1998 mariner 150 efi hard to start

Never had any problems starting it until the last 2 trips out , last night it started but appeared to be running really rich , I tilted the motor up for 2-3 hours and I finally put the throttle half way up and it fired right up and ran great the rest of the night . I started it 4-5 times after and had no problems . Any ideas as to why its acting up after sitting for a day or two ? I run stabil in every drop of gas and occasionally run mercury quickleen per the instructions on the bottle

Is there two yellow/red wires terminated on tha same terminal on tha starter solenoid?

Seems like its not getting enough airflow at start up and it's loading up on fuel , when I engage the throttle it fires right up , kinda makes me wonder if the throttle bodies need cleaning , looking at them with a flashlight the outsides are all gunked up with carbon and grime

Kinda sounds that way. IMO get away from tha Sta-Bil. Look for tha wires I described. Can't remember if tha '98 is that way or not. Most EFI's are. Its an enrichner circuit that engages whenever ya go ta start. It fires injectors (big squirts, tha longer ya grind tha more fuel is pulsed) every time ya hit start. Tha injectors probably need service, all of 'em......You probably have one or more actin' up.....Its something that needs ta be done periodically anyway with tha junk gas nowdays no mater what kinda gas treatment ya use.

Back to tha wires. If you do have both yellow/red wires trace 'em down. One should go to tha ECU harness and one should go to tha motor harness. Disconnect tha one going to tha ECU harness. This will disable tha "choke" effect of tha ECU. Now if you can find tha *yellow/black* in tha motor harness you can tie it to the yellow/red goin' to tha ECU harness and control tha enrichner (choke) function by pushing tha key in instead of it doin' it every time ya hit tha key........

*----It may be buried under tha tape at tha female part.

That's where I'd start but could be a few other things but most would effect tha overall way it would run.....

Just get your injectors replaced or serviced. Replace the, electric fuel pump strainer, fuel lines and manual fuel pump diaphragm. Every injector that I have had serviced, they said fuel line degradation caused the problem.

Update , went out today , boat fired right up and ran all day like nothing ever happened ... Dunno but if it ain't broke I ain't touchin it . It's never given me a single problem till it didn't start the other night ...

Oh and btw when I bought the boat from James Niggemeyer he had new fuel lines already put on , dual fuel filters and it has been maintained very well , I had the diaphragm replaced last year

With the crappy fuel nowadays, intermittent crappy running is going to be common occurances. Im going to start running 89 octane in my old merc, because it just dont like the weak stuff and runs crappy.