If it is HOT where you are this crazy ethanol gas could be evaporating in the carbs if it is doing this when after the first run of the day. I ALWAYS ran a fuel/water seperator on the six hammers I have owned! Never had a problem with water in the gas using them, cheap insurance. If your Yamaha is also a 91 and the fuel pump diaphrams have not been replaced I would go ahead and change them, once again this ethanol gas is eating causing them not to last as long as they used too. Check all your fuel lines under the cowl for leaks or cracks. Normally an engine that is bogging down is either fuel or spark, from what you described Id start with the fuel system, I always replaced my plugs in my Yamahas every two years. Most of the time they still looked like new (I always ran Amsoil and ring free) but once again cheap insurance, a set of plugs from NAPA was like 12 bucks. If you have an EFI a dirty 02 sensor is a likely suspect, and of course an HPDI I would take in for a ECM diagnostic, and a fresh high pressure filter (since you described lots of smoke I ASSUme its not a HPDI) Like I said, Ive had six Yamahas over the years, never a mins problems out of em with GOOD routine maintenance. My last one… well my last one was a HPDI, 100 hours or so hours on it are not enough to really say, the feller that now owns her is loving it. The one before that was a 97 carb Vmax 150. I fished out it in Jan 06 and it had 600 hours on the meter then. The guy that owns it is just now changing fuel pump diaphrams, nothing ever done to that engine but routine maint. Trust me if it can happen to a boat motor it WLL happen to me LOL. Id be willing to bet if youve taken care of yours its just something simple. Two trouble free four strokes on “family owned” pontoons between these boats, we still have one of em. It gets used by anyone in the family who wants to take it out IE: it may as well be a rental unit. I put a water/fuel seperator on when I rigged the engine, not a seconds problem just change the filter every six months. Good luck, hope this helped some?
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