Merc 20 hp gas problem

[Gerald K4NHN]

My sons 1972 Merc 20 hp won't make WOT... It'll rev up in neutal real good... When I put it in gear it acts like it's run out of fuel when a load is on it... Like we've all heard before, "It ran well the last time it was out".. About 2 -3 months ago..

I took the carb off and cleaned it well. I've been in the carb and fuel pump several time over the many years I had it before I gave it to him. It had some junk in the bottom of the bowl and the hi-speed jet was closed off also with junk... I cleaned all the passage ways. I've checked the fuel pump and it's pumping well. I disconnected the line to the carb and pulled the plugs and turned the motor with the electric starter and it pumped LOTS of fuel..

Pumping the bulb when the motor is sagging doesn't help either.. The bulb stays hard. I can't find any air leaks. I set the float level by the Mercury manual. I even pulled the pick up tube from the tank to see if it was cloged up, but it was clean.

The fuel doesn't smell bad. Although it's about 2-3 months old. The motor sounds like it's just starving for fuel when you give it the throtle, like a stoped up jet...The plugs are new.. It doesn't sound like it skipping either, just starved for fuel...

I haven't check the compression, but with the plugs in, It takes a lot of effert to pull with the rope starter. It cranks easy and idles well... Am I over looking something?

Gerald K4NHN
Cayce, SC

 

[Plainsman]

Re: Merc 20 hp gas problem

Do the RPM's stay high, or do they drop as well?

 

[Gerald K4NHN]

Re: Merc 20 hp gas problem

The RPM's start to rev up and then the RPM just sag at maybe 1500 or so...There's no tac on a old 20 Merc like this, so it's just a guess at the RPM's...

I backed it into the water and left it tied to the trailer when I was tring it after the first carb cleaning. I've went throught the carb again tonight and will try it again tomorrow...

You can hear it trying to suck real hard through the carb. I may have to try and feed some fuel into the opening of the carb to prove that it is fuel starvation.

I think I'm going to pour out the gas and try a fresh tank of fuel tomorrow... I've taken care of this motor for years and this it the first time for this....Carb, fuel pump, coils, and water pumps, and wireing harness rebuilding have kept this old boy going in the past...

 

[Plainsman]

Re: Merc 20 hp gas problem

OK, I thought maybe the prop was spun. Sounds like you have a plan, keep us posted.

 

[Gerald K4NHN]

Re: Merc 20 hp gas problem

Yea, I've been down that road with the props before.. I carry 2 in my fishing boat because of where I fish..

When I had my son's boat I've even changed the water pump in the swamp so I wouldn't have to spend the night there.

 

[Motor Boater Bill]

Re: Merc 20 hp gas problem

I've had one cylinder drop out on my 20 and it seems a lot like what you describe--idles, but no power when you try to speed up. Are you sure you're getting spark? Might use an inline spark checker ($6 at Harbor Freight) if you've got one or look at the plugs right after you shut it off to see if one looks different than the other. Just a thought.

Good luck, and keep us posted!

 

[Gerald K4NHN]

Re: Merc 20 hp gas problem

Both plugs have good spark with the plugs out of the engine. I looked at the plugs after I got back home and they both looked the same.

 

[JUSTINTIME]

Re: Merc 20 hp gas problem

sounds like the float height is not set right and maybe the timing is off
might want to clean again and check fuel filters

 

[Gerald K4NHN]

Re: Merc 20 hp gas problem

Well I may have the "OLE Motor" doing it's thing again.. I went back through the carb again and didn't really find anything I could say "Well That's It" I did change the two adjustment that work the float level for up and down.. I had set it by the manual the first time. I went back to the orignal setting which were larger that the manual called for.

I did pour out the old gas and loaded up with a new fill-up and added my usual StaBil and RingFree..

I took it to the river and backed it in the water but left it strapped to the trailer. I poured the coal to it in forward and it really put the pressure on the transom and was really grunting and I could see the strain on the motor shock mount.. Not quite max RPM's but the boat was locked down and the motor was under the max strain I think that it could be under.. Occasionally it would cavatate and the RPM would increase some.

I lean to maybe the gas was the problem the last time and not the carb. There wasn't any question the fist go round the carb was in really bad shape.

I'll know more Thursday when we head down the river with several other guys for about a week fishing and camping.

Thanks for all the suggestions and support from you guys.
Gerald K4NHN
Cayce, SC