TR2/3/3A SU Carb Tuning

I've got enough time on the car to get a baseline and start fine tuning everything. I have a mixture issue I need to address to the SU Gurus out there.

I set the idle mixture for the best idle speed, plus one flat richer. The carbs are balanced at idle, dashpots filled with 30w oil, and it will purr at an even 750rpm. The engine starts and runs perfectly, except...

Even when it is fully warm, it wants about 1/2" of choke, or it goes flat on acceleration. As long as the choke is out 1/2" it runs great. Even when it has been idling and the temp starts up towards overheating, it simply does not want the choke fully in, or it goes flat on acceleration.

So, my thoughts are to
...unscrew the idle nut and accept a rich idle to smooth the transition. Or
...See if the needle can be raised in the piston for a bit richer transition. Or
...Order new needles, assuming they got changed somewhere along the cars 60 year history.

Am I overlooking anything obvious?

Moss says standard needle for TR2 (HS4) is an FV. They should be marked on the upper shank that goes into the piston. Do you have the pistons matched correctly with the domes?? How about the springs inside the domes?? They come in different tensions, and can also go flat. There should be a paint color on the top to indicate tension. I don't know what your TR2 should be, but I believe that the TR3 had red springs standard. Here's a chart of the springs for 1 1/2" & 1 3/4" SUs. The length is the compressed length and what the ounce reading should be. Easy to measure. Fold a piece of cardboard into a "V" and cut to length, insert into spring (not binding) and push down on spring while on a small scale.

Lt Blue 2.5 oz @ 2.625"
Red 4.5 oz @ 2.635"
Yellow 8.0 oz @ 2.750"
Green 12 oz @ 3.000"

I'd just try and turn the jets down a couple of flats and see how things work out. My engine is pretty modified, and I adjusted according to an O2 sensor, and it's pig rich at idle, but is fine everywhere else, and runs like a scalded cat!

Edit: BTW, here's a neat little SU needle comparison program you can mess with...try putting in the standard FV needle along with an FM, and you'll see how the FM is richer. You can compare up to five needles at a time.

https://www.mintylamb.co.uk/suneedle/

CJD said:

I've got enough time on the car to get a baseline and start fine tuning everything. I have a mixture issue I need to address to the SU Gurus out there.

I set the idle mixture for the best idle speed, plus one flat richer. The carbs are balanced at idle, dashpots filled with 30w oil, and it will purr at an even 750rpm. The engine starts and runs perfectly, except...

Even when it is fully warm, it wants about 1/2" of choke, or it goes flat on acceleration. As long as the choke is out 1/2" it runs great. Even when it has been idling and the temp starts up towards overheating, it simply does not want the choke fully in, or it goes flat on acceleration.

So, my thoughts are to
...unscrew the idle nut and accept a rich idle to smooth the transition. Or
...See if the needle can be raised in the piston for a bit richer transition. Or
...Order new needles, assuming they got changed somewhere along the cars 60 year history.

Am I overlooking anything obvious?

Click to expand...

I would pop off the domes and check float level in jets.Obviously they should be the same,just below top of jet.
I had one that float that was holding but the level was noticibly lower than the other and after adjusting the level it required significant leaner adjustment than with the low level,which I guess I had compensated for with a richer adjustment.
Tom

Thanks, Art, Tom.

I did the drop test on the pistons to make sure they fall at the same speed, but never compared springs. I assume the order you listed springs would go from leanest to richest transition? Do the big 3 carry the various spring types? I have boxes of Holley tuning parts...now it's time to start collecting my SU tuning parts!

While checking the spring type I have, I'll definitely give the fuel level a check too.

Interesting little app for the needle comparison. Is there a code to the lettering on needles?

Good luck,you may want to also observe if with it choked to run best do the pistons move as they should when given the throttle.
Tom

John, go to teglerizer.com/suneedledb. Then scroll down to the "web based searchable needle index". Great chart. However, I think you'll find that 99% of all needles are .0890" at station one. Which means changing needles will not change the mixture at idle. Run all of those other checks as recommended above first.

Bill aka Aridgerunner

Needle letters are random as near as I can tell.

Given what you've said, I would definitely replace the needles and jets. What happens is that people don't get them perfectly centered, and they rub. The rubbing leads to wear that is very hard to spot; which in turn lets fuel in effect leak around the expected needle/jet gap. Since the leak is more or less fixed in size, it makes the idle mixture go rich. Then when you lean out the idle mixture, the main mixture goes lean.

Jet-Needle centering is very important, of course, as is totally free movement of the piston (when not limited by the damper). If the piston rubs or scrapes or has a tight spot, you'll have to find and fix the issue. You should be able to hear it land with a click after lifting and releasing.

On my H6 carbs, I've discovered that the fit between the dome and body is poor enough that removing and replacing the dome will affect the centering enough to let the needle rub. To work around the problem, I turn the dome lightly clockwise after loosely installing the 3 screws, and hold it there while I finish tightening the screws. The screws against the sides of the holes are a more repeatable location. Of course, you'll have to recenter the jet after doing that for the first time.

Idle mixture is deliberately a bit rich on these carbs; so setting for max idle rpm may not be the best. I use the "lift the piston" test as described in the book. If everything is right, the test is quite sensitive and should show a difference for even one flat of the mixture nut.

Sorry, I don't have my SU books handy; but there is a lot of information on Burlen's web site if you poke around for it. Here's a chart for various piston springs
https://sucarb.co.uk/piston-springs.html

However, the purpose of the spring is mostly to match carb size to engine size (and VE), rather than changing mixture. It does affect mixture of course, but normally you would pick a spring first to match air flow; and then use needle profile to tweak the mixture.

John, any parts that you need for the SU's you can get from Joe Curto. I called him after I made the modifications to my TRactor engine, and he suggested needles and springs based on what I told him, which worked out very well. One of the best when it comes to SUs/ZSs

https://joecurto.com/

I believe the problem is retarted timing. I would not mess with the carb mixture yet; instead I would click the distributor timing up and at the same time move the choke in as I drove around. Sure the idle will rise up also, but I would let that go until the car would pull off without the choke. Then after that, I would work at getting the idle down with the screws until I found a happy medium between retarted timing for idle and advanced timing for moving. I guess tr2 dizzy was a 40480A with a dwell of 49, but again I guess the tr3 or tr4 dizzy would work also. My opinion is these cars are used to a higher auctane fuel, so we need to work with what is out there for fuel.
steve

I knew you guys would come through! A lot of great info. I'm taking a bit of advice from each of you, and called Joe Curto to order parts. I'll be getting a couple different weight springs to play around with, and a couple different needles...the stock FV and a GRE that he said is the next richer. Also a handful of seals and gaskets while I play around with everything.

So, the parts are on the way. Once they get here I'll get serious with timing, fuel levels and mixtures, based on ya'll's advice. Thanks again guys!

PS...one thing Joe mentioned is that the ethanol fuel tends to cause a lean stumble off idle, thus the richer needle recommendation. He said if I ever go with a K&N air filter that it would take an even richer needle.

Are you sure the shoulder of the needle is flush with the bottom of the piston. As a previous pos said all the needles are the same at the top or idle position, but if the needle is in the wrong position then it will never be right. I also agree checking the timing. But it could also be an air leak considering you appear to need more fuel by pulling down the jet by the choke lever.

I am anal about setting the needles, so that is one thing I can be sure of. I am also quite sure it is centered in the jet, but that does not mean it could not have been worn from bad setting in the past.

The air-leak is a very valid point! I'll definitely check that out before I start replacing parts. Thanks!

While I wait for parts, could you guys give me an idea of what you prefer to set for timing at 700rpm?

CJD said:

...

While I wait for parts, could you guys give me an idea of what you prefer to set for timing at 700rpm?

Click to expand...

There are many variables involving timing, as all cars are different. I've found the best way is through road testing. Set it statically according to the book (4°). Advance until it pings under load, and then back off a bit. Use the knurled advance/retard knob. It's the best way to keep track of things.

I agree with Art on timing. With modern fuel and a totally stock, fresh engine, you could maybe start closer to 8 BTDC (about 3/8" at the edge of the pulley), but its safer to be a bit conservative at first.

On the mixture, though, IMO you are spinning your wheels playing with alternate needles and springs. It should run very well with the stock components, even with ethanol in the fuel. After all, the mixture adjustment affects all throttle positions equally, so it will compensate across the board when the fuel takes less air to burn.
I have even driven a TR3A on pure ethanol, just by lowering the jets, and it ran quite well.

IMO, get it running good first with stock components. Find the bugs, whether its vacuum leak, valve timing , distributor issues or whatever. Then you have a baseline to compare with and can try richer needles or whatever. I think you'll find that the factory made a pretty good compromise between performance and economy, its tough to improve one without spoiling the other. (Richer needles cost me about 5 mpg with only a tiny improvement in power.)
Of course the power is worth it for a racecar, but I don't get the impression that's what you are building.

Without a fully calibrated wheel on the pulley it is difficult for me to tell the degrees, I agree with Randall that about 3/8 to a ½ on the pulley. Do you have a stock tr2 dizzy? I think with an air leak you would not be able to get a 700 idle, unless the timing was turned way back, so I think you are good there. I would turn the timing up to feel the performance. It will be evident when it is too far by the engine acting aggressive and not worry about the idle just yet. What do the plugs look like?
steve

I thought more about what you asked John and to get an idle of 700, I would say more like a ¼ inch off TDC. The ½ inch is more like what my car needs for power and more like a 900 idle. I have checked my tach so I think it is right on, but some people say the tachs are off, but everyone I have had is right on.
steve

You're right, Randall. I have ordered the stock FV needle and the stock red spring. I just want to make sure I am starting with a new baseline, and not worn needles or week springs. But I do have the GRE needles on the way, for something to play with later, or if the FV's won't dial in.

Steve, I do have the stock TR2 dizzy, but the springs may not be stock. I should probably take the time to record the advance curve to make sure it's close to stock. I think I'll play with the timing while I wait for the parts. I'll see how far advanced it'll go before it pings or gets an unstable idle. I have one of those "dial an advance" timing lights. I'm not sure how they work, or even if they are accurate, but you shoot the mark and then dial the mark back to TDC. Then you read the timing on the gun dial.

Got the parts in, so tinkered with the SU's today.

1). Checked for air leaks...none found.

2). Checked the fuel level by looking down the carbs with the domes removed. Both the same and about 1/8" below the top of the jet with the jet fully up.

3). Switched in the new "red" piston springs, to make sure the springs were not a factor.

4). swapped out the old FV needles with the new FV needles.

The engine ran much better, and I could push the choke fully in. It did, however, still have a flat spot on initial throttle at slower speeds. It did not fall flat on its face like it was originally, but still felt weak until you got enough rpm to pull out of the flat spot. When slowed, but not enough to get into 1st, it was weak coming on in second until the rpm came up.

5). swapped the stock FV needles for the GRE needles.(one step richer)

Now the car is running just like it should. Choke starts it cold and can come off after a couple minutes of driving. No flat spots at any rpm. Idle is consistent at 750rpm.

So, thanks for all the tips! Art was right about Joe in the link above. The dude knows his carbs!