SU Carbs - I Can't Win

[JT68]

Yeah, I remember when that weber carb came out - they never really caught on. It is literally 1/2 Spanish DCOE so it comes with all the same features and headaches. Just like dcoes, they have a main venturi, main air jet, main fuel jet, idle jet, accelerator pump adjustment all which require proper calibration to the specific engine to work effectively-otherwise lean/rich spots,sputters/coughs/flat spots etc. Most others who have looked at those go a different route for many reasons. As I said before there is no magic carburetor - they ALL require proper set up and tuning. SU's are generally pretty user friendly so I'd follow Keith's suggestions and try to get the SU's sorted. No local assistance possible?

Is this issue on a 1600 or a stroker?

[23yrRebuild]

"The engine would rev on up with the old carbs"

If your old carbs ran better, I would try swapping the old needles into the Z carbs, you should also be able to identify the old & new needle designations for reference. It's not good that you would have to use your old needles in expensive rebuilt carbs, but at least it might give you clues as to what's going on. I would inspect & verify that both float bowl needle & seat valves are set to the correct specification. I've also read that certain types or brands of needle & seat valves will commonly give the same symptoms you describe.

P.S. I just realized,... you might have sent your old carbs in as a core, so my idea might not be useable.....

[GeoffM]

I would say that a needle swap is for fine tuning AFTER the carbs have been set-up adequately. There's no reason that the Z-Therapy carbs with the OEM needles can't get set-up to run well. If you find that you want to tweak certain things afterwards (such as rich idle) then try out the ADQ needles which Keith swears by.
I guess what I'm saying is that it COULD be the needle/seat but I dont think Curtis has got his baseline set-up done yet.

[23yrRebuild]

I agree Geoff, what I was suggesting, is that one doesn't really know what the rebuilt carbs are providing. I've spent many long hours over the decades, trying to chase down gremlins, while thinking "the new part I just put in, couldn't possibly be the problem", ....only to find out I was wrong. I'm not suggesting the Z carbs are faulty, only trying to offer a way to eliminate the Z carbs as being the problem...
Some examples: anything to do with ignition - spark plugs, coils, ignition wires, condenser, cap, and /or rotors can be notoriously defective. Also alternators, starters, fuel pumps, and water pumps, and thermostats. I'm actually embarrassed to admit I've had that kind of bad luck with so many different new or rebuilt components, but that is across 45 years of servicing my own vehicles...YMMV...

[GeoffM]

Totally agree Mike! It's the worst when it's an electrical component like a voltage regulator....can get you down a rabbit hole.
When I was a kid in the 80's I worked on the parts counter. I often took the brunt of it when the customer returned a few hours later...lol

[23yrRebuild]

LOL !...."the parts counter"....I know what you mean, I've worked those too !

[keith0alan]

The carbs are the last thing to adjust. Verify valve lash, dwell, timing, plugs. Total tune up then adjust the carbs.

[Curtis]

The carbs are the last thing to adjust. Verify valve lash, dwell, timing, plugs. Total tune up then adjust the carbs.

All new ignition parts including blue tooth distributor. Timing set. Haven't checked the valve lash for a while, maybe a year. Less than 100 miles of driving.

I ordered an EZ Tach meter. Arrives next week. Have a colortune and UniSyn.

[Curtis]

I think I'm closer. I took off the domes and set the inside to .09 per the write up. Balanced the carbs at idle with the unisyn. One dog bone needed a change.

I put in the ADQ needles. It idles really well now. I still have a flat/stumbling spot mid RPM. and then when it come off that it revs really well. What it didn't do today was give me a big old backfire pop when I shut it off like before.

My suspicion is that I need a thicker oil in the dash pots. I suspect that the newly rebuilt carb pistons work too well.

[Linda]

Good news!
You can always slightly adjust from what you have and Keith mentioned setting the needle out or in as an adjustment too.
At least you are closer
So the 1600 Stroker carb set up is a little different ?
Linda

[Curtis]

Good news!
You can always slightly adjust from what you have and Keith mentioned setting the needle out or in as an adjustment too.
At least you are closer
So the 1600 Stroker carb set up is a little different ?
Linda

I'm not sure since the old carbs did not have this much of a flat spot.

[dbrick]

One thing often overlooked is the fuel level in the bowl, it really affects mixture alot. With the pistons and domes off, pull out the choke all the way, it should drop the jet and nozzle down and fuel should just cover jet. too high, too rich, to low too lean, especially at idle.

Linda mentioned the Magic Number, which gives a great starting point.

According to Bob, (the engine builder) I have a pretty radical cam,B reground toward a C, so virtually no vacuum below 1000 rpm, which the SU's seem to be able to deal with well. I got them adjusted about 95% correct, and now just don't mess with them. Tune for the midrange, live with whatever idle you get, or change needles. Idles like a Harley below 900rpm, and they are a bit rich, but I'd rather burn gas than pistons. The crappy running at idle smooths at 1000 rpm and is good all the way up.

I drove 2 other peoples stock SU 2000's last month, It was on a long uphill I drive on all the time. I was surprised how much more power mine has in the low to midrange, you would think the more radical cam would give up some power in the lower ranges, but the SU seems to compensate well.
I found either 20w shock oil or Dexron ATF work best

I have a set of those Webers, keep wanting to try them, but the SU's run really well. If anyone wants to try them, I got them pretty cheap, will pass on for the same price with a new throttle bracket that fits Roadster and a U20 intake manifold. Just PM.

[Curtis]

Okay, I think I'm a bit closer.

I put in 30wt oil the other day which helped a bit. This morning I got out the colortune and tried it. Based on what it shows the car is running rich. So I lean it out some. Ya, no, only made it worse, much worse. So I tried making it richer than I had it originally, flat spot is better. Doesn't idle as well.

Going to try some 40wt in the pots.

[Gregs672000]

Ya, that is what is so frustrating with SUs... those damn needles. Get it right one place, screw it up in another. It's hard to know what is causing the flat spot too... too lean, too rich? How much timing are you running when it flat spots? Is it a consistent RPM? Without a wideband, you are guessing while you're driving. I wonder what would happen if you were to try adding or subtracting a couple degrees of advance where it flat spots. Are you running the 123 dizzy on this car? I'm no expert, but ignition timing will have an effect on how APPROPRIATE the air/fuel ratio is as you can increase or decrease the amount of time you give the engine to burn the fuel before you exhaust it out. It's a system, one effects the other. You also kinda need to know if it is lean or rich when you're adjusting the timing, as it can take more advance without ping if it is rich, but if you're running the ragged edge of being way too lean you need to be a little careful. If you're not already, run premium fuel while doing adjusting as it will resist pre-ignition better. If you go back to 89 or 87 later you can back off a degree or two if she pings. Octane is about resistance to exploding, not inherent "power" of the fuel. Stock compression? Stock cam?

[spl310]

What needles have you tried?