Reduced power at WOT

[Wingedwheel]

I have an '89 Mercury 40HP, ser# group 9570793 and above. my problem is when I first got this motor I could take off from the dock and it ran OK but seemed like it didn't have enough power. I could stop and sit for a few minutes and when I took off again it would have the power I expected. The last time I took it out, same thing and even stopping and cutting the motor off didn't take care of it. As I was powering back in it was like someone flipped a switch and suddenly I had full power. I'm assuming this could be an electrical issue but I have no idea where to even start looking. Anyone ever had a similar issue or know of one like it?
Thanks

 

[Yesterday]

Re: Reduced power at WOT

yep, sure sounds electrical. I would first look at the feeds into the coil(s), then from the trigger to the switch box. Actually if you go to that trouble, might as well inspect ALL wiring and replace anything thats corroded or exposed. Also is there moisture getting to any of the wiring?

 

[Wingedwheel]

Re: Reduced power at WOT

Everything looks like new under the cowling .No corrosion or any type of indication.

 

[CharlieB]

Re: Reduced power at WOT

4 cyl?

First thing to look at is the spark plugs, are they carboned up bad? If surface gap, use a green scotch-brite and carb cleaner, between the two you can make a big difference.

While the plugs are out it never hurts to verify a hot blue arc of 7/16 inch necessary to reliably ignite the fuel under the increased pressure at full load.

If you do not have an adjustable gap spark tester I believe a search of this forum will turn up plans for a simple DIY spark tester.

 

[Wingedwheel]

Re: Reduced power at WOT

Plugs are new and gap is correct. I didn't check the spark but I have a tester. If it shows a weak spark, what am I looking at next? What's the next step?

 

[CharlieB]

Re: Reduced power at WOT

If you find a weak spark or no spark on one cyl then download, study, and follow the clear directions in the Ignition Troubleshooting Guide

ISSUU - CDI Electronics Practical Outboard Ignition Troubleshooting by CDI Electronics

The usual troubleshooting sequence is;
Check for a hot blue spark at least 7/16 inch
Compression test
Test Fuel pump output
Clean and adjust carbs

Fuel pump should pump a big pulsed stream at least an inch and a half the diameter of the hose i.d.
Second pump test is temporarily install a clear hose to the carbs, run the motor and watch for any air bubbles passing thru, indicating air sucking in somewhere before the pump. A common fault is the O-ring in a quick disconnect hose fitting. It will seal when pressurized by the primer bulb but fail when a vacuum is induced by the fuel pump.

REMEMBER to remove the clear hose once finished testing, clear hose is NOT rated for fuel, only as a very short term testing tool.