68 Roadster 2000 SU’s Running Rich

[halvernaz]

Hi All,
Looking for some help/things I should check. I have a set of freshly rebuilt SU’s on my 68 2000 that we can’t lean out. Even fully adjusted to lean they still run very rich.

Thanks
Henry

[Gregs672000]

I'll defer to the SU experts, but in the meantime I suggest a search of the postings as there are a number that discuss proper needle settings/position etc. As far as I know, that's what it's all about with SUs, assuming the needles and nozzles are correct and matched.

[nismou20]

Correct needles? Nozzles all the way up? Choke cable returned to home position does not always send the nozzles up. They can get sticky. You can adjust needles slightly proud of damper base. Maybe needles were sanded by PO which richens mixture? N17 were stock, N25 were slightly richer. If the profile numbers are illegible I would get a stock one to compare. I found that various size button holes can help in finding a profile at various points along the needle. Calipers work but with taper needles they are approximate.

[Linda]

Lots of threads on this .Who rebuilt the carbs, a mechanic or Z Therapy? Big difference. What got replaced?
Worn nozzles ( hole ovaled out), worn fuel valves, bent needles, throttle plate bushings, all can contribute.
I like the “magic number” for nozzle setting found in Tech Wiki write up
Check for any vacuum leaks from all relevant gaskets, that’s easy.
Good luck
Linda

[todd lorber]

You also need to pay attention to your float bowl settings. If they are getting too full, they will dump too much fuel onto the venturi bridge and cause a rich condition.

I had temporarily set up a clear tubing apparatus and ran the carbs to see where mine were. This was a bit of a pain since the carbs had to be installed and removed twice, but it helped. For some reason this keep rotating the photo sideways, but you can get the idea.

Float Bowl Measuring.JPG

Unfortunately, on these carbs, the bowl drain is more toward the engine. On the 240z it is on the outside of the bowl, which makes it easier to rig up a site glass tube to see the dynamic float bowl level.

[Linda]

I never really understood how this method worked. Any size tube? where do you make the mark?
I thought you do this on the car?
Linda

[todd lorber]

You DO do it on the car. But it was hard to get a decent photo in the engine bay. You make the mark at a certain level below the top of the float bowl. I can't remember the spec now but it may say in the tech wiki section what that level is. Basically you want it just slightly below the bridge.
Tube size does not really matter. It just needs to be vented to atmosphere and the tube needs to be higher than the fuel level or it will spill out. I made a "T" so that one tube fed carb and the other went upward to show the level.
All it is doing is showing the level that the fuel is seeking while running (idling). Obviously, when you gun the throttle, the bowl will temporarily drain down a bit until it is replenished by the fuel pump.

[nismou20]

Linda, If you hold a clear hose in front of you with both ends up and droop in the middle, fuel,water whatever fluid will find its level. Works as a crude carpenters level. Same principle. You never want fuel higher than the carb body bridge.
Todd makes good points on the float.

[keith0alan]

The choices are pretty much:

Float level too high
worn needle or nozzle
needle installed incorrectly
incorrect needle
nozzle sticking down with choke

What parts were changed when they were rebuilt?

[halvernaz]

Thanks for all the suggestions, I'll check them all. Hope to have it figured out over the long weekend.

Henry

[RBMann]

......Nozzles all the way up? Choke cable returned to home position does not always send the nozzles up. They can get sticky. .........

This just happened to me. The nozzle on the front carb would stick in the full choke position. When the choke knob was pushing back in the cable housing would come out of the its holder on the carb. Even when the nozzle finally popped back up the cable housing caused the choke to be partially engaged until it was reseated in the holder.

[keith0alan]

For sticky chokes I recommend spray carb cleaner to clean then silicone spray to lube. WD-40 or anything like it is to be avoided. They gather dirt and gum things up. Silicone spray is slippery but drys dry and doesn't attract dirt.