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Discussion Starter · #1 ·
I have a carbureted Merc 150 with nearly 500 hrs on it. Over about the last 100 hrs we have been having unusual issues with the motor acting like it isn't getting enough fuel. However, it is internittent and only in an RPM range of 2500-4000. Ruled out water in gas tank, changed fuel filter, water sperator, anti-siphon valve, etc, and we have run ~100 hrs with the issue. The motor runs fine for the most part, but bogs down between those RPMs after the motor has been running for a while. For example, today we ran 30 miles at ~4500 RPM with no issue, when we backed down to start trolling, the motor bogged down until 2000 RPM, then would bog down when accelerating until 4500. After being tied up to a rig for an hour, the motor ran fine at all RPMs until we ran for another hour or so at 4500, then started bogging down when slowing the engine.

Anybody else had this issue, or have any idea what may the cause. It seems maybe the carburetor is having issues or perhaps electronic engine controls. Any thoughts would be appreciated.

Thanks
 

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Discussion Starter · #3 ·
no alarms, just slows down. then when it gets down to about 2000 rpm it runs fine. Then after it idles for a while, all seems normal until it runs at higher sppeds for a long period of time. go figure.
 

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Had a similiar problem once, crud in the tank would build up on the pickup tube screen engine would bog. When I slowed down(less vaccum on the fuel system) all the crud would fall away, when I increased the throttle crud would start to build up again and the engine would bog once more. Replace your primer bulb, possibly a bad check valve, get a good quality one from merc or yamaha.
 

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Does the alarm horn still work? Don't know if they have a vacume switch on the fuel system, but that or heat may be putting the power pack into a rpm limit mode
 

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Discussion Starter · #7 ·
alarm horn hasn't gone off, but the engine will still get up to speed, it just bogs down at the mid-level rpm's. when it is bogging down, I am able to push the throttle all the way forward, and it gets up on plane and runs fine until I bring it back down between 2000-4000 rpm. seems like an issue with either fuel line/screen or carb. it's just odd how it runs fine, but when it has been run on plane for a while it acts up, then is fine after idleing for a while. Thanks for the input.
 

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That is scary, mine is doing the same

I also own an 04 150 Merc. Saltwater and also experience the very same issue. I hope we can find an answer soon. I will try to clean my gas tank and will post the result for you to see "feeling green".
 

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The 2-stroke "effect" of 2-stroke motors where the exhaust ports pull on the intake and create all the horsepower doesn't really get going on those 150 Mercs until about 3K rpms. At that point the engine starts to breathe well and really starts to cook around 4K rpms. If the ambient temperature is 95 or above with the high humidity, you may just be experiencing a lag in horsepower. I have twin Merc 150 carb'd engines on my boat and these engines also experience the low to mid-level torque issue when I have them propped with 21 pitch props during hot weather. When I have 19 pitch props on, the problem is almost non-existent. Of course once the temps drop to below 90 degrees, I don't see the issue at all. Take this info for what it cost you and wait for cooler weather to see if it applies. You might want a smaller pitch prop during hot weather if it really bothers you..
 

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It could be fuel related

If your tank is aluminum the ethonal fuel will wash off built up varnish which built up over time. The alcohol in ethonal fuel will wash this off and it could create a problem with the pick up or in the carbs. If you have a fuel/water separator this will also restrict flow as the varnish clogs the filter. I changed filters several times until all the varnish was removed and haven't had any problems since.
 
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