I had many issues with a Volvo Penta Gi TBI engine when i first bought my Four Winns Vista 238.
Over time i got used to checking Trouble codes on the ECU with a paperclip and LED.
You will have to do some research to find out if this will work with your engine
before jumping straight into it, but if the ECU is a MEFI 1 - MEFI 4 (around yr 2001 and earlier) it should work. You will see the pictrure on the below link of the data plug - check this is the same; if so it should work.
Details are available here:
https://www.offshoreonly.com/forums/do-yourself-boating-budget/103342-make-marine-efi-code-tool-less-than-%241-00-a-2.html and there is also a list of codes on there so if any flash up you can identify them.
Of course this is all at your own risk and i would strongly suggest checking your Engine's ECU is the right MEFI Variant before proceeding. Also double checking all wires and connections before the procedure.
there are other places online that may help with this too - try googling "DIY MEFI Code Reader" or similar.
The TBI Engine was an absolute headache for me, and i say that as someone with above average knowledge of these things and also a half decent DIY Mechanic - I hope you have better luck than me.
Here is a brief list of possible reasons for not reaching 'normal' RPM, whilst some can be immediately and easily discounted, it may help to give you an idea of other things to check:
1. Fuel condition. Type and Octane possibly old fuel
2. Propeller pitch or diameter, damaged blades
3. Restricted fuel pickup tube or anti siphon valve Fuel System Test
4. Crankcase oil volume, high oil level can cause aerated oil and lifter collapse
5. Marine growth on hull and outdrive
6. Wrong gear ratio in outdrive
7. Restricted carburetor air intake (clogged flame arrestor)
8. Restricted exhaust system (broken exhaust shutters/flappers) in engine transom shield or drive
9. Poor cylinder compression Compression Test
10. Carburetor defective, or wrong type. - Also Mixture (MAP Sensor in your case!)
11. Fuel pump pressure and vacuum
12. Boat overloaded, improperly loaded, or improperly trimmed.
13. Engine Overheating
14. Engine timing and ignition system operation
15. Remote control cables and linkage for proper travel to open throttle plates fully
Hope you get it sorted soon.....