Recent Boat Questions

mercrusier 3.7 1987. tach runs eractic after 3000 rpms. Replaced with new tach,same results. I replaced the points and had the alternator rebuilt since the last time the engine was ran and the tach was acting ok then. What do you think? Thanks

I have a 2000 model year Mercury Marine 2-cylinder 4-stroke 9.9 HP "Bigfoot" remote steer/remote throttle control outboard that has trouble idling.
Serial number is XXXXX Model 9.9EL 4ST. Max RPM is 5,500.
Made in Fond du Lac WI by Brunswick.
The motor is used exclusively in fresh water.

This trouble has been present since the motor was first activated from winter storage a month ago.
I've had the motor for four years. This is season #5.
Always ran great on 87 octane 10% ethanol gas that I religiously treat with Sta-Bil marine formula ethanol treatment/stabilizer.
Winterizing has always involved running the carburetor dry with the engine in water; changing the motor oil and lower drive oil; then cover the motor with a heavy plastic bag down to the drain plug so no rain, snow or blown moisture gets into the cowl area.
Motor oil is full synthetic 5W-30.
It is then stored outside all winter in freezing weather.

This spring when started the motor could not maintain constant idle RPM or remain running at stable RPM despite stationary throttle position at low-to-mid loaded-or-unloaded speeds---up to 2/3rds throttle.
At low-to-mid speed the engine tends to sputter, but generally remains running. RPM can fluctuate wildly despite static throttle position.
At unloaded idle the engine usually stalls.
At full throttle it seems OK.
Bringing the motor RPM down off full throttle typically results in wide RPM swings (despite constant throttle position) and an eventual stall.
It immediately restarts OK, but will struggle within seconds to remain at constant idle without stalling.

So I figure it's a fuel problem.
I first started the engine this spring with 87-octane gas leftover from autumn---although the gas was treated with marine fuel stabilizer. I've never had a problem before.

So I drained the carburetor; replaced the inline fuel filter, and got some fresher gas.
Problem did not improve substantially.
I totally disassembled the carburetor and intake. I did not see any sludge or varnish in the carb bowl or throttle body. All throttle parts and carb float mechanisms move freely. There was a little bit of crankcase sludge in the intake from years of use as the crankcase is vented into the intake, but not enough to suspect intake blockage.
Cleaning the carb ports, jets, and the intake surfaces---reassembling the carb and then running the motor with fresh gas has not greatly improved the situation.

I again switched gas to fresh 89-octane and ran the motor with a load for about a half hour--- varying the throttle speeds to see how it would run. It seemed to run better at mid-to higher speeds…no more variations in RPM and no sputtering.
But it still doesn't seem to idle without stalling.
I adjusted the position of the throttle rod to have it idle at a slightly higher RPM. That doesn't seem to help it avoid stalling.
The idle stop screw on the carburetor seems Loctited in place. I cannot tighten the screw to increase the idle speed. I'm afraid I'd only strip the phillips screw trying to overcome the Loctite.

There is no gas, motor oil, exhaust or cooling water leaking from anywhere, so I don't think there's a compression issue.

Is there something in the ignition system that could degrade over a cold winter which might affect the spark advance? Are there any sensors within the ignition system that might rely on optical or magnetic pickups which might be compromised from dirt or debris under the cowl cover? It almost seems like there could an ignition timing- or spark advance issue.

This motor is on the back of a pontoon boat.
It's a real pain to disconnect all the remote control cables and rods and get this thing off and bring it to a repair shop.

I'd rather spend a few dollars replacing components that might be suspect before I undertake hauling the unit to a repair shop.

The shift interupter switch on a mercruiser is that supposed to always go to the center after shifting into forward or reverse?