Dentonation (89 Evinrude 90 HP VRO)

[jcanker]

What can cause dentonation in just one cylinder? It happened just after I had someone rebuild the carbs (#3 piston) and now after the rebuild, it happened right away on the #1 piston. The ignition coils may have been swapped during rebuild. <br /><br />I thought at first that it might be bad gas, but why would that only affect one piston?

 

[Dhadley]

Re: Dentonation (89 Evinrude 90 HP VRO)

The cylinders do not run exactly the same combustion temp. Its possible for one piston to have damage before another. Although the others are not far behind.<br /><br />Describe your damage. Is the damage on the top of the piston or on the sides?<br /><br />What spark plugs did they install? Did they reset the timing?

 

[jcanker]

Re: Dentonation (89 Evinrude 90 HP VRO)

Dhadley to the rescue again! <br /><br />The damage to the piston is only on the top. Most of the combusted material flakes off in 1 inch pieces, except for the the area on the exhaust side of the piston head(the flat portion, not the sloped face.)<br /><br />We have not readjusted timing, but all timing and linkage was marked and taped and returned to the same position they were in. It was running on Champion UL77V plugs, which my Clymer manual says to use for "sustained high RPM use"

 

[Dhadley]

Re: Dentonation (89 Evinrude 90 HP VRO)

Damage like that is usually from an ignition source. It may be a bit hard to find if the motor isnt able to run now. If it still runs, you might degree the flywheel and hook a timing light to #3. Run it on the trailer under a load back it down the ramp) and see if that cylinder is double firing or advanced too far. <br /><br />Mark each cylinder at TDC, total timing and its number. When running you should see only the 3 with the light. If it starts picking up another number, its double firing.<br /><br />A bit involved -- I was hoping it'd be easier to find, like the wrong plugs. Those should be OK, especially if you were using them before. I still opt for the QL77JC4's.