So I recently encountered an issue with my GSL-SE after getting a catalytic converter installed behind the presilencer. What was happening is everything below 4K on the revs was fine, then the ports would open and be okay until 5.5K where the engine would lose a lot of power and not want to go higher. Even keeping my foot in it the revs would reach 7k but not enough to trigger the buzzer (this is in every gear). Prior it had happily gone up to 7.5K or 8K with no complaints, it was even fine in the initial first weeks after getting the cat welded in. I tried removing the air filter to see what would happen at it helped a bit, now it dies past 6K to 6.5K and will hit the buzzer but it is clear the issue is still present. This dying is also accompanied by random small surges and a bizarre suction sound coming from the rear of the vehicle.

I know it is not the fuel pump or fuel filter as I replaced these with new units within the last year. Does anybody know what might be causing this? Shops near me won't touch the engine and the closest rotary shop is a good 4 to 5 hour drive away. Is this something I'm gonna need to make that drive for or is there something simple that I'm overlooking?

Thanks

 

Probably 5th and 6th ports not opening fully or carboned up

wire them open, spray carb cleaner through the intake (trans fluid works too) and take it for a rip

it will smoke until things clear up

 

84stock's post makes me think about the 5th+6th ports and your new cat. Did you perhaps delete the air tube from the exhaust? The exhaust pressure plays a role in opening the 5th+6th ports and if the air tube is missing then you would not have the required pressure to open those ports.

This is the connection to the cat, from my 83 GSL, 12A


This is the air pipe from my 12A, the 13B is prob similar...

 

There are a few things to check, since you mention the exhaust we can do that first...

Exhaust: there is a little hose going to the 6 ports, Tee a pressure gauge into that hose (boost gauge works), go for a drive, you need to see about 2.75psi, but more than 3psi, and you're loosing power.
Ignition: how old are the plugs? the spark plug wires should be under 16k Ohms in resistance (new is around 7k). cap and rotor should be replaced if they aren't excellent.
Fuel is next: the main filter should just be replaced. there is also a teeny little one in front of the pump, which can be cleaned/discarded.