Engine stopped high end on water

[Nolster]

1991 Force 90hp 2-cycle 3 cylinder motor has been working great until we were on the river and now it does not get high RPM or speed. Launched the boat and started cruising about 3/4 throttle. Throttled back for about 30-45 seconds and tried getting back to cruising speed but only about 5-7 MPH and 2900 RPM. Motor used to run close to 5000-5200 and 32-33 MPH.

Changed prop and hub, went through carbs and changed fuel lines and connectors.

Runs at 5200 RPM on muffs in the driveway. Launch into water, back to 2800-2900 RPM and 5-7 MPH.

 

[cyclops222]

Checked the sparkplugs ? All look perfect ?

 

[Nolster]

Checked the sparkplugs ? All look perfect ?

I forgot to mention, also changed plugs. They looked fine but first time with the marine flat style plug

 

[cyclops222]

Ignition works at high & low speeds before you switched to the flat type ? Still has high & low ignition.
Check the ignition timing with a timing light. Should generally be 0 to 8 degrees at idle speed. About 25 to 30 degrees at maximum throttle position. If not ? Flywheel to crankshaft KEY is broken. And you may be running very retarded when a load is placed on the engine in real water.
Has engine ever backfired ? Had a very hard slow engine cranking time ?

 

[Nolster]

Ignition works at high & low speeds before you switched to the flat type ? Still has high & low ignition.
Check the ignition timing with a timing light. Should generally be 0 to 8 degrees at idle speed. About 25 to 30 degrees at maximum throttle position. If not ? Flywheel to crankshaft KEY is broken. And you may be running very retarded when a load is placed on the engine in real water.
Has engine ever backfired ? Had a very hard slow engine cranking time ?

They were the flat style plugs and worked for 2 years until last weekend. Motor runs great on muffs but I would say nothing in water compared to what it used to. I will try to get my hands on a timing light. I don’t ever recall a backfire but I feel the cranking time is a little longer than it should be.

 

[cyclops222]

I am waiting for my marina mechanic to get free enough. To replace my broken flywheel key. The sure symptom of a sheared flywheel KEY is starting taking several seconds AND AND having to use the choke to start the engine in the morning. After engine warms up ? No choke is needed.
Anyone know what size & number of POINTS the socket should be ? Can me and a small friend force the nut loose wearing gloves ?
Can a pipe STRAP wrench hold the flywheel in place ?
What direction are the shaft threads ? Is a impact wrench used very carefully a good option ?
2 very old guys are going to do it.
How many pound feet of torque should be used ?

Buy a pair of oars ? as a back up ?

 

[Nordin]

Have you tried running without the hood on?
Maybe you have an exhaust leakage, check the rubber boot exhaust outlet at the back of the midleg.
As you can rev it up in the drive way but not in water.
In the drive way you do not have any back pressure from the water for the exhaust but in water you have.
There can be an exhaust leakage at the adapter plate between engine block and midleg too.

 

[racerone]

In gear and on muffs is still the same as neutral.----The prop does not need to do any work moving a bit of air !----Does spark jump a gap of 3/8" on each lead ?----Fuel pump diaphragm checked ?----And you can not find a sheared key using a timing light !

 

[topgun3690]

In regard to running WOT-in gear-on the muffs.......not a good idea. Using a test wheel/prop and a large test tank would be the way to go.......or put the boat in the water......the prop needs a load on it to keep things from "blowin-up".

 

[Jiggz]

Follow Nordin's advice first before delving any further. The symptoms points to exhaust leaks. So try running it on the water without the top cover and see if that'll make a difference.

 

[racerone]

Sounds like it is running on 2 of 3 cylinders.----Test the spark.------Tested your overheat warning horn ?----Test and post the compression values you have now after this " loss of power " incident .