Mercury Model 40 4 hp ?

[JAG59]

I have a Mercury Model 40, 4 hp (serial # 9191451). it is made in Belgium and someplace between a 1976 and 1979, depending on which source you read.
I've only had it for a year. It has always been a temperamental engine (which I understand is common with this model). Lately, it has gotten worse, being way down on power. It will barely bring my 13' aluminum boat to trolling speed at WOT. I have read previous postings about making sure that the passages behind the idle mix control knob are clear, making sure the idle screw is set right, etc. My problem: Where can I find a diagram that identifies these different controls and screws? Also, it sounds like the biggest problem with these engines is poor fuel pressure, due to a funky carb and gravity feed. My next question is: Can I remove the line from the tank to the fuel inlet, connect a remote can with squeeze bulb and overcome the inconsistent fuel pressure problem? I know why they called these things Gnats. This one is bugging the &#*@ out of me.

 

[Paul Moir]

Re: Mercury Model 40 4 hp ?

If you're only testing, that will work. Naturally if you stop pumping it'll soon die from lack of fuel since it's gravity fed. If I remember correctly, you need three lifejackets' height above the top of the engine to maintain the feed from a remote tank.

But go for the simple test first. Remove the line from the carburetor, open the fuel tap and see how fast the fuel runs out. Just a dribbling is fine, but if it's just a drip it's not.
If it drips, there *ought* to be a fuel filter just above the tank fitting, or the fuel tap could be screwed up. The fuel tap is often the problem and I'd check that first. Note what the firing ends of the spark plugs look like (ideally tan coloured insulator on each).

If that checks out, try a few things. Tap the side of the carb and see if the engine picks up. Pull the spark plugs and check spark & compression. Even a rough idea on compression is fine - aka the thumb over spark plug hole test. Well, it's kind of hard to get a thumb in there but you get the idea.

The carb can be cleaned very easily. The bowl is sealed with an o-ring so often you can pull it and set it up with no additional parts.

If the power loss is significant, it's not the idle circuit. The idle mix control is on the starboard side of the carburetor and should be linked to the "rich/lean" control on the front of the outboard. The link just pulls off and then you can unscrew/screw in the control freely.

My brother's Gnat has been the picture of reliability and congeniality this year. It must have been all the threats!

Helpful resource (4MB)

 

[JAG59]

Re: Mercury Model 40 4 hp ?

Paul,
Thanks for the helpful resource. It's always good to have a starting point.

 

[JAG59]

Re: Mercury Model 40 4 hp ?

I pulled the carb off this puppy to clean it right and found that it was clean as a whistle. After reassembly, I started looking for a bad spark. I was sure it was a fuel problem, but ... Anyhow, when I pulled the bottom plug wire from the coil I found that the wire was actually out of the coil socket even though this was hidden by the spark plug wire boot. I reattached the wire and tested the engine on the lake. It ran fine for awhile but then it was back to it's old ways. The only other thing that seemed logical was a coil breaking down. This was backed up by a different post and the good help of some other iboaters. It looks like I burned the coil out by running the engine with out the wire connected. No place for the charge to go. I put a new coil in and the engine purrs like it should. Lesson learned: Make sure your wires are tight and don't take anything for granted.
When dealing with electrical systems (and other systems, as well) if things aren't right and you continue to operate the engine, chances are that you are going to mess something else up.