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TR2/3/3A SU Carbs

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Skidude

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My 59 TR3A has SU carbs from Moss Motors

Model # AUC 878T

Problem is there is no connection for the vacuum advance on either carb.

Certainly, my car cannot be the only British car with this issue.

What are you supposed to do about the lack of vacuum advance?
 
Normally that connection is located on the underside of the front carb.
The vacuum advance is not necessary. It is a help to improve fuel mileage. You might want to increase your timing to be able to run at least in the middle of the range .
Charley
 
Unfortunately, the vacuum on the TR2/3 is placed so it sees no vacuum at idle, but then begins as the throttle valve opens and uncovers the hole in the throat. In other words, if you just hook it to the intake with full time vacuum signal, it will have a flat spot as you accelerate off idle. Unless you are very careful, drilling a tap into the front carb could end up badly.
 
CJD said:
Unfortunately, the vacuum on the TR2/3 is placed so it sees no vacuum at idle, but then begins as the throttle valve opens and uncovers the hole in the throat. In other words, if you just hook it to the intake with full time vacuum signal, it will have a flat spot as you accelerate off idle. Unless you are very careful, drilling a tap into the front carb could end up badly.
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I should have added that my friend, a TR4 owner and retired professional mechanic does not use the vacuum advance on his car by choice.
Have you thought of returning those and finding a used original set and rebuilding them? I know that I have a set that I cleaned up but did not rebuild as I chose to use a set of original HS6's instead. They are available cheap.
Charley
Charley
 
The original distributor for the normal-compression TR4A was a 25D4. model no. 40795A. Its vacuum advance spec is 2-6-3, meaning that it starts at 2" vacuum, max advance is at 6", and max advance is 3 degrees at the distributor. I suspect that there is very little difference between using and not using vacuum advance if you have the stock setup.

Even though the engine is nearly identical, the TR3 may have a very different advance spec. If you have an aftermarket distributor, you might have some generic vacuum unit with yet a different spec. For starts, you could take a look at the data on the British Vacuum Unit site, and see if you have an advance spec that is great enough to make a difference, and go from there. BTW, the advance spec is usually stamped on the advance unit, often near the vacuum connection.

I must say, I'm a little surprised by the variety of mechanical- and vacuum-advance specs for engines that are basically the same. It gives me the feeling that the factory was continually experimenting with these specs.
 
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You can run without the vacuum advance. It is for economy under part throttle only. You will lose no power, but will get worse gas economy. It will not hurt anything.
 
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