Help!! 2000 25 HP Yamaha 4 Stroke Issue - Cuts Power

[robertjw]

Help!! 2000 25 HP Yamaha 4 Stroke Issue - Cuts Power

2000 25 HP Yamaha 4 Stroke – F25ELHY
Manufacturer December 1999
Problem: Initially the motor starts and runs perfectly. After it warms up and it starts cutting down on the power. I back off the throttle and it continues to run, any attempt to increase and it wants to kill. One time I had the throttle full open and it ended up killing the motor, engine flooded. Timing of the event is very erratic.

After talking to a few technicians. Decided to attempt a few items myself before bringing it in.
Replaced: Fuel - Fuel Line – Gas Tank – Gas Filter

Friend attempted to troubleshoot – former Marine Mechanic:
Connected up a fuel tank (source) and gravity fed gas directly to the carburetor. Issue still exists.

It has been to 3 repair shops, 2 of them are Yamaha dealers - Attempted by the repair shops:

Carburetor Cleaned
New Fuel Pump
New Computer
Engine is not overheating.

Additional Background: The issue seemed to appear after I had to remove the motor from the boat. Ended up laying the motor horizontally. I did lay it on the correct side.

Opinion: Based on the way it is acting, it seems more electrical. I am thinking of trying the Thermo Switch. Apparently the job of the switch is to drop the RPM’s down to protect the engine. I and the shops have verified it is not overheating when the issue arises.

This issue has been going on now for close to 2 years. I am getting very frustrated… If anyone has any ideas or opinions. Maybe any additional resources. I would GREATLY appreciate any HELP.

Thank You!!
Sincerely, Bob

[greasyshaft]

Has the Thermo-electric valve been tested?

[rodbolt17]

find a real tech.

it goes like this.
suck
squeeze
bang
blow.
yes its that simple.

can it mechanically seal?
that takes care of the suck and squeeze part.
can it produce spark at the correct time?
can it introduce fuel in the cylinder at the proper A/F ratio?
those two take care of the bang.
can it exhaust?
that takes care of the blow.
that motor was made by Mercury marine all the way down to the Motorola electronics.

so test the cylinders sealing ability,leak down test is preferered.
then test spark.
then test CDI output at the failure RPM and load.
if they pass keep chasing fuel ghosts.
I have seen the odd small motor that the CDI fails to advance ign timing.

[robertjw]

I have not tested. I know I saw a procedure.
I will pull out and test.

Thanks for the replies.

I will try it all and let you know.

Thanks, Bob

[robertjw]

Forgot to mnetion.

They did try another computer - CDI. Issue still came up.

[rodbolt17]

splatter gunning only works for bird hunting.
find a real tech.

[99yam40]

I have not tested. I know I saw a procedure.
I will pull out and test.

Just what are you planning on pulling out to test?

[nitrousmikey]

run a spark tester on both cylinders and see if they are dropping out maybe the cause of the flooding. crank sensor could be intermittent as well.

[greasyshaft]

It's all made by mercury so anything could have failed

[almetelo]

F25 was also made by mercury like F30 and F40 were ?

[nitrousmikey]

where are you located?
Run a timing light to see if it is erratic.
I would scope the crank sensor.
I have personaly seen a bad coil back feed and cause havoc on a ecm.

[nitrousmikey]

one more thing a cold engine with a tight valve lash could be a hot engine with too tight to run.