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New Yamaha F70 low RPM's

R

RTolli

New member
Bought a new Starweld 16' with a F70. First time out with a 15 pitch prop couldn't get above 3200. Changed the prop to an 11 and the RPM's went up and I was able to plane an get to suggested WOT (5800). The problem I'm having is after trolling for a while with that motor I can't get back up on plane. The RPM's stay at about 4200. It has done this just about every trip out. First thing after leaving the dock it runs great then after trolling for while I can't get my RPM's up to get back. Any suggestions?
 
Does the weight load or the weight distribution in the boat change while you're out on these trips? That's the only thing I can think of that could explain this behavior. If it was fuel-related, I don't see how it would start off fine each morning. If it were prop-related, the rpms at WOT shouldn't change over the course of the day.
 
Nope, stays the same, even try to make sure it's evenly distributed. Mostly just 2 guys too. I'm starting to think I may have an air leak in my fuel lines. The engine mounted fuel water separator was only half full of fuel when I got back last time. Priming the bulb filled it back up. Is it normal for air to be in the top half of that clear housing?
 
As far as I know, the filter should be pretty much full all the time. A good test would be to prime the bulb after you notice the lower rpms at WOT, then see if the rpms rise when you bring it back to WOT.

That wouldn't confirm that there's a leak, but it will certainly eliminate other non-fuel-related possibilities (such as spark, ignition timing, bad prop, etc.).
 
Haven't had a chance to get back on the water yet but I tried an experiment. I primed the fuel bulb several days ago, it filled the filter housing and the bulb was rock hard. Several days later without running the boat I checked it and found the bulb soft and air in the top half of the housing. I primed it again to where the housing was full of fuel and the bulb was hard, the disconnected the fuel quick connects at the motor to see if I could isolate the air leak if there was one to either the motor side od the tank side. Lone behold a day later there is no air in the housing and the bulb is still hard. So....are my quick connects faulty???
 
Wow, that's surprising, but it certainly sounds like a leak of some kind.

If your quick connects were faulty, I would think you'd see fuel leaking out somewhere or there'd be a strong smell of gasoline. But it could just be the valve in the bulb is leaking. When you disconnected the hose, I assume you left the quick connector attached to the end of the hose? If so, and the bulb stayed hard while the hose was disconnected, then that would mean the quick connector is fine (because it won't allow fuel to run backwards if it's fully closed). So maybe when it's connected to the engine, the quick connect valve opens, and then fuel starts running backwards through the leaky bulb valve. Maybe try just replacing the bulb/hose assembly?
 
The bulb was actually the first thing I changed. Nothing changed. I think the problem may be when the connectors are pushed together they aren't sealed tight. I don't know. I'm going to bypass it and see what happens.
 
Bypassed that quick connect fitting and the same thing. We did notice the fuel level in that clear housing drop all the way to the bottom edge of the filter, so the fuel level was actually below the filter. I'm not sure if the motor could get enough fuel with the level that low. Priming it fixed it and I was able to plane. Could my fuel pump be under powered to suck through a yamaha 10 micron fuel/water separator and the smaller engine mounted one? Do I really need 2?
And would a lift pump help? Thanks.
 
No, you shouldn't need 2 mechanical fuel pumps, regardless of the extra fuel/water separator filter. I would take it off and pump the button on the bottom of the pump housing. If it makes a wheezing sound when you pump it, you should replace it (or at least replace the diaphragm inside).
 
Checked the pump, no sounds, seemed in good working order. I set up a tub with water to watch the fuel filter housing. After idling for a several minutes the fuel level creeps down to the bottom of the filter and with the hood off I can hear a slight knocking sound. Prime the bulb and the fuel level in the housing goes up and the sound goes away.
 
Even at 7500 feet?

I'm taking it in Monday to the dealer. I get the feeling that I'll get the "We couldn't find anything wrong with it" response though. I'll post back on here when I get it back.
 
The knocking sound might be the computer trying to close a valve that's stuck open. I too would reluctantly take it in to get looked at.
 
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