Yamaha 98 C150 OB Starving fuel or spark?

Oh but of course it does not happen all the time making it hard to trouble shoot. The motor will run great all day, and whammo it is hard starting, having to choke it. I will give the best description as possible.
1. Cold start with choke and a little advance throttle, it will fire up, but I don't think it is firing on all 6. after a minute or two, it will start to race to 5000 rpm
2. Once warm and settled it will run great for hours, even with constant restarting for drift fishing, and will idle at 700 rpm. then WHAM. I have to choke the carbs till it coughs, open the choke and it will start, but not idle under 1500 RPM without stalling not easy docking under that sitch.

I changed Fuel/Water filter, and on board filter, with no signs of water, and fresh premium gas filled tank [BTW this is a premix setup]
I suspect not all 6 cyls are working when this happens, yes it runs rough when it does happen. I just don't know how to tell if it's fuel, or spark. where do I start, especially being it won't happen in my driveway [trailered boat] Thanks for any help.

Re: Yamaha 98 C150 OB Starving fuel or spark?

it kinda sounds like crank seals or some kind of air leak.if it revs up on cold start. then later. seal/air leak is somewhat sealed by gas oil mix.while running on hose spray carb clean on crank case halves if it kills motor or slows rpm you found leak. also spray carb gaskets.and try it's not easy spray under flywheel

Re: Yamaha 98 C150 OB Starving fuel or spark?

woody66912 said:

it kinda sounds like crank seals or some kind of air leak.if it revs up on cold start. then later. seal/air leak is somewhat sealed by gas oil mix.while running on hose spray carb clean on crank case halves if it kills motor or slows rpm you found leak. also spray carb gaskets.and try it's not easy spray under flywheel

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Thanks for the tip, I'll try that today ,, Mike