I just bought a 1974 datsun 260z about a month ago. It is stock for pretty much most of it, except its carburetors are holley 2 barrel. but i recently found that i was firing only on three cylinders. So i did the most basic thing i could think of; I changed the spark plugs.

I then found an increase in power i guess ( might have been in my head, cause i did not check anything with gauges) but i started to have a popping in the silencer, as though the engine was running rich. I tried to change the settings on the carburetor, with no luck. For the most part, I have a pretty good idle. But when i started to move, the popping got worse.

I then was told to check the distributor. but before i do that, I was told to check the valve clearance. So i did a valve adjustment (cold) using the repair manual settings of (E-.30mm I-.25mm). This did not help the popping and i think it got worse. I tried to adjust the distributor to the lowest setting, and this helped a bit, but i would have to control my accelerator and clutch very precisely. And I would not be able to ride the clutch.

I did a couple of trys at valve adjustment again once hot and once cold with settings i found on the internet. Still i was not able to fix the issue. I have now thought about my filters and i have put an order for a new air filter. I have changed my fuel filter already though. But my engine is still running bad.. probably worse than before.

Is there something i am missing? are my valve adjustments wrong? I think that is the issue, because the major problems started after I did my first adjustment. I have never done one before that, but I have gotten better now i guess. Any kind of advice would be very appreciated guys. I have not much experience with cars but i am very patient and want to get this right.. I can put up pictures and videos if you need them.. Kinda bummed out that i have not gotten anywhere in a week. and I am out of ideas.