2 stroke 25/40 loosing power after 10-15 min (when warm)

[ElBarto69]

Like mentioned, I have 1996 (?) 25QEO, deristricted to full 40 hp, its loosing power when gets warm. On wot and cruising speed, it slowly slows down, till i'm not planning anymore. If i shut it off, pump it ( prime it) the pump feels full, it starts from first two turns, then for a minute or two runs ok again, then again starts to slow down.
Carbs have been taken apart and cleaned, filter/water separator removed and hose from the gas tank put directly to the fuel pump.
The fuel line has been put directly to the gas in the tank ( eliminating bad fuel snorkel line and its connector).
What else I can do?
Any advice apriciated....
Kind regards.

[robert graham]

Sounds like a bad fuel pump….check/replace if needed….

[99yam40]

to tell the truth I have never heard of a 25 that was also a 40 hp motor.
how did you modify this thing to make it a 40?

[ElBarto69]

Its Italian version of 700 cm3 3 cyl. Engine, throttled down so it can be selled in Italy. Maybe its evwn 50 hp, dont care. They (Italians) dont need any license, insurance, papers what so ever for boats under 6m and under 25 hp. So the major manufacturers sell 40/50 hp engines throttled down to 25, and they know, every one will putt bigger nozzles in carburetors and remove brearhing restriction gaskets and will have full operating power. Same shit as 1000cm motorcycles with 25kw...

[99yam40]

interesting.
I would think not only jets but intake of air along with the fuel and probably exhaust would be modified too.
did they change the timing also?

since they had a 25 already designed, I do not see why they would detune a 40-50 to get a 25 with the extra weight.

Anyway, you will need to find out if one or more cylinders is loosing spark when the problem shows up.
or check with a timing light to see if the timing is staying in spec.

actually a timing light will show if spark is getting to the spark plugs when hooked to the individual plug wires.
if spark and timing are good ,then it has to be a fuel problem.

does this motor have an alarm system to notify you if an overheat occurs or low oil if it is oil injected?

maybe the motor is going into RPM reduction mode due to a oil or overheat problem

[ElBarto69]

Engine has "limp mode" security system which shuts down one cylinder if the oil level is too low, or is overheating. I dont have a strobe light since im on my holliday. Sparks are ok, the running feels and hears smooth like its suposed to. The water jet is strong, doesnt feel too hot, maybe like a warm coffe or really hot jacuzzi.
I'll try to get a fuel pump where I am, I hope to get it.
Can I rule out the pump problem so, that I make a 2% oil mixture and put it in a 1,5 lit. Plastic bottle and hold it above the carbs, so the fuel will be forced trough gravity to the carbs, like on a small motorcycle

The detuned 50's are sold becouse the real 25 hps would do any good on 5.99 m boats.
Every one will derestrict restricted engine. Its like on wild wild west.

[99yam40]

if the primer bulb is still firm while this problem shows up, the pump is not the problem.
has anyone tried pumping the bulb while the problem is there to see if it helps?
pumping it takes place of the pump, but then if the pump diaphragm has a hole it would flood #3 crankcase making it too rich

and to answer your question on gravity feeding, no that would not keep up . not enough pressure so not enough flow.

find a timing light.
timing being retarded would loose power
need to know if it is in spec and advancing as it should, and it will tell if spark is dropping

can you spray a mix of fuel and oil into the individual carb throats to see it that would help or at least tell you if one or more cylinders are running to lean?

does this motor have a choke or prime start enrichment?
Is it set in proper position?

also the tell tail stream is just and indicator that the water pump is pumping . it will not feel hot because it is before going thru the main part of motor

[ElBarto69]

I can squeeze a primig pump barely.
its not empty and squished like the air screw on the tank will be closed. So if thats the case, the fuel pump is ok?
I tried to pump it, but no change what so ever. Tommorow i can get a new fuel line, maaaybe a pump. Im in a holliday resort, no chance for strobe light here.
how could i spray a fuel mix into carbs? Doesnt have any spray bottle here. I can get a start spray (usualy ether) at a local fuel pump.
Choke - it has a small switch (lever) near the ignition key, under rhe throttle lever. if i activate it when the problem begins to occur, it doesny help, it gets even worse.
The tail jet - I lost a screw on the leg couple of years ago, which is ment for washing the engine with salt water (didnt tighten it properly when flushing the engine after holliday) and the next year, it felt out. Then, when reaching some specific speed, the cavitation sucked all ( or enough) water out of this unscrewed hole, not providing the water stream up the leg to the engine cooling flow. Then, the tail jet was realy hot, forcing the engine into the limp mode (rev limiter or one coil shutting down), so i can surely say, the engine is not overheating now (since i know how overheated engine feels on the nozzle jet).
the jet is strong, replaced the water pump last year (I wish my flow was so strong and long in my mid 40's...)

[99yam40]

any type of spray bottle should work.
ask the cleaning crew at the resort, they should have access to cleaning spray bottles
or find a store to see what you can find.

a plastic water bottle with screw on lid might work.
poke a few hole in the lid with a pin, clean water out, and add some premix fuel/oil, put cap on tight ,then squeeze when stuck close to the carb throat.

have you removed the cowling and silencer to watch what is happening in side the carb throats?
is there too much fuel or not enough fuel flow that you can see in any of the carbs?

did you try covering the individual carbs with your hand to see if they all act the same way.

you are going to have to find out if you have a spark, a timing, or a fuel problem

By brother in law has a 50 Yamaha I looked at for him one Sunday last year.
all I found was #3 carb had some debris in the main jet
it would idle but bogged when given throttle.
does not take much to cause problems

it is easier to find which cylinder has the problem before finding out what is causing it.

I am not sure what you meant by
" its not empty and squished like the air screw on the tank will be closed."

[ElBarto69]

"" its not empty and squished like the air screw on the tank will be closed.""
if you forget to loose the screw on the gasoline tank filler cap, the priming pump is sucked togeather due vacuum in the fuel cycle

[99yam40]

OK that makes sense, full of fuel and not flattened

Good luck, I am heading to my brothers to work on his generator.
will be back later.
let us know what you find out

[ElBarto69]

I took the air duct down, the carbs from I can see and know, looks like they are doing fine. I managet to get a movie, and will put it on youtube and post a link.
i tried to spray it with wd40 (burns quite well) when the power loss apeared, no noticable difference..
But then I could return to our camp at wot with full speed.
I will replace the fuel hosing from tank anyway, its some years old, replacing would not be a problem.

[ElBarto69]

Today I checked the charging voltage, thought it could be low... in fact, its over 30V, replaced the regulator/rectifier, same shit, over 30V..... wth?

[99yam40]

what are you calling charging voltage?
charge coil feeds the ignition system/spark
lighting coil feeds the RR battery

are you checking the peak voltage outputs of the charge and lighting coil with the proper voltage measuring devices?

[ElBarto69]

I started the engine and then took negative cable from the battery, so it was in the air... Then i measured the voltage on tha cables which would come to the battery. Over 30V, it gave ma a little shock...