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TR2/3/3A SUs Running Rich

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frankfast

Jedi Trainee
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My SUs are running rich at idle. Is this normal? I've gone through all tuning adjustments and can't seem to keep the plugs clean. Manual states that mixture adjusters are to be backed out 2 1/2 turns. I've read here that mixture adjustment screws only affect running at idle although others claim they affect running throughout the RPM range. Which is it? I've tried screwing in the adjusters a couple of flats without success. Car runs fine off idle. Car will idle at 750 RPM before it loads up and drops RPM until it stalls. Floats have also been adjusted.
 
Frank - take a look. There are a *lot* of variables in setting the mixture. Here's SU's own needle selection suggestion for the carbs on the TR3.

https://sucarb.co.uk/technical-jet-needle-recommendations

Note the "Rich", "Standard", and "Weak" (lean).

The ID is usually on the head (top) of the needle.

Tom M.

Edit: carb needle info:

https://www.mgaguru.com/mgtech/carbs/cb120.htm

https://www.mgaguru.com/mgtech/carbs/cb119.htm

Note the reference to verifying you've got the right needle, and how to "lower" it compensate for rich mixture that doesn't respond to normal adjustment.
 
If you haven't already I would check that the adjusted floats are holding and at the correct level in the jets.If the level is just below the jet top then crank it over without ignition and again check.Sometimes float valves leak by enough to cause a flooded restart or rich idle while less noticeable at higher speeds.
Tom
 
I could not find any info about adjusting on those links, so here are a couple things I would look for: make sure the choke returns fully, make sure the carbs are in good mechanical order; the needle jet cannot be oval. The jet has to be perfectly rounded so fuel does not get around the needle and force a rich running system, or that the piston does not return fully at idle. Do not have too much fuel pressure. If you have an electric fuel pump make sure that it is set at 2 psi or has a pressure reducer in the line and make sure your distributor is working correctly, so the advancement is returning with the throttle.
steve
 
Thanks for the info on the needle. I'll pull them out to see what I have. If it's the standard needle, maybe it's worthwhile purchasing the lean needle instead of lowering them. Either way it seems to be a compromise to what is correct. It's possible that both the jet and needle need to be replaced due to wear, a good indication of need for a rebuild. That's something I was thinking about doing this winter anyway.
 
frankfast said:
Thanks for the info on the needle. I'll pull them out to see what I have. If it's the standard needle, maybe it's worthwhile purchasing the lean needle instead of lowering them. Either way it seems to be a compromise to what is correct. It's possible that both the jet and needle need to be replaced due to wear, a good indication of need for a rebuild. That's something I was thinking about doing this winter anyway.
Click to expand...

:encouragement:
 
frankfast said:
Thanks for the info on the needle. I'll pull them out to see what I have. If it's the standard needle, maybe it's worthwhile purchasing the lean needle instead of lowering them. Either way it seems to be a compromise to what is correct. It's possible that both the jet and needle need to be replaced due to wear, a good indication of need for a rebuild. That's something I was thinking about doing this winter anyway.
Click to expand...

+1

For a stock motor the standard jet and needle are what you want. It sounds like one or both of yours are worn from not being centered.
 
Check also for air leakage around the throttle shafts. If you have leakage, you tend to adjust the jets rich. Then if at some throttle position the leakage decreases, you're suddenly running too rich.

If the float valve is leaking, the fuel level will be high, and that will make the carbs run rich, too.

Before worrying a lot about needles, I think you need to make sure that everything is right with the carbs. If you suspect they need rebuilding, they probably do.

Finally, don't be too concerned about that 2 1/2 turns admonition. The point where the correct mixture is set is often far from that value.
 
Sarastro said:
Finally, don't be too concerned about that 2 1/2 turns admonition. The point where the correct mixture is set is often far from that value.
Click to expand...

I wondered about that. I was afraid to turn those adjusters more than a quarter turn because it has to affect mixture throughout the RPM range. If I lean the mixture out at idle, it may be too lean throughout the rest of the range.
 
frankfast said:
I wondered about that. I was afraid to turn those adjusters more than a quarter turn because it has to affect mixture throughout the RPM range. If I lean the mixture out at idle, it may be too lean throughout the rest of the range.
Click to expand...
That is a concern; and I suspect is sometimes the source of overheating problems (caused by mixture too lean at cruise conditions).

Part of the problem (IMO) is that most rebuild kits do not include new jets and needles, so many don't replace them. In theory, they shouldn't need to be replaced, but in practice both of them can wear if the jets are not perfectly centered or the piston rod & bearing are worn. And wear is almost impossible to judge.

Note that if the upper gland (the cork washer inside the jet housing) is leaking, it leaks fuel into the carb throat.

Also worth noting that the "lean" needles do not idle leaner than the standard ones do, the lean mixture is in the mid-range.

At least in my experience, throttle shaft wear doesn't upset the mixture too much until they are really badly worn (enough to cause other problems).

Make sure your jets are centered. If you lift the piston with a finger and let it go, it should land with a distinct click. The motion is slowed by the damper (if you have it installed), but you should still get a click. If no click, the needle is rubbing on the jet, which will upset the mixture.
 
Centering the jets is a little tricky. When I thought it was centered, I'd tighten the gland nut and the piston would stick. Had to loosen that gland nut a number of times before it would return properly. Coming from motorcycles where the needle had notches and could be adjusted and the jet was fixed, I think this system is unnecessarily complex. It is what it is.
 
Frank - if you're still in doubt about centering, there are various devices you can buy/borrow to do it for you. But trial and error is the usual method.

If you lift the piston up and let it fall, you should hear a "click" when it hits. That's the common test for verifying the needle is centered and not rubbing. By the way, back when I first got my TR3, I discovered both needles had somehow been bent, and never would "center". Replaced the needles and all was well..

Tom M.
 
I removed the needle today and found it labeled "SM". So it is the correct needle. It has been centered. At this point before I take on rebuilding the carbs, I think I'll raise the jets by turning the adjustment screw a few flats and then do a few plug checks both at idle and running at higher RPM. If the readings are still not acceptable I'll have to do a rebuild. By the way, the floats have been adjusted with new float valves because one was worn and leaking.
 
My $.02 worth- 2-1/2 turns for the static adjustment seems excessive. I believe most manuals suggest 2 turns and even that is richer than my final setting. To start with, the jets should be even with the bridge before lowering. I like to use a dial caliper for the initial setting and lower the jets to .060". Also, as has been mentioned, make sure the jets are fully "home" when the choke is off and that the float level is correct before attempting the adjust the carbs.
Berry
 
Plus the pistons fall a lot better/faster with a warm engine. You can google a test that shows how to check both pistons are falling at the same rate. You basically hold them in your hand with the vent and oil hole on top plugged and then time the speed it takes for the piston to drop down. I think 3 to 4 seconds or something was the happy number, but balance is important. I had one so wore out or miss matched that it just fell fast with no drag or vacuum.
steve
 
It's a distinct possibility. Any real reduction in exhaust restriction tends to make the mixture go lean. But lots of other factors are involved, including what altitude you typically drive at.

Only way to be certain though, is to run on a dyno with an exhaust sniffer; or install a wide-band O2 sensor.
 
Tightened the front carb mixture screw a 1/2 turn with good results. Did a plug check and the front plugs were a lot cleaner. Will do the same with the rear carb. Had to readjust the idle since it was running around 1200RPM. Didn't noticeably affect performance.
 
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