good compression but piston looks bad on top

[pratt_man1]

I have a 1965 evinrude 40 hp motor. Took top spark plug out and it was melted bad. Took cover off so i can retap threads and found top of piston pitted and not shapped the same as the lower one. compression is still good. Do i need to replace it or adjust the gap on my new spark plug. Any thoughts of what caused this? Any help would be greatly appreciated.

 

[Joe Reeves]

Re: good compression but piston looks bad on top

I assume that the bad piston has a melted dome appearance, and if so, that is caused by pre-ignition.

This pre-ignition would be caused by either a small water spray entering the cylinder, points set wrong so as to create a too high spark advance setting, or a lean running carburtor (yes, a single carburetor model can have a setting that will affect only one cylinder).

If on the other hand, the piston does not have the melted appearance but rather has a huge waxy build up on it, that is caused by what is known as "lugging", This is cused by having a propeller with too much pitch which prevents the engine from hitting its top rated rpm when the spark advance is set to full and the throttle butterfly(s) are opened to full throttle.

 

[pratt_man1]

Re: good compression but piston looks bad on top

there is no build up of any kind. i dont have a manual yet. it is on its way. how do i adjust the point setting? should i replace piston? how can i tell if water was entering?

 

[Joe Reeves]

Re: good compression but piston looks bad on top

If the piston is damaged, it should be replaced (if the engine is worth it to you). Also check the cylinder walls for scoring/damage.

Water can enter via a failing head gasket, a slightly worn sealing surface on either the cylinder head or the block, loose head bolts, a failing exhaust baffle gasket or inner plate, etc. Water droppets are usually found on the spark plug and in the cylinder area.

(Point Setting Of Magneto Models)
(J. Reeves)

Set the points as follows. Have the flywheel key aligned with the fiber rubber portion of the ignition points. Adjust the gap so that a .020 gauge will pass thru but a .022 will not. Should there be any question of the points being dirty (touching the contact with your finger would cause them to be dirty), clean them with a small brush and acetone or lacquer thinner.

NOTE: Should the operating cam have a small portion on it with the word "SET" imprinted, align this portion with the fiber rubbing portion instead of the flywheel key.

Installing the flywheel and nut...... the nut must absolutely be torqued to 105 foot pounds, otherwise the flywheel key will shear and cause damabe to the crankshaft taper etc.

 

[pratt_man1]

Re: good compression but piston looks bad on top

Thank You so much. where can i find a new piston for this motor. i can find them for 1970 models on up. will that fit? The walls on old piston are fine the only thing wrong is the dome is burnt away. thats it nothing else. can it be used or should it be replaced? There is no sign of water entering at all.