loss of RPM's and knock

[Mtgbker]

While running back to the ramp with my 1975 model 50 hp Evinrude twin cylinder RPM's slowed and after reducing power a knock was heard in the engine. After pulling plugs and checking compression the top cylinder indicated 145 lbs and the bottom indicated 115 lbs. Any thoughts about bearing or piston damage. The engine may have run up to 5300 RPM at times (seldom) and gas/oil mix is always 50-1. Columbia River spring chinook season is peaking and the fish cooler stay empty with the boat parked.<br /><br />Thanks

 

[Reel Appeal]

Re: loss of RPM's and knock

Sounds like you may have overheated.Check the water pump(impeller) for damage.

 

[DJ]

Re: loss of RPM's and knock

Mtgbker,<br /><br />Look at the spark plug, very closely, of the cylinder with the low compression.<br /><br />Any signs of metal deposits?<br /><br />Either way, I think I would remove the head and have a good look inside the cylinders.

 

[Mtgbker]

Re: loss of RPM's and knock

Your suggestions sure help. I installed new plugs just before this incident and they looked like normal burn for the compression check. I changed plugs because the choke seemed to quit choking even though the electronic choke was moving the choke linkage. I didn't check for full travel. I used a squirt of ether to start

 

[Hooty]

Re: loss of RPM's and knock

Lose the ether. It's real bad for two stroke engines. Not only does it not have any lubricating properties, it washes the oil off the cylinder walls and bearings. Use a squirt bottle of premix.<br />I'd pull the head and see what's happening.<br /><br />c/6<br /><br />Hooty

 

[Chinewalker]

Re: loss of RPM's and knock

With a 30psi disparity between top and bottom, your motor is coming apart - whether on its own or by wrenching it apart. Wrenching it apart will have a better chance of getting her healthy again. If you let it come apart on its own, then you'll be looking for a new motor...<br />- Scott

 

[Dhadley]

Re: loss of RPM's and knock

What brand and type spark plugs did you put in?<br /><br />Just curious....

 

[Dhadley]

Re: loss of RPM's and knock

Just wondering how you made out.......

 

[Yepblaze]

Re: loss of RPM's and knock

I think you said the choke was beginning to not choke much anymore. <br /><br />That would be a sign of fuel starvation which could in turn cause piston failure do to lean condition under power.

 

[Mtgbker]

Re: loss of RPM's and knock

plugs are NGK B7HS. Sorry for the delay. Have to make living too. I worked with the choke and found that the solenoid is weak and not fully closing off the air. <br />I plan on pulling the power head and opening that up. When I started the motor yesterday the knock was still there at idle and no cooling water. Impeller must be trash and overheating caused the problem. <br />Any idea on parts source and parts manual?<br /><br />****

 

[Hooty]

Re: loss of RPM's and knock

Here's your source for the factory shop manual and parts list.<br /><br /> Kencook <br /><br />There's lots of places for parts on line. Here's one to try. He's pretty reasonable<br /><br /> Rickracer<br /><br />c/6<br /><br />Hooty

 

[Mtgbker]

Re: loss of RPM's and knock

Thanks to all the replies. You are all a wealth of assistance and guide to honest and reasoned diagnostics.