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SU carb springs

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Adrio

Jedi Knight
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I am trouble shooting the SU carbs on my TR4A. I used the colortune for the first time (great tool) and found that one carb goes richer and one goes leaner when I open the throttle. Both carbs have new needles and jets and gaskets. I found no vacume leaks and the butterfly closes well.

What I did find was that the springs in the suction chambers are not the same. So I know there is a problem there. All the books I have say the car should have the "red" springs. Given the age, there is no indication of what colour the springs were.

I have four sets of springs from my spare parts carbs and all the pairs match but are all different from each other. I am asking if there is a way to tell what springs I have by measuring the wire diameter, the spring diameter and counting the number of turns.

The original problem that got me on to this was that the car would be at the correct mixture at idle but would hesitat when accelerated heavily. As if it was going lean on me.
 
Adrio,
A friend of mine who restores high-end British cars (and has old Bentleys etc...) claims the springs don't have as much effect as one might think. Early SUs didn't even use springs.

Probably more important is the fluid used in the dashpots. It is okay to use a thicker oil in the one that goes leaner to even out any differences in springs, as well as tolerances/wear in gaskets, pistons and jets.
 
Thanks Sam. I was having such fun with the colortune I wanted to get this fixed right. I have one more data point I should have given. The engine in this car is a TR3 engine with a TR4 head. So the displacement is 1991. But the carbs and needles are those for the TR4 engine. I wonder if that is making a difference? I hear a lot of people say that needle selection is "fine tuning" and now your source says the same about springs. My condition is not a fine tuning item. The performace of the newly rebuilt engine in the TR4 is noticably poorer then that of my non rebuilt tired old TR3. I have something that needs fixing and I am going to find it some year.
 
The other thing to check is that the pistons are moving freely and equally in the dashpots, and there is no binding.
You can test this by taking out the springs and dampers and watching hem rise. It might run (read should run!) lean, but it will help eliminate a possible source.
The early SUs that had no springs had heavy pistons, too, BTW
 
There are so many ways to tune out the differences. You can also adjust the float bowl needles to allow more or less fuel.

The first thing is just to get the idle mixture right on both carbs. Then, if under throttle they are different, play with different oil in the dashpots. If that can't get them good and even, you'll need to try different needles.
 
[ QUOTE ]
My condition is not a fine tuning item. The performace of the newly rebuilt engine in the TR4 is noticably poorer then that of my non rebuilt tired old TR3. I have something that needs fixing and I am going to find it some year.

[/ QUOTE ]
Hi Adrio,
The things that could cause major power loss would be:

1- Low compression - I would do a compression check first.

2- Ignition timing retarded. Advance it a bit & see what happens.

3- Very lean fuel mixture - Assuming you have the correct heat range plugs, run it at load for a bit, do a clean shut down, & check the plug colors.

4- Wrong cam timing. Not as easy to check, but very important.

5- Less likely would be restricted exhaust.

Good luck,
D
 
I had a similar problem once and it turned out that the piece that connects the two carbs together was slipping. I had to look real close and I noticed it grabbed at about 50%. The back carb/carbs worked well at idle, but the engine had low power from fuel starvation during acceleration. It was a weird one.
George
 
Hi George,
That certainly is a possibility. Had a friend who made his own carb to carb connector shaft. Got it a bit undersized. It would not clamp & he was running on one carb only. About 150 miles into a trip he was complaining of no power. It was easy to spot the problem immediately. A bit of beer can shim in the connector joint solved the problem until he could get the correct shaft.
D
 
One bad vacuum leak can ruin your entire day. After changing springs and needles and and playing with oil viscosity I still was not seeing any inporvment. So I hunted for vacuum leaks. It turns out I had one through/around a "plug" that was covering a vacuum port. It seems better now though not perfect, but I think I am in the relm of fine tuning now and that may have to wait for spring as winter is setting in. And I have an other problem that I have to solve before I drive the care to storage (see my new post on the control box).
 
Hello Adrio,

a point that has been made before about S.U. needles is that any needle of the same group (jet size)will have the same idle setting, but as the needle lifts then the profiles differ. I would be surprised if a TR4 needle is that much different to a TR3 though. Certainly, as Roger suggests, I would check the lifts of both pistons as you open the throttle. If they are more or less lifting the same amount the other possibility is a fuel flow restriction on the leaner carburettor. I'm afraid that I do not agree with Sammy regarding using fuel levels to adjust mixture, that is correcting (roughly) a wrong with a wrong.
I also don't think that the springs have much effect but they are easily swapped from front to back to see if there is a change.

Alec
 
Alec,

I have done some research and found that the TR needle is leaner then the TR3 needle for all stations other then the forst two. That is the once close to idle. The Hayne SU carb book has all the needle profiles.

As to the springs. As best I can figure the spring determins the depression on the constant depression that these carbs run at (anyone feel free to correct me as this is just my theory). But that get cancelled out with the mixture setting at idle. That is to say if you have a weaker spring, the piston will sit higher at idle but the jet will have to be moved up to make the idle mixture correct.

With the vacuum leak stopped up things are better. Not perfect yet but better. I think the fine tune will have to wait for spring.
 
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