No power from 20HP Merc at full throttle

[JFNStuart]

Dear All, <br /><br />I have just installed a 1981 blueband thunderbolt Mercury 20HP onto my boat (12 foot)<br /><br />The engine runs well at idle and will rev thru the range in neutral, however, when I put it in forward gear the engine will not rev above about 1500 to 2000 and will only power the boat to about 5 knots. This engine should be capable of making the boat plane. I have been told that it may be the carb, but I am unsure whether the prop hub is slipping or if the gear train is kaput. <br /><br />Has anybody had any similar experience.<br /><br />The fuel bulb does not seem to get very hard, but there does not appear to be any fuel leaking anywhere. <br /><br />Any help or advice would be appreciated, <br /><br />Cheers<br /><br />Stuart

 

[threecylinder]

Re: No power from 20HP Merc at full throttle

The first thing I would do is replace the fuel pump diaphram.

 

[Chinewalker]

Re: No power from 20HP Merc at full throttle

Sounds very typical of a motor running on one cylinder. 2-cylinder motors can "sound" very good running on one cylinder - that is until you put a load on it in gear and try to come up to speed... Run some tests to determine if you've got spark and fuel to both cylinders. <br /><br />Start and warm up the motor. Shut down. Remove one of the plug wires from it's spark plug, and ground the wire to the block. Attempt to restart the motor. If the motor doesn't start at all, it's likely the wire you removed was the one that it was running on. If the motor does start, but souunds and acts EXACTLY as it did with both wires attached, then you've removed the one that wasn't firing. Next step is to determine whether it's a fuel or spark issue...<br /><br />- Scott

 

[Daquet]

Re: No power from 20HP Merc at full throttle

I have the same Prob let me know what you find, I have replaced my Hose and had a tune up done on it when I took it out after the tune up it did the same thing,

 

[UNCC Grad]

Re: No power from 20HP Merc at full throttle

I have just went through the same troubles on a 40hp Mariner. After checking the fuel and electrical systems out, it ended up being a clogged exhaust port. Evedently the previous owner had used it for trolling a lot and caron deposits had choked the exhaust to 1/3 it's original diameter.<br /><br />The motor feels like new now and will really move my 16' aluminum tunnel hull.

 

[gramkaran]

Re: No power from 20HP Merc at full throttle

How did you clean the clogged exhaust port?