Ok. I have a 2003 F150. It has around 350ish hours now. I bought it three years ago with 42 hours. I had to change all the fuel lines, all the filters, new injectors, etc when I bought the boat. It has been pretty flawless up until lately.

here’s the symptoms.

first start of the day is ALWAYS perfect. Absolutely starts and runs perfectly first time. Just tap the key.
I can run it hard to the first stop and it runs great. 2 minutes-2 hours it doesn’t matter, I go to start the motor and it will not start on the first try....ever. It will, however ALWAYS start on the second attempt. Even if I wait 5 seconds or 5 minutes. It starts immediately on the second attempt. Never on the first attempt. It’s like that the rest of the day. I can do nothing to the motor...don’t even touch the fuel lines. I can load it onto the trailer, then drive home. Come back to the lake the next morning, and it fires on the very first tap of the key.
rinse
repeat

Once i was fully trimmed and wide open, and it shut off. Completely. Tidal wave over the transom. Lol. Fired right back up and went the rest of the day....starting on the second attempt of course.

is this fuel line collapse issue?
fuel filter issue?
fuel pump issue?
ignition issue?
VST float sticking?

I don’t know how long it would take to be called a heat issue. I have been fishing every weekend and it has been cold. I think the engine is COMPLETELY cooled down within a few hours, still doesn’t start on the first attempt. Second only. The motor has no power loss whatsoever. Still jumps on pad and effortlessly gets my TZX190 to 60-61 gps. Idles smooth. And quiet.

primer bulb is never hard, but I’ve been told that doesn’t matter much on a fuel injected 4 stroke.

Any ideas? Anyone had anything similar with a similar motor?

Check/replace your fuel injector driver relay(s). I had the same problem with an HPDI, it would start fine first crank of the day then intermittent starts the rest of the day. Found out as they would heat up they would blink out and not fire the injectors. Replaced them and all is good.

I’ll look into that tomorrow. Thanks for the suggestion.

I'd want to look at the low pressure fuel pumps. The diaphragms begin to leaks in and causes flooding and the hard starting on the later starts vs fresh at the ramp. They don't last that long and should be replaced ever so often. Got mine at boats.net

Thanks! Are those the round looking things in the back of the motor, with three 8mm bolts holding a round plate over them? Bad description, I know. Ha. But there is two of them, one above the other.

Looking into it further, need the exact model you can find it here https://www.boats.net/blog/find-yamaha-outboard-serial-number
The F150 didn't come out till 2004 and are meant to rebuild I'd say since they are expensive! I'd look int checking them
Boat.net 2004 F150 , Video, DiagramTest Post

I have a 2005 F150 and had similar issues as you. I am not a boat mechanic but, the F150 is really easy to work on.

CADD had a good suggestion and one way you can tell if the low pressure fuel pump is an issue is to pull your oil dipstick and smell for gas. Typically, there will always be a faint smell of gas but if it really smells of gas then it is possible that you have gas leaking from the diaphragms as CADD said.

Another question is about the primer bulb. when you say it is never hard, does that mean you can't pump it to get it firm? I have heard, but never experienced a bulb going bad with the check valve leaking back.

however, I would highly recommend you look at the injectors and either replace them or have them cleaned. I bought my boat used like you and as part of my preventative maintenance on a new to me motor I went through everything step by step and lake tested after each step to see the difference. I changed plugs, VST filter, main fuel filter at front of motor, and had the injectors cleaned. (as well as upgraded the harmonic balancer as motors of 2012 and older can see failure and cause huge issues. If you have not done so, I highly recommend) I ended up purchasing a set of refurbished (cleaned) injectors online.
Result: Even though my motor was pristine and all the plugs, filters, VST canisters etc. were pristine and in like new condition and I had the "hard starting" issue as you describe. After replacing the injectors the motor starts up every time regardless of circumstance. I sent my injectors off to be cleaned and got a report back showing 1 injector clogged by about 10% and another about 2%. So even though all my filters were new and everything was clean a slightly clogged injector can be the issue. I now have the refurbished injectors installed and had the originals cleaned and keep in the boat as spares. The injectors are super simple to uninstall/install and cleaning services take a day or so. I would also change the front fuel filter at the front of the engine at the same time if you go that route.

Hope that helps you and good luck!

Had a 2002 VMAX150. So 2-stroke. Same exact problem you're having. All plugs clean and in great shape. Thought it might be corroded fuel lines but they checked fine. Lines not crimped/collapsed. Fuel filters all clean. VST float not sticking. Fuel bulb seemed to be ok (would get tight and hold pressure) but since I hadn't replaced it in several years I did that while it was in the shop. Cheap. And that totally fixed the problem. Fuel injectors would have been next on the checklist but simply changing the bulb is all it took.