Hi, 

first of all i want to thank everybody on this forum for always pointing me in the right direction with my Z so far. I am still in the process of learning and understanding the car and trying to identify and fix as many issues as possible.

The car is a 280Z with a F54 L28 in it with P79 head and Ztherapy SU carbs (4 screw). Exhaust header is a Zstory Street into Zstory 2.5" full system. The carbs were tuned with the help of a colortune until it showed a bunsen blue (first 1 then 4). 
This resulted in the fuel mixture nut to be turned  a little over 2.5 revolutions down (measured both sides with calipers: 2.7mm down). As mentioned in an earlier post the idle is quite stable around 800rpm. 
There was some valve ticking from 5 and 6 which disappeared after valve adjustment according to the FSM (cold 0.008/0.010).

The car runs fine as far as i can tell (maybe burns a little too much oil => 3/4L on 1300 miles). Revs fine, sounds awesome, but i am still anxious that something is not right and that someday the engine will blow up.

Especially the fact that when i remove the oil filler cap and or the dipstick nothing changes. No stumble no drop in RPM, nothing. The valve cover gasket is new, albeit one bolt in the middle of the driver side is missing, but i cannot see any oil leak and hope it is sealing. I checked for potential vacuum leaks but found nothing so far. Also checked the pvc system which seems to work fine. The spark plug colors after some longer highway trip were as follows: 1. dark brown; 2. dry black; 3. wet brown; 4. brown with a hint of white at the tip; 5. same as 6. dark brown. Timing is 10 BTDC, mechanical and vacuum advance seem to work. With vacuum advance disconnected and around 3000 rpm it was at over 20 and with vacuum reconnected even further of the scale. 

My question now is: what could be the reason and a possible strategy to fix the issue with the non stumbling lady? 
Any help is very much appreciated.
 

If you're  running carbs, I am not sure I would expect much of a change with the oil cap on or off. Maybe someone else will confirm this

 No problem with the oil cap or dipstick removal. The only thing I ever noticed when pulling the oil cap was more valve train noise. The plugs are a bit too dark. I'd start with new NGK BP6ES's. A couple of things to check are, choke cable adjustment, sticking nozzles (either one will prevent the nozzles from returning to their top position. Try turning the mixture screws (secondary mixture adjustment)  to 2 1/4 turns and check the plug color after a few miles. If none of these suggestions work I'd look at the float adjustment, which is the primary mixture adjustment.

 The oil loss is likely from poor valve stem seals. Not a difficult job if one has the right spring compressor. These engines are very reliable. It would also be a good idea to take a compression test to see if theirs any problem with the valves or rings. The results will likely set your mind at ease.

Johnny, As eluded to above...

If you're running carbs, there's no effect when you remove the oil cap when the engine is running. That's only an issue for the fuel injected cars.

Carbs could care less.

Ok, thanks for the clarification. I thought that this is a generell behavior.
I will try with new spark plugs and recdo the carb tuning. 

As soon as i have time to go to my workshop (sadly not in the next 2 month) will do the compression test.

Regarding the sticky nozzle, i sometimes run into the problem that the front nozzle is stuck in the downward position eventough the choke is closed, a gentle touch (sometimes it is doing it by itself) will let it jump up. I had to adjust the choke linkage before, because both nozzles wouldn't stay up. Is there a special method how to bend those linkages to make sure that the nozzle stays up and is not binding? 

As to the valve stem seals: What would be the "right spring compressor" for the job?

  I doubt that one could adjust the choke linkage properly with the carbs installed. The flat bar linkage that physically pulls and pushes the nozzle up and down needs a little twist to ensure the attachment point on the linkage is parallel with its mounting surfaces. You may want to try pulling the nozzles down with the choke, cleaning the exposed brass surface on the outside of the nozzle and lube it with a bit of lithium grease before pulling the carb.

43 minutes ago, Johnny280 said:

As to the valve stem seals: What would be the "right spring compressor" for the job?

https://zcardepot.com/products/valve-spring-compressor-tool-l-l24-l28-l16-l18

 

 It doesn't look that difficult to fabricate. I have removed sp[ring retainers with two, long, square shanked screwdrivers. Put the tips under the cam and pry down on both sides of the retainer. It does help to have an extra set of hands to pull the keepers as well as face shields. Be sure to raise each piston to TDC and stuff enough rope or cordage thru the spark plug hole into the combustion chamber to ensure the valve doesn't fall down into never, never land.

I had stuck nozzles on a 240 I bought once. The previous owner had used small fuel line which hardened instead of the proper thin wall pre-shaped hose. I bought 2 from Nissan for a few dollars. Problem solved.

Ok, thanks for all the advice. I found some similiar tool available in my part of the world and sourced some elring valve stem seals (original Nissan ones go for 16$ per piece over here...). I will update this thread after the compression test and valve stem seals/spark plug change.

I finally had time to work on the car. Valve stem seals are done, new NGK BP6ES's installed + a wideband O2. Mixture ~2.75 rev down showed a pretty rich idle of 11.2-11.4. I went back to 2.5 revs down and now the car is idling at 11.8-12 when hot. Which seems to be inline with experience other people have with SUs on this forum - idling a little on the rich side. I still have to drive a little bit more and check the plugs. And write down the air-fuel-ratio for different conditions. 

If someone from Europe reads this: The "Ventilfeder-Montagewerkzeug" (valve spring removal tool) from KS Tools (150.0918) works like a charm for removing the valve springs.