Idle adjust nute RH or LH

[Cahhh68]

My 1600 is all stock 1968 manufactured and having studied and maintained the carbs for 29 years, I've always considered the idle adjust screw on the bottom of the nozzle shaft to be Right Hand threads. I've been plug reading for years and have found right hand tightening the screw on the rear carb always results in carboned plugs on the cylinders controlled by the rear carb, even when the screw is almost to max tightening. Front plugs are burning correctly, even when adjusting screws on both carbs are opened same number of turns (2). An answer would be appreciated to the question as to whether the screws are RH or LH on these carbs. I have always considered them RH threads, thus if looking at the carbs from below, the screw is turned clockwise to tighten or close or raise the nozzle. The throttle valves are synchronized as I have used the SK Synchrometer and set them so the drafting is synched at idle and synched at about 2500 RPM's and about 3000. All unplanned potential air ingress locations are tight. If the screws are RH, the only other reason I can guess the rear 2 plugs are carboned no matter at what screw setting, then the issue may be a float bowl over flow. But I smell no gas and see no overflow anywhere. Any ideas or help on resolving this fuel issue is appreciated.

[Gregs672000]

There are many here much more experienced with SUs, however a few things come to mind: Adjusting the screw on the bottom raises and lowers the nozzle. If your remove the domes etc you can observe the position of the nozzle and compare them side by side, as well as the effects of your adjustment, left or right. Needle position in the carb "slider" (?) will also determine ratio, as will the needle itself (could have worn differently). There could be some air leaking past one carb throttle shaft or elsewhere more than the other, requiring different adjustments. Since you have them synchronized so well it's probably not a valve adjustment issue or a couple of cylinders with low compression, but a compression check wouldn't hurt.

[JT68]

They are all RH.

[Bwk2000]

Typically both mixture screws are backed off 2.5-3 full turns(ish) when properly set. As for the plug fouling, I would suspect an improperly adjusted dog bone causing the back butterfly valve to remain a bit open at idle, the float set a bit too high or a worn metering valve (needle) allowing fuel ‘dribble’.

[Cahhh68]

Thanks for the various insights. I will use them all.

[Cahhh68]

Domes removed, confirmed jets are adjusting properly with the adjusting screws, throttle butterflies are in sync, dog bones equalized. Then removed float bowl covers. Both floats float. There is a significant difference in the angle of the float valve tab when laying the covers upside down. If the photo can be seen (attached) the cover on left is from the front carb which is the carb that is performing as designed, confirmed by plug reading. The cover on the right is from the rear carb that is burning too rich no matter what setting the jet level adjust screw is. An adjustment to the rear carb tab to match the angle of the front one is my thinking but based on the front float tab logically allowing the float to rise higher would be allowing more fuel to that carb although it does not. Any guidance relative to this would be most appreciated. Another issue with the higher angle tab (on right) is that the hinge pin is rounded over on both ends like a rivet. The pin on the left carb is free to be removed. I suppose the right carb captive pin could be filed on one end to be removed. Is there a recommended tab bending procedure other than just pushing/bending it down without removing? I should probably remove the float valve and check it.
This troublesome carb being the rear one, and this being a 68 1600, there is the reed valve fitting that allows for extra fuel to be routed back to the fuel tank. Inspection of the reed seems that all is properly in place and blowing it allows pressure to open it although it is a very stiff reed. Could this reed element cause a problem that would enrich the rear carb? Thanks for any insight to these issues.

[Cahhh68]

Reconsidering the reed valve atop the rear carb, I'm unable to blow through that valve when the float valve is closed. Not knowing what pressure is required to open the reed valve, my lung pressure does not reach the minimum to open it. Could this be the problem with rear carb always being too rich? Perhaps the fuel pump generates enough to open the reed when float valve is closed?

[Cahhh68]

Dug out Gordan's Garage notes and found the answer then looked through the Datsun factory manual and found it there as well. 14 to 15 mm is the measurement where float just touches the float lever.

[Cahhh68]

Found a good float lever to replace the bent one and replaced float valve and noticed improvement but still rich. Today lowered the jet needle about 1/32" and that brought more improvement but still not coloring the plugs on those two cylinders same as front two. From here I'm expecting very slight adjustment in idle adjust screw and/or lowering the jet needle maybe 1/64".

[drieseck]

Cahhh68, thank you for taking time to keep us posted with your questions and findings (& photos), very helpful and appreciated. And, all the community "experts" offering suggestions and tweaks, good reminders and trouble shooting for everyone. Really looking forward to seeing if the 1/64" makes it perfect. You address two of his suggestions, I'm still curious if you found Bkw2000's: "the needle could be worn differently" or if they look even? I just finished cleaning/replacing a series of Solex Jets and was surprised at several worn differences.

[nismou20]

I’m most likely sure this is not your issue but when I first acquired my car I could not get them even. After much head scratching I found that I had 2 different needles. Also, it’s always good to pull the domes and check the butterflys at idle position visually to see for certain the valves are at equal opening. Sounds like your fine tuning is working. Good luck.

[Cahhh68]

Update on rear carb richness: Jet needles look very good - no visible wear and straight. They were at even heights before I made the first lowering adjustment on the rear one. I've proofed out most all suggestions and have a few left but yet to readjust the rear needle down additional 1/64" as the speedometer went crazy while test driving for next plug color checking after the first lowering of 1/32" that resulted in a little improvement. Received today a replacement yellow 16 tooth speedo pinion and will will install it (attempt!?!?) soon after Christmas. Hopefully by January 1 I'll be back on the rear carb detail. The pinion arrival is like an early Christmas gift because it will allow me to get back to tweaking the Cahhh. Gotta have a challenge. Merry Christmas to ALL.

[Cahhh68]

After lowering the rear carb jet needle a hair more than 1/16" now have plugs very close to color matching from front two to rear two. Years ago when I began this venture into color tuning carbs, I read somewhere on line from I believe a dragster forum that checking plug color must be done immediately after stopping from a run - no idle time can be included to have a good check. I'm not a dragger but at the end of a few miles test drive I'll cruise into the hood at about 2500 RMP's in second, pull into the driveway and shut it off. If let idle for any short amount of time before checking the color the truth of the mixture will not be shown.