How are your SU carbs set up? Looking for a baseline for adjustments.

[marcus550]

I've got a 1970 SRL311 and am trying to set up the SU carbs. It was running mostly OK before I started this process, just some rough idle, a little stumbling and full-throttle roughness. But now I'm pretty far down the rabbit hole and it just keeps getting worse. Valve adjustment is done, new 'Gary Boone' distributor, coil, wires, cap installed last week.

I balance air at idle, then at fast RPM. I then started to adjust the fuel mixture. The rear (#3/#4) carb was turned all of the way in against the carb, so all of the way rich. The front carb was only a turn leaner. I installed a Colortune on #2 and started to lean out both carbs (to attempt to keep it balanced.) I'm watching the spark color and it is bright yellow as I lean it out one turn, two turns...10 turns. How far can I go? No change, but the engine is bucking at this point. The Colortune says it should be bunsen-burner blue. Never saw that! That front carb felt like the piston was sticking so I took it apart, cleaned it, light lube, and back together. Same with the back carb. Now I need a good starting point to try again.

How far out is your air adjustment (idle screws) and how far out are your mixture screws? I need a baseline! The idle screws have springs on them that the screw bottoms out on. I'm 6 complete (360 degree) turns out from that but it isn't terribly happy right now.

MUCH appreciated if anyone can help.

Mark

[dbrick]

One thing to check is if you have vacuum leaks, either at the throttle shafts or intake or carb gaskets, these will make you crazy, as the mixture keeps changing and you are constantly fiddling with it. Spray carb cleaner on the ends of the throttle shaft, if idle changes, you have a leak. Sometimes it's well worth the time savings and headaches to have the carbs redone, set them up once and enjoy it.

With the caps removed from the carbs, if you lift the pistons and let them drop, they should fall equally with a nice metallic thunk when they hit bottom, and with pistons in and oil in the domes, should fall slower and equally to the bottom.

Also float height affects mixture. A quick check is to remove the domes and pistons and look down into the jet nozzle. If you pull the choke out to full choke the jet drops down and when it reaches bottom, that should be where the fuel is sitting. Hard to describe, but easy to see. As you pull out the choke, the nozzle goes down and at some point the hole will fill with fuel. That should happen at or very close to full choke. While the pistons are out, be sure the needles are seated properly, the lip/shelf should be even with the base of the piston. I loosen the screw a bit, pull them out a little and use a razor blade to push them in and end up flush.

Changing to non smog needles helps too, but I'll leave that to the experts.

My car has a pretty radical cam, a reground B cam heading for a C cam, so almost no vacuum below 1000 rpm and is always rough at idle. It's much easier to set mixture and balance at 2000+ rpm and then let it idle wherever it likes as long as it runs. If you do have a B cam, it will idle rougher than an A.

[GeoffM]

Mark,
I'm sure you've read this write-up from Keith but I know from experience that I benefited from reading it 15 times and Its been an invaluable guide.
http://www.311s.org/pmwiki-311/pmwiki.p ... sNewSuTech

The initial nozzle setting for a 1600 is .087" below the venture height. Not sure what it is on a 2000 though.

[marcus550]

Geoff, you're right. I had read that only four times, but reading it AGAIN I notice I've got the mixture screw going in the wrong direction. I think. I had printed that out when I was doing this work yesterday. Regardless, I couldn't get the blue flame at any setting so maybe a refurb is in my future. I feel like every single problem that could exist with Roadsters came with my Roadster!

[keith0alan]

I think you were backwards on the mixture knobs. Turning them up, ccw from the top leans the mixture. If they are turned all the way up, the nozzles are actually all the way up also, and you are still rich then you may have worn needles, nozzles, or the float level too high. With the colortune installed and engine running lift the carb piston a bit. There is a pin on the bottom for than or you can catch the edge with a fingernail and lift it. This will lean the carb. If lifting it speeds up the engine and turns the colortune blue then you are too rich. The yellow is very bright and easy to see. The blue is very hard to see in very much light.
Remove the domes and look at the metering needles. They should have a matte finish. If you see shiny spots then they are worn. Also the nozzle hole should be round. If pear shaped it is way worn. With the dome and piston removed you can check the float level. With the choke off you might see the fuel a ways down the hole in the nozzle. with the choke on the nozzle should be just below the float level and there will be a little puddle of fuel on top of the nozzle. If you have fuel running out of the nozzle when the choke is off then the float level is way too high or the float is sunk. This test is done right after shutting the engine down.

[Gars68]

also, this initial setting helped me out:

Step Two: Remove the dashpot covers, springs, and plungers with needles from each carburetor. Make note of which one is front and back. Be careful of the needles as they damage easily.
Step Three: Peer down into the center of the opening and you will notice two brass pieces. The center one is the one we are going to set and it should be slightly below the outer brass piece already. You could take the caliper and check the heights before you begin. I did this on mine and they were surprisingly equal-not bad for doing the adjustment by feel.
Step Four: The magic number for the 1600 SUs is 0.087 or 0.09 in. Since you have already checked the settings you should know whether you need to lower or raise the idle mixture adjusting nut. Raise or lower to meet the magic number. Do this to both carburetors until they are both set to the magic number.
Found here: http://www.311s.org/pmwiki-311/pmwiki.p ... ion.SuTech

[marcus550]

Spraying carb cleaner on the throttle shaft exposed a pretty big leak there. Knowing that, I've got them set up to be tolerable. Idle is still rough but it pulls OK. I've put 10 miles total on the car, ever, so I just want to DRIVE it and stop spending MONEY on it. If possible. This week is cold and rainy, so hopefully soon.

[cktrap]

I experienced most of those problem and I replaced the nozzles and it made everything better. If they look even a little distorted replace them

[notoptoy]

I have been helping with a Roadster in the area, and could not get it to idle well for anything. Ran the carb cleaner spray test and found the leaking throttle shafts. One is so bad that it is basically impossible to balance the carbs. Will probably seek out a cheap used set or see if the owner wants to splurge on a rebuilt set. In the meantime, he is driving it with it's temperaments knowing it's set as good as it can be and what the problem is.

[spl310]

Tom, is it a 1600?

[notoptoy]

Yes, it's Ken's and we haven't touched it in a while, need to get back over there!

[Linda]

One thing overlooked is that you need a good seal with good gaskets on the fuel bowls, which means making new ones I have found.
Replacing or checking the fuel regulator in the bowls if necessary can be helpful also.

14285049996442.jpg

Linda

[spl310]

Yes, it's Ken's and we haven't touched it in a while, need to get back over there!

I have some spare 1600 carbs... Maybe even two sets...

[marcus550]

OK - Update. Had the carbs cleaned and re-bushed with Delrin throttle shaft bushings. Put it back on the car and spray the shafts with carb cleaner. Very little change, so good result. STILL can't get it to idle right. Pulled the floats out and they were set wrong, and different, so I adjusted them both to 0.54" for the 'H' dimension (base of the lid to the bottom of the brass float/top of the curved plunger hinge.) Cleaned the needles there and noticed they were of different design, and noticeably different looking in age. Back together. Still can't get the car to idle smooth. The rear carb wants to be full lean. The front carb is only one full turn richer. Pulling the plugs shows all four are covered in black soot. So...I assume the carbs are too rich even though they're set full lean.

Causes? Needles are worn? Time for an expert or should I go for the ZTherapy rebuild kits? Hate paying someone to work on ANYTHING.

[spl310]

Needles and jets are suspect. If the needles are of different design, I would get a pair of matched needles. I would also look at the jets to see if they are worn or even mismatched. It sounds like you may have scrambled carbs that are from different cars.