Ghosts at idle

1999 Merc 50hp 4cycle tiller...motor has been fine since I got it. Also has low hours(65) and has been "overly" well maintained. Last outing it suddenly started acting haunted. As soon as it was started the rpm started rising to 4000 in about 3 seconds elapsed time with the throttle still at minimum until I hit the kill button. The motor has an "auto start solenoid" that I think is engaging an idle solenoid that moves the carb linkage. In the pic the autostart is the bottom solenoid with the idle solenoid right above it. Both have wiring that goes to the opposite side of the motor connecting to the electronic controller. Any ideas how to diagnose what part is actually failing? Especially since they seem to be priced like gold!

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OEM service manuals are your friend.

Unplug the solinoid and make sure it's not stuck. Try it again. If it still does it look for a vacuum leak. A cracked hose, bad gasket or any small leak will give enough air at idle to make it scream. Spray cans of anything like parts cleaner to WD-40 on suspected leaky areas will change the RPM and are how to find a leak.

I can watch the piston slowly push out as engine speeds up. I'll try the vacuum investigation tomorrow and also see what happens if I unplug the autostart.

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Well I went over every split, joint, mating surface and bolt head one at a time with carb cleaner while holding the air baffle linkage for the carbs. Nothing seemed to affect rpm at all. The only vacuum lines come from a 4 line pull off that sits above the intake manifold and carbs. So I proceeded to unhook those 2 solenoids after watching the upper one slowly push open the front butterfly baffle which made the engine rpm rise steadily till I either shut the baffle linkage or hit the stop switch. Something like 750 rpm to 5000+ in maybe 3 seconds... More than a little nerve racking. I removed the bottom one(auto start) and checked the wires for breaks but both were good and I had continuity across the wires. Put 12 volts to it and nothing happened no push or pull, no click or hum of any kind. Started motor and watched the upper solenoid first jump out and motor raced. Manually grabbed linkage and slowed it down to 1000 to warm motor up again and after several minutes saw the solenoid trying to pull back but the only way to get it in far enough for a normal idle I had to assist it manually. Took it off and put 12v to it and it slowly applied. If I cut the power it tried to pull back a little but unless I applied pressure would not pull anywhere near far enough. Seems nuts that both have failed at the same time. Best pricing I could find for them was $185.88 and 97.36 at Boats.net. Now waiting for parts. Oh ya with both off motor acts like it should except hard starting cold. Fingers crossed.

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Did you check ebay? It should be a yamaha one. That is who made your motor.

nccatfisher said:

OEM service manuals are your friend.

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I have the OE service manual. Problem is it has NO Diagnostics.

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