V6 185HP Piston failure

[orca]

Recently my Evinrude V6 185HP 1984 Detonated No. 6 piston. The motor was rebuilt, everything was checked, Carbys stripped and cleaned, Timing checked, Everything appeared OK. the motor avhieves 5500RPM so the prop pitch is OK. after about 1 Hours running No.3 and No.1 pistons have detonated. A mechanic thinks that the fuel hose may be too small for the motor and the motor is running lean at high speed. he reckons that OMC made a vaccuum switch for these motors that would cut the motor if the vaccuum was too high, has anyone else had a similar problem with these motors.

 

[Dhadley]

Re: V6 185HP Piston failure

Remember that if the piston is detonated, its not a piston failure. The piston damage is a result, not a cause. Since the reason for the first rebuild was a damaged piston due to detonation and now there's more piston damage from detonation, the cause of the detonation has not been found and corrected.<br /><br />5500 is OK but barely. That motor needs to be set up at around 5800 for max life. A lot of the set up will depend on which pack it has. I dont think this was a factor to detonation but its something that needs to be addressed at some point.<br /><br />The absolute #1 most important thing in any rebuild is discovering and correcting the cause of the first failure. Its also the #1 most overlooked part of a rebuild.<br /><br />If the pistons are starting to detonate after only 1 hour the cause should be very obvious as the motor should have only been run slightly above low speed range. Since the motor probably still runs have the mechanic start diagnosing the problem now rather than after another rebuild. That way no more new pistons will be hurt.

 

[rodbolt]

Re: V6 185HP Piston failure

hello the vacum switch was for sounding the overheat alarm if a fuel system restriction occurs. test it with a mity-vac pump or equivalent.<br /> good luck and keep posting

 

[orca]

Re: V6 185HP Piston failure

Thanks for the input guys, The big V6 OMC motors used 3/8" fuel hose rather than 5/16" that was used by the smaller motors, could this be a factor in detonation, my motor has 5/16" fuel hose.

 

[rodbolt]

Re: V6 185HP Piston failure

could be. suzuki 225's came with 6mm and all yamahas come with 8mm lines. but OMC recommended 3/8. but I would definately find the cause before butting it back in service, a quick way to test the vacum alarm is to install a vacum gauge at the engine filter and pinch off the supply line while watching fuel system vacum at idle. the horn should sound at about 6"Hg.