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Engine Cutting and Surging at High RPMs

kaydano

kaydano

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I was out the other day, and noticed the engine cutting out and then speeding up between 5700 rpms and 5800 rpms.  It would hit 5800, then drop back to 5700, then speed up to 5800, etc.  I did this about 3 cycles per second or so.  It only did it at WOT.  I thought I might have been hitting the rev limiter, but I don't know what happens when you bump up against that exactly.  5800 rpms is the upper limit on the motor, but I've been right up to 5800 many times and have never had this happen before.  All other conditions were the same as many other times I've been out on the boat.

Anyway, this happened after burning maybe 5-7 gallons of gas off a full tank, so I decided to unscrew the gas cap, and a BUNCH of air rushed in.  The engine didn't pulse anymore after that.  So, I assume it was a fuel starvation type thing going on due to the vacuum in the tank from gas being used but not being displaced by air.

Anyone had this happen?  Also, I thought there was a charcoal canister under one of the seats with a hose that vents to the atmosphere under the boat.  Is this a "one way" type of thing where pressure in the tank can be released through the canister (it filters fuel vapors - an emissions thing), but a vacuum will not pull air in through it?  Or do I have some kind of problem going on here?
 
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I am not familiar with the specifics of your fuel system, but fuel caps used to allow air to be drawn in to keep this from happening.

I guess i would start there, check the cap to see if it is damaged, or had a mud dobber building a nest in it, etc.

I also have to ask, when you filled up, did you 'over-fiil' it by chance? Sometimes overfilling can flood the vent system.
 
Maybe something in these pics will help you
 
Thanks for posting all those pics Derrick.  Everything you posted has a "pressure valve" in the system, so I assume it's a one-way valve that lets air out but not in (or why have a valve in the first place).

Pretty sure my system has the valve right at the filler neck.  I remember several years ago, when fuel wouldn't go in, the vent line had a "p trap" in it unintentionally just due to the way they wire-tied everything up at the factory.  Once I smoothed out the slope of the vent line and removed the P trap to let the gas drain back to the tank, fuel went right in.  Long story just to say I'm certain the valve is at the filler neck...

I do fill it quite full before storing for the winter.  My logic in doing this was it was only going to get colder from the day I stored it, so any air trapped in the tank/line would drop in pressure and suck more humid air in (which I don't want).  Plus more air allows for more condensation, so I fill it quite full, to the point that I click the fuel handle over and over to add just a little bit more.  So, some could have easily gotten in the vent line and flooded the canister or something.  I've only had the boat out 3 times since taking it out of storage, and I had not opened the gas cap until after the motor started acting up.

Anyway, all your comments help.  I'll just watch it and not fill it as full and see if it ever happens again.  I'm feeling a lot better its not a motor problem, since the motor has been running just fine at WOT since I took the cap off, plus the comments here.  Thanks guys!  I'll sleep better tonight, and not worry if I'm going to have to call for a tow while out on the lake.
 
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Well if you do I'll pm you my number. Nothing worse than being stuck on a giant mud hole,
 
Ha ha. That's right! Thanks for your number. I will pm you mine. Call if you ever need help.
 
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