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1998 Merc 90 2-stroke dilemma

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SentientSkeleton

New member
Greetings,


A friend and I recently purchased a ‘83 Mako 171 Angler with a ‘98 Mercury 90 2 stroke. We did a sea trial and the boat ran fine.


On our first trip out post purchase, we were heading out of the inlet and I had given the boat some throttle after clearing the no wake signs. There were some bigger waves from an oncoming boat, and I’m new at this and still not comfortable so I backed down on the throttle before hitting the wave, at that point the motor made a strange noise and sort of bogged down. I also thought there was a burning smell, but that could have been from any number of boats in the inlet I suppose.


From that point on, we’ve had issues with the motor. The above might be unnecessary information but figured it could hold relevance to one of you with more experience.


The issue we’re having now is in higher RPMs. The boat starts fine, idles good hooked to a hose, but tends to sputter some when idling in the water for some reason. But the main issue or thing that is weird to me is that you can cruise out from anywhere from 1K-2K RPMs all day with zero issue. And then if you give it throttle, she’ll ride beautifully for all of 10 seconds at speed, before starting to choke out and slow down on her own. Then, if you lower the throttle and ride around at 2K RPM for another 10 minutes, you can open the throttle and take her fast again for another 10 seconds before the issue repeats. Sometimes, it’ll even stall if you back down to hard on the throttle or stay in neutral for too long.


Sorry if this is a long-winded explanation, just figured the more details might help one of you diagnose the issue.


Really hoping to get out some days during the summer when it’s calm enough to help clear my head and socially distance the best way I know how.


Definitely appreciate any of you taking the time to read this and provide any feedback.
 
Could be a clogged fuel filter in the engine, Port (left side of engine white tube 1" dia., 2" long)....cut it open and check contents.....report back. Or could be vent to fuel tank vent clogged.....engine's fuel pump sucking fuel out of the tank creates a vacuum on the fuel line and on into the tank if not vented. At high fuel removal rates the existing vent can't keep air replaced in the tank fast enough....apparently at slower speeds it can keep up. If you have a water separating fuel filter between the tank and engine it may need servicing.

If you cut your fuel filter open and find it full of tan hard material, some in the form of a semicircle, you need to replace the fuel line to the tank. If it has black bits in it you need to replace the fuel lines in the engine....and if that's the case you might as well put a kit in the fuel pump while you are at it....diaphragm hardens over time.
 
We think we fixed the problem. It’s running perfectly, unless there’s still some underlying issue.

We went to change the primer ball, and when cutting into the fuel line, my friend discovered it had some sort of inner liner of thin plastic. He said he never saw fuel line like that (and I’m still a googan when it comes to these things) so he wanted to get some more ‘normal’ fuel line. We changed it out and now it’s running perfectly. Upon further inspection it did seem there was some kinked-up area that I suppose was sucking shut with the vacuum created.

New to this site too, but I’ll try to attach a photo
 
Yep the Mercury gray fuel line. Doesn't like ethanol fuel after 3-4 yrs. I've seen it (totally broke down with small pieces in vst chamber) or a flapper as u described.
 
And another thing I'm preaching is onboard fixed tanks. This is only my opinion and our shops opinion. After 2,3, or 4 yrs. of using ethanol fuel ( stabil or any fuel additives) fuel still. Makes water depending on humidity.. As it sets in tank. ( don't know if . y'all have seen YouTube video , of 2 mason jars set in front of a fan , 1 with ethanol 1 without in 90% humidity. Fan blowing over mason jars? If u haven't u need to watch) it collects water from humidity. So that being said. I've already seen this problem in like 3 Tritons bass boats . like 4-5 center console boats.2 stroke with hpdi systems ( Optimax or v- max yammys) but especially on 4 stroke motors. Runs fine one hose all day long. Water is setting at bottom of tank not mixed. Anti-siphon tube is 2 " from bottom of tank. Runs fin on hose! Put it in choppy water and water start's mixing. So now u have watered down fuel. Just trying to explain what I've seen and fixed.
 
Half of my answer is based on what you guys are talking about with the grey fuel line. I had the same problem on fuel lines when they crossed 10 years on ethanol and 17 years on gasoline; different engine, same year.
 
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