40 HP Merc Losing Power

[sprinter144]

I have a 1994 40 HP Merc 4 cyl oil injected. It starts and runs fine at idle and will run fine unloaded, however when it's in the water and I throttle up it picks up and then as you continue to throttle it dies. I've changed plugs, gas, fuel filter and sprayed the carbs. What next?

 

[snapperbait]

Re: 40 HP Merc Losing Power

I have had a similar problem on a 50 merc.. The needle was sticking in the seat in one carburator thus allowing no fuel to flow on that carb...Time to rebuild carbs... Spraying won't help...

 

[sprinter144]

Re: 40 HP Merc Losing Power

Thanks. I think down deep I knew it was coming to this.

 

[mellowyellow]

Re: 40 HP Merc Losing Power

my similar problem was caused by weak fuel pump<br />diaphrams. it's an easy/inexpensive rebuild to<br />do. I would do both.<br />good luck,<br />M.Y.

 

[sprinter144]

Re: 40 HP Merc Losing Power

Just found out that cylinders 1 and 2 have no spark. Cylinders 3 and 4 do. Suspected something when spark plugs I just put in were factory clean on 1 and 2 and some deposits on 3 and 4. Now what's the most probable cause?

 

[snapperbait]

Re: 40 HP Merc Losing Power

How did you determine that you have no spark?? Does your motor have two carbs??

 

[sprinter144]

Re: 40 HP Merc Losing Power

I have two carbs. I disconnected the spark plug wires. Then while turning the ignition key placed a screwdriver in each plug wire, one at a time. I placed the screwdriver close to the engine block to see if a spark would result. Nothing on cylinders 1 and 2. 3 and 4 sparked. I also swapped 1 and 4 wires (along with what I believe to be their coil) to see if it made a difference, only 4 sparked again. So I now believe the wires are OK and something else is wrong. Unless the engine is supposed to start using only 3 and 4 cylinders. I'm new at this and don't have a manual and will be ordering one, but tried to follow advice from others on the troubleshooting to help narrow down the problem.

 

[aeronutt29]

Re: 40 HP Merc Losing Power

If you swap the coils and get spark on 1 & 2, then you've narrowed it down to a bad coil. If you still get spark on 3&4 only, try swaping switchboxes. You will eventually find the part that is causing the problem. Consider yourself lucky that your motor has two of everything. It gives you a "limp home" capability when something fries!

 

[sprinter144]

Re: 40 HP Merc Losing Power

Did all the resistance tests on Stator and Trigger and found only 1 item out of spec. The high speed coil resistance was 100 ohms and by the manual it should be 56-76 ohms. I also switched the stator wires (blue with blue/white and red with red/white) and still only got spark at cylinders 3 and 4, so I'm going toward it being the switch box. I'll give all this info to the Marina before I spend $250 on a new switch box. Any other ideas?

 

[sprinter144]

Re: 40 HP Merc Losing Power

Put a new switchbox in today and it runs fine. Thanks for all the help. All it cost was $250 for a switchbox and $30 for a manual. Gave me some confidence and I really feel like I know a lot more about my Mercury.