This measures the air leakage rate between the piston & housing which is an indication of how closely the piston is clearanced to the housing. Not particularly important on single carb installations, but very important when matching 2 or more carbs.

In your case, it does appear that one piston may slightly undersize. Other causes of different drop/lift timing under ACTUAL operation could be a metering needle binding in it's jet, check jet centering, or a restricted vacuum passage in a piston.

The early MG carbs which have no return springs can be especially critical in this area. Adding the lightest available return springs "might" help but should not be needed.

If there is "adequate" oil in the damper chambers, level slightly below the inner chamber top, oil levels should not be critical. Both damper sleeves should have a bit of free float. They are actually a one way valve which restricts piston rise but allows it to drop freely.
D
 
Pistons and bodies are matched at the factory. They are not interchangable so swapping them will not give a definite diagnostic result. Be sure you keep them straight! As Dave points out, tolerances are far more critical on early carbs than later ones with return springs. THe springs were basically a cost saving measure. If you look at the really early ones, such as on pre-war MGs, they had beautifully machined brass sleeves and other really nice components. As time went on, they got cheaper and springs instead of accurate gap tolerances were a way of saving machining time.
 
I wondered about those springs. Makes sence to me.
 
There are several different pistons for the H2 carbs, as Steve noted. The earlier ones were brass, then they went to aluminum with a brass or steel weight in them (which seems most common for the ones in the TD) and later to plain aluminum with a spring in the H4 carbs for the TD MK II and the TF. You might want to compare the weight of the pistons in your two carbs and see if they are the same weight. Also, as Steve noted, the piston/vacuum chamber come as a matched set and should not be mixed and matched. It is also imparative that the jets be centered perfectly as Dave advises above. This is a fussy procedure, but must be done in order for the carburetors to perform properly. An improperly centered jet usually results in the needle catching on hte side of the jet before the piston is all the way down and does not allow the piston to go all the way to the bottom of its travel.
Good luck,
 
Many thanks to all for your observations and suggestions. Excellent info. Bob will do the drop test tomorrow, and I'm willing to bet he will report a failure on one carb. It's probably worn and not holding the vacuum as well as it should. I'll keep the list/thread posted.
 
This "matched set" thing can easily be over-emphasised. Sure they're matched - like every other mass-produced carb! Remember, they were never just for Sports cars. If they're not too bad, corroded or distorted you can match a pair with patience, metal polish, and persistence.
Alexm ,since you never tried the drop test, do that first, but the symptoms you describe fit perfectly with slightly bent needle.
Steve, another reason for SU introducing springs, by the way, was to provide a much easier adjustment than making pistons of different weights.
 
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