Get the most out of your Mercury 25 hp EFI 3 Cyl 4 stroke
My Merc 25 hp EFI 3 Cyl 4 stroke has always been lacking in power to get my boat up on plain. This is how I fixed the problem. This motor is on a heavy 14 FT fishing boat, with floor, live well, power tilt, elect stare and etc. It currently will go 22.5 mph with a 530 lb. load in it.
1; Replace you gas line going from tank to motor. The ethanol in the new gas breaks down the inside liner of gas line and starves the engine of gas. This may actually get worse through the day as the temp get warmer. The new line should say it is good for gas with ethanol in it.
2; Get the new Merc. 4 Blade spitfire prop. This really helped. Get the same pitch that you are currently using or one size greater. Your engine should run between 5500 and 5900 RPM at full speed with your normal load in the boat. I am running a 9.3 x 9.5 Spitfire prop. and I am thinking about trying a larger pitch. If you do not have a tac, get one like this,
https://www.amazon.com/Motormeters-...fu2p6khjvr8qj_ttl?_encoding=UTF8&s=automotive .
3; Put a 1000 ohm (1 K Ohm ) Resistor in series with the air intake temp. sensor (MAP sensor). You could use a pot like I did, this way you can adjust or turn off the resistance when you want. This makes the engine think it is about 50 deg. F colder air than it really is and will make it run richer. By doing this my time to 20 mph went from 26 sec. to 20 sec. with the spitfire prop. This may not sound like a lot but when in waves going against the wind it will really help. This did not increase my top speed as my motor revs at about 5850 RPM at top speed anyway. If your engine is lugging, you may see a higher top speed. I do not know if this effects gas milage, but I doubt it will have any meaningful effect.
Two things I did that had little or no effect.
1; Matched the intake ports. This was not worth the effort.
2; Drilled out the idle speed exhaust port to 3/8 if an inch.
My Merc 25 hp EFI 3 Cyl 4 stroke has always been lacking in power to get my boat up on plain. This is how I fixed the problem. This motor is on a heavy 14 FT fishing boat, with floor, live well, power tilt, elect stare and etc. It currently will go 22.5 mph with a 530 lb. load in it.
1; Replace you gas line going from tank to motor. The ethanol in the new gas breaks down the inside liner of gas line and starves the engine of gas. This may actually get worse through the day as the temp get warmer. The new line should say it is good for gas with ethanol in it.
2; Get the new Merc. 4 Blade spitfire prop. This really helped. Get the same pitch that you are currently using or one size greater. Your engine should run between 5500 and 5900 RPM at full speed with your normal load in the boat. I am running a 9.3 x 9.5 Spitfire prop. and I am thinking about trying a larger pitch. If you do not have a tac, get one like this,
https://www.amazon.com/Motormeters-...fu2p6khjvr8qj_ttl?_encoding=UTF8&s=automotive .
3; Put a 1000 ohm (1 K Ohm ) Resistor in series with the air intake temp. sensor (MAP sensor). You could use a pot like I did, this way you can adjust or turn off the resistance when you want. This makes the engine think it is about 50 deg. F colder air than it really is and will make it run richer. By doing this my time to 20 mph went from 26 sec. to 20 sec. with the spitfire prop. This may not sound like a lot but when in waves going against the wind it will really help. This did not increase my top speed as my motor revs at about 5850 RPM at top speed anyway. If your engine is lugging, you may see a higher top speed. I do not know if this effects gas milage, but I doubt it will have any meaningful effect.
Two things I did that had little or no effect.
1; Matched the intake ports. This was not worth the effort.
2; Drilled out the idle speed exhaust port to 3/8 if an inch.
