40hp power loss on long runs

[newbie600rr]

I have a early 80's 40 hp merc that looses power on long runs. It runs great for the first 20 min or so then looses all power. at full throttle it will only go like 5mph. If I let it sit for a hour or so, it starts up and runs good again. It pumps water good and the primer bulb stays firm. Where should I start looking. Any info will be appreciated.

Thanks

 

[Laddies]

Re: 40hp power loss on long runs

It sounds loke your loosing a cyl. you need to check for spark when the engine is acting up. It probably is a coil or switch box.

 

[bowman316]

Re: 40hp power loss on long runs

Try disconnecting each cylinder one at a time, and seeing if if runs worse, without one cyl. If it does not run worse, that means the cyl is dead, or the coil is dead. I would say 80% chance it is a coil, 20% it is the cyl.

 

[newbie600rr]

Re: 40hp power loss on long runs

I took it out today and it happened again so I disconnected one ingition wire from spark plug and it would not run. Its a 2cy so it narrowes it down it what coil it is. every once in a while It would arc between the two posts that wires connect to. I will try replacing the coil unless somebody thinks its something else.

Thanks

 

[OldMercsRule]

Re: 40hp power loss on long runs

I took it out today and it happened again so I disconnected one ingition wire from spark plug and it would not run. Its a 2cy so it narrowes it down it what coil it is. every once in a while It would arc between the two posts that wires connect to. I will try replacing the coil unless somebody thinks its something else.

Thanks

Yer doin' a hit or miss with yer wallet.

Ya need to eliminate the problem fer sure.

Pull the plug on the non working side, [(both) ta make pullin' it easy now that I ponder it a bit]. I did this with me 25 horse Merc!

If it is electric start hook up a water hose ta save yer water pump.

First verify the hot spark side kicks both plugs, then verify that each coil sparks by switchin' the wire from the hot side from the switch box to the non sparkin' coil side.

If the coil is bad no fire, if not switch box!

Be sure to leave the wires in the factory positions! Da yup; Da yup: a gooooood ider fer sure!

Then ya know where ta spend some dollars.

Good luck. JR

 

[newbie600rr]

Re: 40hp power loss on long runs

I had a coil from an old motor so I put it on and it still ran funny. So I pulled the wire and plug and it sparked. Upon further inspection of the plug it had little dropplets on it. I would guess water. So I put a compression test to her and I got zero, not low, zero. I put it on the other cylinder and it looked to be around 150. This sound real bad. Is my motor done. This motor is on my hunting boat and there is still a lit of hunting left. Somebody please tell me this motor is not toast.

Thanks

 

[OldMercsRule]

Re: 40hp power loss on long runs

I had a coil from an old motor so I put it on and it still ran funny. So I pulled the wire and plug and it sparked. Upon further inspection of the plug it had little dropplets on it. I would guess water. So I put a compression test to her and I got zero, not low, zero. I put it on the other cylinder and it looked to be around 150. This sound real bad. Is my motor done. This motor is on my hunting boat and there is still a lit of hunting left. Somebody please tell me this motor is not toast.

Thanks

Something serious is wrong. JR

 

[bowman316]

Re: 40hp power loss on long runs

you can still use the motor, you just have a 1 cyl motor now, and have half the hp.
i don't know how you would fix the compression, maybe the piston is stuck, or you have bad seals in the cylinder. That is something for a mechanic to fix.