Yamaha 1999 VMAX Question (P150TLRX)

[ChickaYeah]

I have a new-to-me 1999 VMAX 150. Runs great, starts easily and idles fine at 6-700rpm. Recently I was cruising at ~5200 when it started to drop rpm's on its own to about 4000 (first time this has happened), I brought it to neutral out of concern and ran it up again to around 4 and haven't been much past 4 since out of fear of causing irreversible damage. I changed spark plugs and fogged her over the winter (plugs looked pretty dark with slight black residue), replaced water/fuel separator, and gearcase oil, but that's all. I have a feeling it is fuel related or dirty carbs/fuel lines that would have caused this so I plan on replacing/fixing things one piece at a time.

I am going to replace fuel filters, I have the small filter element that sits on the outside of the engine (part # 61N-24563-10-00) and will change that this week, but I am struggling to find the VST filter.... is there no VST on this engine?! Using SimYamaha and a service manual and can't find a mention of VST anywhere nor parts online for the p150tlrx, seems odd as it sounds like a pretty important system for an outboard.

Also considering seafoaming her through the intake, but I already use Startron and Ring Free and don't love the idea of so many additives in the system.

Any other troubleshooting suggestions would be appreciated as all of my local marinas are backed up beyond belief and can't spare a moment to take a look.

[TownsendsFJR1300]

VST's are used on fuel injected engines, not carbureted... So correct, there is none.

A parts diagram of your carb's for reference:
https://www.boats.net/catalog/yamaha...999/carburetor


Parts diagram for your engine:

https://www.boats.net/catalog/yamaha.../p150tlrx-1999

[ChickaYeah]

VST's are used on fuel injected engines, not carbureted... So correct, there is none.

A parts diagram of your carb's for reference:
https://www.boats.net/catalog/yamaha...999/carburetor


Parts diagram for your engine:

https://www.boats.net/catalog/yamaha.../p150tlrx-1999

That would make much more sense. Thanks!

[99yam40]

dropping a cylinder will affect the motors output.
compression or leak down test will tell if all cylinders are doing well.
spark plugs are the easy and less costly thing to try or an inline spark tester to make sure all are getting spark as they should.
not really easy to do while the problem shows up,
but an inductive timing light oved to each plug wire might help.
but you will have to recreate the problem while testing

if everything on those is good, then you can try to make sure each cylinder is getting the proper fuel to air ratio, which can be difficult to do