TOPIC: '71 Merc 500 cuts out at WOT

Hi. Title says it all, my 50 HP mercury 500 seems to die at wide open throtle. It seems to have just started happening and is now happening more and more frequently. It doesn't happen at low speeds, just when I get it up on a plane at WOT or close to it - I hear it start to starve for fuel (I think) and then die. It starts right back up when this happens. Seems to happen more when it's warm, and it seems to be getting worse. Pumping the bulb seems to help.

I think that what's going on is that the fuel pump is nominal / starting to go out... but I'm not sure.

I'm not a mechanic, so I'm wondering if their are other things I should check myself before going to the boat mechanic?

I looked at another thread on this forum, but I'm not sure if it was exactly the same issue. Any help is greatly appreciated.

One more thought... when I say it seems to be happening more and more, it may be that it happens more the closer to empty the tank gets. When it first happened, the tank was almost empty and I thought it might have just been the angle of the boat as it went nose up getting up to a plane - thought it couldn't pick up fuel.

I put it in the shop for a new impeller, change the oil on the lower, checked carbs, compression, and installed new plugs. Mechanic said all was fine. I had warned him the fuel tank was almost empty and he used the shop tank.

I mixed up a new tank, and went on vacation to a nice lake. All was good for an hour or so on the fist outing... then it happened again and it seems to happen with increasing frequency (as the tank get's lower?) Now, the tank is almost empty again and it happens as soon as I go to WOT - still starts back up and cruieses just fine nice and slow, putting around the marina and such... but try to go to WOT and it dies.

Someone on the other board made a suggestion to me. That if the primer bulb helped the situation that I check the fuel lines from the motor to the tank. Is there a pick-up tube in the tank? Mabey check that to see if there is a good connection.
Just a thought. Hope it helps.

check the fuel pump diaphram.replace it while you are there. check to see if your air vent is open

Sounds similar to the problems i was having with my 1350...which turned out to be a clogged inline fuel filter.

Id start by making sure your gas takes are spotless on the inside...including the pickup. Double check all your fuel lines..replace anything questionable. Replace any fuel filters you might have. Might not be a bad idea to tear the carbs down and give them a good cleaning if it hasnt been done in a while...could be crap floating around in the bowls clogging the main. Make sure your floats are adjusted according to factory spec.

Okay - I do have an inline fuel filter and I haven't checked that - I'll replace that and check the lines and the tank, and see if that helps.

Thanks for the info.

Another silly question - is it "safe" for me to contiue to operate this motor with this condition? Like if I change the filter and mix up another tank of gas, I can go test run it again and see if it's helped? Is it still okay to use at low speeds with this condition until I figure out what's up?

Pacific Mariner wrote:

Another silly question - is it "safe" for me to contiue to operate this motor with this condition? Like if I change the filter and mix up another tank of gas, I can go test run it again and see if it's helped? Is it still okay to use at low speeds with this condition until I figure out what's up?

I think you'd be OK at lower speeds...or for quick tests. The only way your going to figure out if you found the problem is by trying it.

Obviously if it starts to crap out at WOT, dont keep trying to replicate the problem over and over. Sounds to me like its starving for fuel...two stroke motors require the oil in the fuel mixture for lubrication...run it out of fuel and your running out of lubrication at the same time. Combine that with the high RPM and it could be ugly if you keep doing it over and over.

same thing just happened to me last weekend out.
noticed a fuel leak around inline filter after a stop to shore,,
i have since fixed this but have not ran it again....

mine was cutting out at WOT, but seemed to do so when I hit a wake/wave? never died once just cut out ..i'd back out of it...ease back into it..it was fine?

hoping it was sucking air in at the loose conection ...if you find another culprit please post.