Good idle, but runs rough when throttle is opened

[RandyEvers]

I have a 1973 Mercury 20 H.P. 2-stroke Electric Start. This engine powers a 14' aluminum fishing boat. Motor has always run good, but last outing on the lake, the motor ran extremely rough when the throttle was opened past idle. Starts good and runs good at idle. Past idle starts to run rough and after about 1/4 throttle, opening the throttle further adds no more rpm. The obvious answer is the motor is nearly 40 years old--get a new one. But on a fixed income retirement, I just can't afford $3K for a new one. Any suggestions other than perhaps change the spark plugs (which is about the limit of my mechanical ability. I'm using the Walmart outboard 2-stroke oil for fuel/oil mixture--16 oz. per 5 gallons gas. Could I be using the wrong gas/oil mixture? Would this account for this sudden engine problem?

Any help/opinions would be appreciated.

 

[MacDaddy21]

Re: Good idle, but runs rough when throttle is opened

16 ounces for 5 gallons is too much...I think you should be running 50:1 mixture. which would be about 13 ounces of oil for 5 gallons. You probably need to have the carburetor and fuel pump rebuilt. Sounds like lack of fuel. Walmart oil is low quality. On those older motors I recommend running high quality oil through them. I use the Mercury Quicksilver 2 stroke oil in my 1979 Merc 40 hp. Its 8 or 9 bucks for a quart but it is much better for those older motors in particular.

 

[RandyEvers]

Re: Good idle, but runs rough when throttle is opened

Thanks for your help. I had hoped just changing the plugs would do it, but the more I think about it, I'm afraid you're right about the carb/fuel pump. $$$Ouch!!. Also, I looked at the owner's manual (on-line) and it says 12 oz of Formula 50 Quicksilver per 5 gal. so you were right about that too. I'll take your advice and go to the Quicksilver product.

Thanks again,

Randy

 

[MacDaddy21]

Re: Good idle, but runs rough when throttle is opened

the carb and fuel pump shouldn't be too pricey as far as labor goes. Parts for the rebuilds are extremely cheap. Maybe 20 dollars max. A Mercury shop will probably charge you an hour's labor, which could be anywhere from 50-100 bucks. The Merc shop in my town charges $65/hr. I do all my own labor though. Just giving you that information for reference.

 

[dlplost]

Re: Good idle, but runs rough when throttle is opened

try the new plugs first, and change to better oil, the wallyworld stuff is junk and will foul the plugs.
Even a 2 cylinder can idle fine on one, but will have no power in gear.

 

[Mi duckdown]

Re: Good idle, but runs rough when throttle is opened

Check for spark on both cylinders. Check the easy stuff first Compresion,spark, fuel.