View Full Version : Mercury 50 High RPM problem. Sounds like rev limiter but isn't. 2 stroke.

10-27-2015, 01:34 PM
Hey everyone. New to the forum. After doing some research I can't seem to find a real solution to my problem. Mainly because most results bring up propeller issues.

1996 Mercury 2 stroke 50hp 3cyl serial# 0G346628

I'll start from when the problem occurred. I was out on the bay (around weedon island in FL), going full speed (around 32mph) on my scout 155 sportfish. Hit some shallow area which caused my motor to lift a bit. The trim stopped working. Thought I ruined the trim motor but it ended up being the trim motor wire disconnected from the main harness on the inside of the motor. Easy fix, But that jolt caused that wire to come loose I imagine it could have caused other things to happen. I did notice a bit of fuel puddled inside due to one of the carb gaskets leaking. So the inside had about an inch of fuel sloshing around the bottom.

It wasn't shooting a strong beam of water from the pee hole and the motor was stuck somewhat up until I got home. At the time I wasn't aware of the manual release screw for the trim motor and didn't assume it was such a simple fix with the wire. Idled it home. so I dropped the lower unit, cleaned out the water pump (there was a light amount of sand in there) Once reassembled it was peeing strong. Also ended up replacing that carb gasket when I got home.

So I take it for a test run. pulls strong out of the hole but once on plane, (20 mph) It starts to sound like it is hitting the rev limiter at about 3/4 throttle. motor bounces side to side. Go full throttle, same thing. It does rise in rpm's, but not fully and still bounces and will not go past 20 mph. Normal speed is around 32 by myself. So I thought I spun the prop. I went ahead and bought a new prop and hub kit since my prop was looking pretty rough anyway. Bought the same pitch and size. Get back in the water, same thing happens! Strong out of the hole, but sounds like its hitting a rev limiter at only 3/4 throttle and up. Could this be a timing thing? Maybe that original jolt knocked something else loose? I checked the wires and am not seeing anything else disconnected.

Some recent things I have done prior to this which came with great test results: New water pump repair kit, fuel pump gasket kit, spark plugs, carb gasket kits (cleaned carbs very thoroughly before and after this problem just in case. Main jets all clear and clean.) Installed a Sierra fuel/water separator (after problem occurred) and tested.

Inline fuel filter looks clean, carbs are clean, hoses do not seems weak or damaged (although I did have a problem with one of them before but replaced that a while ago)

Spark plug test: Full RPM under load and cut the motor. Checked the color. The middle one looks the best (dry, brownish middle) the top and bottom were not brown, but a bit more grey. Not very oily or anything though. Assuming since it pulls strong out of the hole that the plugs are good.

The next step I assume to check is electrical. What do you guys think? A friend of mine mentioned I should test the stator, trigger, and switch box. Do you guys have any information on testing these on my motor or have any other possible things to check?

Appreciate any help. Thanks!

10-27-2015, 02:31 PM
Check your flywheel key or could be a clutch dog in the lower unit.

10-27-2015, 03:48 PM
Check your flywheel key or could be a clutch dog in the lower unit.

Do you think this can be caused by the gear spline getting miss aligned when putting the lower unit back on? Like if I moved the throttle a little because it wasn't slipping right back in place? It shifts fine though. Just a thought.

10-28-2015, 02:35 AM
That probably is not the problem whatever you hit probably is. Your lower unit would not go back together if they were misaligned.