Re: "Newer" 25/35hp OMC compatibilities

[enrico-italy]

quote johnyrude200:

Question – I noticed the older late 70’s models seems to just have a carb swap to convert from 25-35, blocks appear the same on parts schematics.

Note: My answers are referred to European product line, but are suppose to be valid also for US motors:

quote :

What year did they stop allowing this? I ask because earlier in the year I saw both a 35 and 25 block that had been gutted, and the intake/exhaust ports were different on the two models, despite everything else being the same.

In 1984 the original 35 was derated to 30 and a new 35 came on… this motor has larger port holes (two oval passage instead of 4 circular) and bigger Venturi carb.
In 1977-1983 M.Y. the 25/35 block are exactly the same, and also the carb is the same: the 25 had a white plastic restriction on the Venturi.
Simply removing it ad enlarging the original 49D main jet to 59 D, the 25 carb is changed to the 35 one.

Note: The Powerblocks are identical between 25/35 but differents for differents M.Y. I.E.: The 1977/79 blocks have differents bearing size and inner exhaust flange from 1980 … and the 1981 /1983 blocks have different shift linkage holes.

quote :

The cylinder heads start changing as of 1980 it seems too, with the provision for the temperature/overheat sensor on the motor…

Both 25/35 heads starting from 1980 have pressure controlled thermostat and sensor provision… but only the CD2 equipped motors ( starting from 1986 or so) have really the temperature sensor mounted and cabled.

quote :

In the middle of fixing up a 1978 35hp, revived from long time sitting in a barn (about 8 years)…have it up and running but it’s only turning 4800RPM at WOT, and surging up and down after the fuel gets used up in the bowl…time to check out the float level, fuel pump…and then if that fails…timing of the ignition system.

Check also the upper/lower crankshaft bearing seals… sometime the rubber lip leaks and the air came in …