Help. 90hp Mercury engine - low power and only 4000 RPM.

[Whoaru99]

My 96 Merc 90hp suddenly started having low power and will reach only 4000 RPM instead of normally about 5500 at max throttle.

It's really doggy when trying to get on plane, but it starts fine and idles OK.

The plugs look normal but tried a new set and no change. The fuel filter had some junk in it that looked and felt like rubber particles so I changed the fuel filter and it made no difference.

When I had the fuel filter off (it's the in-line one under the motor cover) squeezing the primer bulb sent a good stream of fuel out of the line so it does not seem like a fuel restriction.

I've also checked that the throttle is pulling the carbs to full open and that seems fine too.

The engine doesn't seem like it's misfiring and does not seem to run poorly - except for the low power and low RPM.

Any thoughts on other things to check before bringing it to the shop?

Thanks in advance.

[Surface Tension]

When you said you had what looks like chunks of rubber in the fuel filter my first thought was you fuel hose is deteriorating. You might be able to squeeze the primer bulb and get a good flow but thats because your using pressure. A fuel pump sucks gas, that suction could be collapsing your hose.

Replacing the fuel line is cheap and something I would try before bringing it in. If you aren't replacing the fitting you should back flush those as well.

[eurolarva]

You may only be getting fuel on one carb. If you have spark tested all the cylinders and verified you are getting spark on all cylinders try choking one carb at a time when up to full throttle. ( Might want to do this with two people on the boat and be prepared for the surge) You can manually choke one carb by covering the air intake with your hand. Because of the black chunks you mentioned you may have dump in the carbs causing the motor to run lean.

[BEMIDJIJOE]

When the motor is running try squeezing the ball. If this helps it means you have a restricted flow or a bad pump. I had a similair thisng happen with my Merc 75.

[Whoaru99]

Well, I took apart the whole fuel line and found nothing wrong. The in/out check balls on the primer bulb are working, the hose itself is wide open with no apparent degredation inside, and I can easily start a siphon of fuel from the line that flows at full diameter.

Used a multimeter and checked the ignition coils, trigger coils, charging coil, and stator coil. All were just about dead center of the resistance measurement specs.

Put it on the water and took it for a spin, same thing, low power and only about 4000 RPM. I pushed in the key switch to engage the fuel enrichener/primer and that made it fall down even more.

I'd have thought if it were lean, it would have picked up some hitting the enrichener. On the other hand, it does not seem too rich becasue there is little smoke and it starts easily as in not flooded. Also, there is no heavy smell of gasoline or wetness/slobbering anywhere.

Still seems run pretty smooth at 3000-4000 RPM, but between about 1000 and 2200 RPM is does shake a lot and sound maybe like misfiring.

Got her hitched up and ready to take to the local Merc dealer.

Any more ideas before I haul her away?

[eurolarva]

Pull the spark plugs and look at the tips. If you find one that is clean that is the bad cylinder. Hopefully if that is the case it is a blown head gasket. Sounds like one or more cylinders is not firing or getting water.

[JohnMickish]

Do a compression check and I bet you won't like the results. I had a 90 Merc that did the same thing. Middle piston went south.

[Valv]

Quote:

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Do a compression check and I bet you won't like the results. I had a 90 Merc that did the same thing. Middle piston went south.

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Mnfishingguy is right on, compression is not good on your motor, check it with a tester before taking it to shop.

[Whoaru99]

Oddly enough, a compression tester is one of the few things I don't have (not really an engine mechanic, but I did stay at Holiday Inn Express... ). Electrical and hydraulics are more my thing...

Anyway, got sort of fed up messing around and hauled it to the dealer.

Before going though, I did pull the plugs and they all looked about the same, all were just a little wet looking, but none particularly cleaner than the rest. Also looked into the cylinders using a flashlight and the piston (as best I could see) all looked about the same and had a light coating of carbon on them. Didn't appear to be any holes at least from what I could see.

I'll post back what the dealer says.

Any opinions on the 90hp's in general? Personally, I was never really keen on a 3-cyl engine, but it seemed to get along pretty well until about a week ago.

[marine_man]

The 75 and 90 Hp Mercs are pretty bulletproof in my opinion.. the 115 leaves a little to be desired in my book.. but all in all still a pretty bullet proof motor.

Let us know what they find...

marine_man