Re: 88 , 150GT Johnson
Check the serial number on the top of the powerhead. The high performance 150's (GT/XP) were all stamped 150S on that top casting plug. The S in the serial number indicates that it is a GT/XP model which comes with the larger-throated 175/(early 200) carbs. You should see 1 5/16" throated carbs on that engine. The carbs have their size cast into the front lip of each carb. That S model engine will make 10% over a normal 150 hp from the factory, or nearly 165 hp. Won't help to jet those carbs up even more, unless you change heads, port it, etc. I'd follow up on Jonesg's comment and check the spark.