TR2/3/3A SU Piston Drop Test

[TR-3rg]

Hi, I tried the SU piston drop test that measures the time for the piston to fall in the air dome. The application is my 1960 TR-3 with H6 Carbs. The time for the piston to drop should be 5 to 7 seconds. Mine are 5.5 and 4.

Is this worth worrying about? Any way to compensate such as different springs? Is it worth the trouble of finding in spec parts? I like the throttle response I have, but I have never driven another car with SU's for a comparison.

Thanks, Roy

 

[charleyf]

Roy,
Although theoretically they should be as you stated. I have just built three sets of HS6 carbs from parts from several sets. I mixed and matched pieces to try to get the piston drop response within the stated range. But I am dealing with 50 year old parts. So what I did was to try to get the two pistons and domes for a set to at least be close to each other. None of the parts I used got to the 7 range and most were more in the 4 to 5 second area. In my opinion your 5.5 and 4 are just within range of each other. But are within the ranges of what I am using. But if you are happy with the throttle response you are having then I would say leave it alone. Remember that the throttle response is for acceleration and the piston is going up against the spring and the oil in the dampers. So those two things pretty much dictate how fast the piston goes up. The piston coming back down happens when you let off of the accelerator. At that time there are a number of other factors involved in how the car responds. Namely the flywheel and the inertia of all of the other engine parts.
So I would say leave them alone and consider the piston drop test as only thing that you would do to try to find a real problem. In this case ---"if it ain't broke don't fix it."
By the way two of those sets are currenly on my TR4's and a friend borrowed the third set and changed out the float bowls and needles and is running them on his
TR6
Charley

 

[CJD]

I would worry less about the number, and more about the consistency between the 2 domes you plan to use. If they are different, then your part throttle mixture will vary between your front and rear cylinders.

 

[Geo Hahn]

Is this drop test done with the pistons in the carbs - or just in the carb valve body removed from the carbs (no springs etc)? I have seen it both ways but never heard a spec on how long it should take to fall.

 

[Marvin Gruber]

Geo
the way I have always done it is, dome and piston in hand, no spring. Never had one go 7 seconds, more 4/5 sec.
Marv