1986 four stroke 9.9 kicker won't stay running

[brythefishguy]

I have a FT9.9ELJ that came on the boat that I just bought. The previous owner said he never used it he just had it there incase the main motor went out. I opened up the case and it looks brand new. I changed the spark plugs. replaced the fuel line and replaced the gas in the tank and it started right up. when I had it in my yard hooked up to a hose it ran really good and for over an hour. so when I was out on the lake I started it up and it ran good until it all of a sudden the RPM's dropped and it wouldn't stay running. I turned the motor off for a bit and it started right up and then the same thing happened again. When i went home i hooked it up to the hose and it ran no problem for well over a half hour? When it slowed down i looked at the oil pressure light and it was still green. Is there something simple i am missing? What would be my next step into getting it going?

[TownsendsFJR1300]

Crap in the carburetor that came lose and varnish.

Pull the carb, REMOVE all jets, inspect and clean.

The jets MUST be removed as some have sideways orifices (as shown below).

Here's a link to your carburetor (blow up and zoom in): https://www.boats.net/catalog/yamaha...retor-9-9j-15j


Pictured below is a low speed jet for a F8 I just got running. The jet was completely clogged, as was others (their very small):

[pstephens46]

Was this the first four stroke made by Yamaha???

[99yam40]

32 year old motor that was never used, and who knows if it was ever stored properly.
it could be anything.\
what kind of case was the motor in?

but if it will run on land but not in the water, maybe the back pressure of being sitting in water is what is causing it to die.

how long did it run while in the water before dying?

[panasonic]

it appears 1985 was the first F9.9 listed in the parts on boats.net....a surprise to me.

I agree with Townsend there is probably crap and varnish in the carb breaking loose.Maybe in the fuel tank as well if original.
Probably a good idea to change the fuel pump diaphragm as well.

Is the engine peeing alright Fish guy?

Yami I think he meant the engine cowl when he said "Case"

[rodbolt17]

both the fuel AND cooling systems need to be addressed. doing one without the other is simply stupid.