1979 Evinrude 85 hp throttling back

[Atreides]

I recently bought an old 1980 Blue Fin 18 foot runabout (My first boat big boy boat with a 1979 Evinrude 85 hp motor. BIIIIIIIG learning curve on this thing. Anyway...

The boat will now run WOT at 42 mph on the speedo, but after about 15 minutes at WOT it suddenly cuts power back to about 35 mph and stays there the rest of the day. I have new spark plugs in and have checked them for fouling, running Sea Foam in almost every tank.

What could be causing this? Plenty of water in the tell-tale and sitting for two hours with the motor off makes no difference, 35 mph is all I get.

Thanks

 

[Atreides]

Re: 1979 Evinrude 85 hp throttling back

Anybody have any ideas?

 

[emdsapmgr]

Re: 1979 Evinrude 85 hp throttling back

Weak ignition components tend to fail when the engine reaches normal operating temps. Posssible that one of the cylinders is dropping out when the engine gets warmed up. Take an inductive timing light with you to the lake. When the engine slows, pull the cowling off. Check each plugwire with the timing light. The flashes in the gun will indicate if you have weak or no spark on each wire. That engine has two identical power packs which can swapped for testing purposes if you find weak spark on one plugwire.