1979 merc 115 suddenly lost high RPM

[Rayford]

Help! I was out on the lake sunday and flying across the lake at high RPM, suddenly the engine began to slowly lose rpm, steadly until it leveled at about 2000 and now will not go above that. It starts fine, idles but will not exceed 2k so naturally will not go to plane. Is it something obvious? or is ther a logical progression of things I should check?

 

[Yepblaze]

Re: 1979 merc 115 suddenly lost high RPM

Compression test to see if it scored a piston. (major engine damage)<br /><br />If they all check out about the same, start looking for failed ignition components. Run on hose and ground out one plug at a time looking for the one(s) that don't drop RPM. Sometimes though an ignition component may check out OK till it has run and warmed up a while.<br /><br />If all those check out ok then start looking towards carburation. Or fuel delivery problems.

 

[JB]

Re: 1979 merc 115 suddenly lost high RPM

Howdy, Rayford.<br /><br />You are probably down a cylinder or two. Don't let a smooth idle fool you.<br /><br />Disable the spark on each cylinder, one at a time. The one(s) that don't cause the engine to slow are dead.<br /><br />Check the spark on the dead cylinders. If they have no spark, do some part swapping to see if you can get the problem to follow a part.<br /><br />If they have spark, squirt some fuel mix into the carb(s) for the dead one(s). If the engine picks the cylinder(s) back up you have a fuel problem. Overhaul that carb.<br /><br />Good luck.

 

[Rayford]

Re: 1979 merc 115 suddenly lost high RPM

OK went through the diagnostics as recommended, compression is 124-126, however in checking the spark I found no life in the middle plug of the lower bank of three coils, the plug was like new. <br />could that have caused the problem I described or is it just part of the problem?

 

[JB]

Re: 1979 merc 115 suddenly lost high RPM

You lost me, Rayford. "Lower bank of three coils" doesn't give me a picture.<br /><br />1. If the plug looks like new you may have water in the cylinder "steam cleaning" it. That would most likely be a blown head gasket. But if that were the case you shouldn't have gotten good compression there.<br /><br />2. If you truly have no spark to that cylinder, try switching the parts with another cylinder to see if the trouble follows the part. If the trouble follows a part, change that part.<br /><br />Good luck and let us know what you find.

 

[dingbat]

Re: 1979 merc 115 suddenly lost high RPM

This is an easy one. <br />Had the same thing happen to me back in December on the way back from off-shore. Replace your stator

 

[tbjtib]

Re: 1979 merc 115 suddenly lost high RPM

Hi,you may have a bad stator,there are high and low speed windings in it. My manual shows a resistance test depending on what type of stator you have. They are fairly easy to replace but expensive(around $200)

 

[Rayford]

Re: 1979 merc 115 suddenly lost high RPM

Thanks ..YEPBLAZE,JB,DINGBAT,TIMB<br /><br />Well, first of all thanks to all for the recommendations, I wrote em all down and added them to my list of things to look at fix or replace. I did all the summary visual checks and found the internal harness wiring decomposing and shredding so I rebuilt it completely, replaced all the fuel lines, hoses and generally tightened up all the loose hardware. I dismantled all the carbs and sdjusted per the manual and replaced the main fuel line and bulb. took about 6 hours but when I took it to the lake sunday the work was woth it. I wa able to make plane at less than half throttle and frankly wa a bit hesitant about going any faster. This really speaks to the knowledge base among the boaters out there. again!<br />thanks folks!!! <br /><br />Rayford