Merc 150 SW loses power above 4200 RPM's

[senator20]

I have a 2000 Merc 150 saltwater series (2 stroke) that has been losing power when I run it above 4200 rpm's or so. It started a month or so ago but wasn't consistent till now. Details and checks to date:

suspected fuel from the beginning, so I have sta-bil in the tank, about 35 gallons in there now (premium fuel)

plugs are new and old ones were clean

visible fuel lines appear solid

new h2o separator
new fuel filter

when getting on a plane it takes off strong, then at 4000 or so it will surge and lose rpms, bulb is still firm..

so, I wanted to test the carbs one by one by taking off air box and I was going to choke one by one to isolate, but when I took off the airbox this time, I noticed oil traces on top of air box and slight oil in carbs. could have dripped in when I took off air box. ran the boat with airbox on to see where oil is coming from and it is coming from the resevoir cap? seems to be on very tight..

all other gauges read fine, temp, etc...

It will hit WOT once in a while. I thought it could be a cylinder but wouldn't that have gone out and been done, not work on occasion?

Trying to think if there is anything else.. Fuel pump has not been changed..

Am I missing something? Doesn't feel like a power pack problem, but what do I know?

Thank you for your ideas.

Senator20

 

[jimc_LA]

Re: Merc 150 SW loses power above 4200 RPM's

I would first look at the oil reservois cap. If you say that it is leaking oil, then the reservois tank may not be holding pressure. If I'm not mistaken Mercs use air pressure to move the oil from main tank to reservois tank and finally to the oil pump. Even if this is not what's causing your problem, it may prevent you from doing harm to your motor from lack of oil injection. Either get a new cap or replace the reservios tank.

Good Luck.

Jim C.

 

[senator20]

Re: Merc 150 SW loses power above 4200 RPM's

Thanks, Jim. I don't know if the oil deal is related or not, but air into the fuel system is one of my suspicions as well, so it could be related. the resevoir tank was replaced in 05 during an annual check before the season but the cap could be compromised. I unscrewed the cap and resealed but still leaked. Only did at high rpm's, though..

Thank you.

 

[j_martin]

Re: Merc 150 SW loses power above 4200 RPM's

On the cap, they often develop hair line cracks, especially if overtightened.

On the fuel, I would remove the fuel line plug on the last carb in the chain, usually port side bottom, and connect a pressure gauge (steal one of grandma's pressure cooker if you need to) temporarily on a hose so you can read it when you're underway. If the fuel pressure drops when the engine balks, you're onto it. If it stays above 3 psi, something else is wrong. This gauge install is temporary. It is inherently dangerous.

The fuel pump is definitely suspect. A kit is only 20 bucks or so.
The pulse line (crankcase to fuel pump) must be clear, tight, and in good condition rubber hose, not soft.

If a fuel hose connector has been replaced with an aftermarket plastic one, that is suspect. I wound up just removing it and running the Mercury fuel line directly to the oil injection T.

For sure don't run this engine like this. Lean running will burn down the powerhead, making the fuel issue a mute point.

hope it helps
John