1968 55hp - Rebuilt with Flat Tops - Impression

[atengnr]

Just an update on a post from months back. I had my motor rebuilt and later found out they used 1970s style flat tops. There was some concern here that the loss of the dome would lower compression ratio drastically causing poor performance.

Well, my inpatience and curiousity got the best of me and I went searching for a lake that was thawed. The engine runs very strong! Top speed was 29 mph, only 2 mph lower than what I got out of my previous 88 60hp. The acceleration is roughly 95% that of the 1988 60hp. I was amazed that the performance was so strong, compared to how it run before the rebuild (had the same accel as my 68 40HP).

I did notice it was a bit rough at mid throttle. Could this be low speed needle adjustment??

Also, throttle does not quite return to idle speed stop. Cable adjustment the issue?

Thanks.

 

[Joe Reeves]

Re: 1968 55hp - Rebuilt with Flat Tops - Impression

If you're speaking of pistons..........

Unless my memory is failing me, the 1968 55hp Evinrude/Johnson is a "loop charged" engine and incorporates flat top pistons.

 

[atengnr]

Re: 1968 55hp - Rebuilt with Flat Tops - Impression

If you're speaking of pistons..........

Unless my memory is failing me, the 1968 55hp Evinrude/Johnson is a "loop charged" engine and incorporates flat top pistons.

Ive got the pistons in hand and they were domed (minimally though). These domes are not like ones I see with the purpose of bumping compression ratio.

 

[Mas]

Re: 1968 55hp - Rebuilt with Flat Tops - Impression

I was told that the "domed pistons" generally help out the the smoothness of the engine...especially at idle...is this true?

MAS

 

[Joe Reeves]

Re: 1968 55hp - Rebuilt with Flat Tops - Impression

Atengnr... The 1968 and 1969 55hp loop charged engines, according to my books had the old style 3 ring pistons, part number 383650 which superseded up to part number 386251, and I do believe that the 383650 piston did have a slight dome effect to it. The 386251 piston (pulling on memory here) was flat.

The 1970 60hp loop charged engine used the new style 2 ring piston whereas the top ring incorporated a bevel as do the later engines and this piston was part number 384251 which also superseded up to part number 386251.

I assume that your engine was rebuilt with the superseded and available 386251 2 ring piston which would be the proper avenue to take. The new style rings with the top ring being beveled would more than make up for the compression loss of doing away with that small dome effect.

Mas.... The smoothness of the engine thing pertaining to that small dome, I've never run across that mention.

 

[iwombat]

Re: 1968 55hp - Rebuilt with Flat Tops - Impression

As someone who's put a few low-compression kits with flat-top pistons in harley shovelheads . . .

The low compression pistons actually make it run noticeably smoother at low RPMs.

Those are long-stroke 4-cycle engines. How that translates to the world of 2-strokes I got no idea, but there's my data anyway.

 

[atengnr]

Re: 1968 55hp - Rebuilt with Flat Tops - Impression

Atengnr... The 1968 and 1969 55hp loop charged engines, according to my books had the old style 3 ring pistons, part number 383650 which superseded up to part number 386251, and I do believe that the 383650 piston did have a slight dome effect to it. The 386251 piston (pulling on memory here) was flat.

The 1970 60hp loop charged engine used the new style 2 ring piston whereas the top ring incorporated a bevel as do the later engines and this piston was part number 384251 which also superseded up to part number 386251.

I assume that your engine was rebuilt with the superseded and available 386251 2 ring piston which would be the proper avenue to take. The new style rings with the top ring being beveled would more than make up for the compression loss of doing away with that small dome effect.

Mas.... The smoothness of the engine thing pertaining to that small dome, I've never run across that mention.

Thanks for the info. This is what I was told by the shop as well. So you think this motor is performing better than it did with the old style pistons?

 

[Joe Reeves]

Re: 1968 55hp - Rebuilt with Flat Tops - Impression

You asked: "Thanks for the info. This is what I was told by the shop as well. So you think this motor is performing better than it did with the old style pistons?"

Without actually checking the engine myself, I wouldn't know for sure BUT it should be. It all depends on what the compression is, spark that should jump a 7/16" gap at cranking speed (with the s/plugs removed), having the carburetors synchronized and absolutely clean, the quality of workmanship that went into the rebuilding, those sort of things. If all of these areas are as they should be, the new 2 ring pistons would be an improvement.

 

[atengnr]

Re: 1968 55hp - Rebuilt with Flat Tops - Impression

Thanks. I trust that the shop did a top quality job. They have been working on these motors since they were new. Im told they fitted the pistons to the bore. Also the motor has new wire harness, coil, cap, power pack, rebuilt carbs...