Logo
FULL SITE MENU

Yamaha Z150 (2-stroke) stumbles - contaminated fuel? Something else?

B

ba58smith

New member
Sorry in advance for the long post - I want to give as much pertinent info as possible.

I haven't had this boat very long. It has Z150 (2-stroke) motors, one from 2001, the other from 2004. The last several times out, the port engine stumbles badly when I'm trying to get to cruise speed, but eventually (after several minutes) it stops stumbling and runs perfectly until something changes. The change could be that I stop for some reason then go again (all the time with the engine running). It could be that I go from smooth water to rough water. It could be that I make it all the way home with no more stumbling, but then it happens again the next time out.

The stumbling is quite severe at times. For example, 2300 rpm down to 1200 rpm for a half second, or even lower than that. More often, though, it's a drop of 3-500 rpm when I'm trying to maintain 2000 to 4000 rpm.

By the way, every component of the fuel system is separate for each engine - tank, Racor, fuel flow sensor, hoses, everything.

My suspicion is contaminated fuel, or a clogged fuel filter. In addition to the on-engine filter, there is a Racor S3213 filter / water separator between the tank and the engine. I never see any water in the clear bowl on the bottom of the Racor, but when I drain a little of it out of the filter's drain valve, I often see some very, very small bits of black something, but not a lot. (It's not magnetic, and my fuel tanks are plastic, so I'm ruling out rust, at least for now.)

BUT - if it's water in the tank, why am I not seeing any in the Racor's bowl? And isn't there a water sensor in the bottom of the on-engine fuel filter canister? If it's gunk in the tank, how does it ever start running perfectly again after it stumbles? It seems like the stumbling from gunk would be because of a clogged filter, and a clogged filter isn't going to unclog itself just by running, is it?

SO - I will replace the on-engine and Racor filters, and I will pump out a gallon of fuel from the lowest point in the tank after the boat has sat perfectly still for a few days. (It's on a lift, and it tilts backwards, so any water would find its way to the lowest aft part of the tank.) But I have a sneaking suspicion that it's not a contaminated fuel problem, because of the "BUT" paragraph above. What else could it be? A fuel pump? An ignition problem? I can't think of anything else that could make it stumble like that, for such short bursts, but over and over again, for a while, and then "fix itself" for a good long time.

I have the harness to connect to the engine and see the data on my laptop. Is there anything I should be looking at there? The problem never occurs at idle, or anything less than about 1200 or so, and it's very sporadic, so it might be very difficult to see any data on the laptop at the time the problem is occurring.

Thanks so much for taking the time to read this long post, and for any suggestions you may have.
 
Well, no one is jumping on this, but I'll continue to post progress here to maybe benefit someone else some day.

I changed the on-engine fuel filter (it wasn't especially dirty) and the Racor filter / water separator (no water in the bowl, and only a little of the "crud" in the bowl), then went out for a spin. Initially - for the first 10 minutes or so - everything was perfect. I even throttled back abruptly a few times to get the fuel in the tanks sloshing around. But then I went out into the ocean where it was moderately choppy, and the problem came back within 30 seconds of the boat gettling jostled around pretty good, and it didn't get any better all the way back home. It stumbled 200 - 500 rpm when I was trying to run it 2100 to 3300, for just a half second, but did it every few seconds, until I came back to idle to pull into my canal at home.

Today I'm going to pump out a gallon of fuel from the lowest point in the suspect tank and see what it looks like. I'll also try to get a look at the pickup tube to see if there's any obvious defect there. And I've ordered some Y-valves so that I can plumb the system so that each engine can pull from either tank, as a way to completely isolate most of the fuel supply system.

I will also run it for at least 15 minutes in smooth water to see if it's possibly a timing thing, and not a fuel-sloshing thing. Maybe an ignition component that's OK until it gets fully warmed up?
 
Pumped out some fuel from every low point in the tank and found no indication of water or gunk in the tank. Then ran the boat in smooth water for about 20 minutes with no stumbling, but then it started. Slowly at first - just 100 or 200 rpm and for less than half a second. Then progressively worse, until it was as bad as it was yesterday. So my theory is that it's something that doesn't happen until the motor is completely warmed up. And it seems like it may be getting worse - yesterday and today, once it started, it never went away, whereas on previous days, it would do it for a while, and then clear up.

Possibly an ignition component? Possibly a fuel pump? Anyone have anything to say on this topic?
 
I finished the fuel system plumbing upgrade, so now I can pull fuel for either engine from either or both tanks at any time. After that, I went out for a 90 minute test. Ran for over 30 minutes in smooth water without a hiccup, then took it out into some choppy water, and the problem started happening almost immediately. I switched to the known-good tank, and the problem persisted, even after running 15 minutes in smooth water.

Next thing to check - is there contaminated fuel in the on-engine fuel filter cup that somehow makes it past the filter when things get rough?
 
The on-engine fuel filter cup had nothing in it but clean fuel. I did discover that the "water in fuel" float switch was not functional - the float was not moving freely up and down the shaft. But that certainly doesn't have anything to do with my stumbling problem. So now I'm out of ideas, and it goes into the shop in two days.
 
Took it to the shop yesterday, about a 30 minute ride on the water. Was hoping the problem would happen on the way there so they could witness it, and hopefully get some computer data on it, but of course, for the first time in over a month, it didn't happen, even in rough water. They're going to fix a totally different issue while it's there, but I told them not to bother with the stumble, because I know they can't fix it if they can't see it happening.
 
My discussion with the guy at the shop raised another possibility - that the main oil tank level sender for that engine is acting up in rough conditions. Since low oil in that tank is supposed to put the engine in "limp mode", it's conceivable that the sensor is giving the low oil signal for very short increments while being jostled around in rough seas. However, I never hear the alarm, and the level indicator on the Yamaha digital tach never shows the level being below full - so that's probably not it.
 
Back
Top