'85 GSL stock everything. Got it after it's been sitting for over 20 years. Did everything I could think of to clean it up. Rebuilt nikki carb and balanced floats. Replaced spark plugs and wires, ignition coils, ignition distributor rotor and cap. Tested the fuel pump. Cleaned out the fuel tank and replaced the screen and fuel filter. Tuned in the timing and idle mixture. Compression is 90 psi.
Everything seems to run smoothly, except sometimes after it warms up, it will start misfiring until it dies, particularly at highway speeds. It's really inconsistent, except taking it on the highway. It almost always starts misfiring around 55-60 mph. Sometimes I can drive it around for like 30 minutes at 40mph and below without issue. Sometimes it will misfire at those speeds within 5 minutes of running.
If anybody could point me in the right direction, I would appreciate it so much

 

Welcome! Check to make sure there are no vacuum leaks, a very common one is the hose that runs from the # 2 anti-afterburn valve to the intake manifold on the back of the engine. It's good you were able to clean the fuel tank and replace the filters. Did you check the screens on the two fuel inlet banjo bolts on the carburetor and the screens under both the needle seats?

Here is the location of the hose that often gets a hole in it.

 

Have you rechecked the fuel filter to see if any sediment that may have remained in the tank or lines is accumulating in the filter? When you rebuilt the carb, did you clean the fuel inlet screens? I've seen those get clogged up and cause issues. Just a few easy things to check.

 

God bless beautiful people like you. I checked for vacuum leaks in the rats nest, but never would have thought to check here. I didn't respond immediately because the battery I recently bought was apparently bad and crapped out right after I posted this. I got it replaced, and then the intermittent issues I was having became permanent. I verified that it was not the fuel pump being faulty with a bench test, and discovered that it is not receiving power. I am now in the process of checking the fuel pump relay. I did check the fuel inlet screens, and while a little bent, there were no clogs and fuel seemed to pass through them just fine.

Fuel filter is new, and I have been checking it pretty much every time I run the engine. Fuel inlet screens were a little bent, but free of clogs. I now believe that the issue was a dying fuel pump relay. I'm going to try to bypass it to verify

 

Figured out my problem(s). The fuel intake line was pressed against the bottom of the fuel tank, starving the engine. After fixing that, I found that the cheap fuel pump was pushing at too high of a psi, which then started flooding the carb once I fixed that first problem. I replaced the fuel pump with one that regulates the pressure. One last problem seemed to be that the ECM was running the fuel pump at the incorrect times, and would shut off, then not kick on until the engine died from starvation. I worked around that with a fuel pump relay bypass.
Runs smooth as a dream so far.

 

Persistence pays off, congrats!

 

Great working finding the issues and fixing them!