"Newer" 25/35hp OMC compatibilities

[johnyrude200]

I’m onto a new learning phase with the 1977-2005 25/35hp motors. While appearances are pretty similar, I’ve already started noticing some of the ‘revisions’ they went through over the years.

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.

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.

The cylinder heads start changing as of 1980 it seems too, with the provision for the temperature/overheat sensor on the motor…say nothing of the changes in the gearcase (thru hub exhaust vs behind prop exhaust) and various shift linkage configuration changes.

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.

I switched the original mag plate (which did not have a charging circuit/rectifier setup) with a 1979 mag plate that had the stator installed for charging…I’m guessing this would NOT be responsible for limited RPMs at the top end. The service manual states the 35hp version is normal between 5000-6000 RPM.

Also checked the high-speed jet and it’s correct (59D).

[fleetwin]

OMC started messing with the ports (block differences) during the middle 80s when hp ratings were measured at the prop instead of the crank. The later 20hp models actually had a restricted intake manifold and smaller carb.
Be careful swapping those early 25/35hp models around, although the powerheads might have been similar, the crank/driveshaft splines were different.
What size prop are you using on your 78 35hp? What type of tachometer are you using when you see 4500RPM? Sounds like there is a legitimate problem if it is surging at high speeds. You might want to remove/check/clean that high speed jet one more time, perhaps some stray crud got lodged in the jet after you cleaned the carb.

[johnyrude200]

It was turning 4800RPM, max, with a test prop. Basically it would hit "max RPM" (for now), hold that for about 8-12 seconds, then lose power. It was pulsating at the top end and acting erratically at anything beyond 2/3rd throttle. I double checked the link and sync and that is all OK. Didn’t get to work on it again yet, but the float was set a bit off, so I’m going to adjust that and then retest. I pulled the high speed jet before typing the last post to insure it was correct, and also clean. The TACH I’m using is a simple one that involves winding a wire around a single plug lead. I will say the motor idles down to a respectable 760RPM before wanting to stall out.

If adjusting the float doesn’t fix the high speed issue (which it seemed fuel starved), then I suspect there may be a clog in the exhaust ports/housing, not allowing the motor to breath the right way. I may try a different fuel pump too, the one I put on there was used but might be undersized/incompatible.

This motor has a story behind it – I went to pull the thermostat cover, only to discover mud dubbers had infested the thermostat cover, clogging up the water outlet at the bottom of the cylinder before it went out the exhaust housing. During the initial run, the motor started to overheat, until a huge glob of gunk/cocoons were ejected out of the exhaust relief ports at the top of the exhaust housing. I pulled the cylinder head completely and cleaned out/used compressed gas on the water outlet at the bottom of the cylinders, but I suspect there may be a lot more crap in there.

Side question – I noticed two holes at the bottom of the exhaust housing on the front side, just above the gearcase. I know the older 20/25’s had water that would gush out of here normally; do these 35’s have that same feature? There was intermittent water spraying out of there, and if there is a big clog below the powerhead/exhaust jacket, then that would certainly provide a major clue as to a clog being the culprit of this issue.

The only other thing I suspect could cause this problem is if the timing is set wrong between the fuel and ignition systems; I switched the mag plate out from a 1979 model which had an alternator; the original did not have this. I haven’t used a timing light on the motor yet, so if all else fails I’ll look at the ignition timing.

Why is it everytime I go to start on a new ‘family’ of motors, the first one always has the most issues 🙂

I remember the first 9.9/15 I tore apart earlier in the year…that ended up being a 20 hour job with all the issues it had, but when I was done, I knew those motors inside and out pretty well!

[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 …

[fleetwin]

Will try to address each of your questions, let me know if I mess up:
OK, you were running with a test wheel. I am not sure what the RPM is that you are supposed to see when running with the correct test wheel, so we don’t know if perhaps 4800RPM is the correct RPM or not. Nonetheless, you mention that the engine was pulsating anywhere above 2/3 throttle. Let’s keep in mind that sometimes our test tanks are our worst enemies, it may just be a mixture of breathing exhaust and test tank water ventilation causing the pulsating RPMs you are seeing. It is always best to put the engine on a boat with a prop and try it before "getting in too deep".
In any event, I don’t think swapping the mag plates is going to cause the engine RPM to pulsate, even if WOT timing is off a few degrees. I realize you cleaned the cleaned the carb, but again, there may have been stray debris in the line or somewhere that might have lodged in the high speed jet after you cleaned it. It only takes a few minutes to remove/inspect/clean the jet, without having to remove the carb (if you have the correct tool), so this might be worth doing. Keep in mind that a loose high speed jet (or not completely seated) can cause a lean/pulsating condition as well, use a pipe cleaner soaked in engine tuner/carb cleaner to clean any residue from inside the jet, it sounds like you have already readjusted the float. It sounds like you changed the fuel pump, but it may not be the correct one. What was wrong with the original pump? How about the screen/filter in the fuel pump? Could be an air leak in the inlet fuel lines somewhere also. Keep in mind that running the engine in a test tank raises it high above the fuel tank, which may exceed a normal operating fuel pump’s ability to lift the fuel so high. In other words, the fuel pump is OK, but it is unable to "lift" the fuel that high. Simply place the fuel tank on a table near the test tank if this issue is in question, make sure the fuel line does not sink to the ground after leaving the high fuel tank though. And there is always the simple method of pumping the fuel primer bulb while the engine is pulsating to confirm pump/air leak issues, it is best to have an assistant pump the primer bulb while you run the engine. You could always feather the choke partially closed while the engine is pulsating too, a positive result may indicate carb restrictions/problems.
OK, back to the mud dobbers. Yes, this could surely have created and exhaust blockage down in the gearcase. Pull the prop and hub off, have a look inside the prop shaft bearing carrier (exhaust area) for signs of mud/crud. You could always use a coat hanger to probe up inside the gearcase exhaust area for signs of mud/crud without having to pull the gearcase off. Have you had this gearcase off? I’m guessing no, or else you would have seen mud/crud. I would recommend a water pump job, but I don’t want to open another "can of worms" for you, let’s just say these early 35hps had cooling issues.
And finally, yes, these engines have those two water relief holes just above the gearcase hidden behind the lower mount clam shells, seeing some water come out of there is normal. The driveshaft tube is sealed (or at least supposed to be), so I don’t think mud dobbers could possibly get in there. In other words, seeing water come out of those relief holes has nothing to do with mud dobbers.
PS- I just looked in the 1976 35hp service manual, but could find no reference to a test wheel, or test wheel RPM, probably an early edition of the manual. Again, please be sure the pulsating isn’t just a test tank/exhaust issue before getting in too deep. And yes, you have found another one of OMC’s gems, those early 35hps were not great engines, even though they did become bullet proof in 1980. At least the one you are working on has CD2 ignition instead of the beloved "low tension" system.

[enrico-italy]

Talking about the "low tension system"…

I had several engines with this magneto version (now I have in my garage two late ’70 22cu 25 hp and a ’76 6 hp.)

I had no problem for more than 20 years but I used them just a bit in the last 10 years… now, all the external coils I have appears to lost their power… I’m unable to combine a decent set of two for ONE motor.

Is it a known issue for these coils, as the well known " age cracks" in the older internal coils?

[johnyrude200]

if they crack, they can still work, but I’d use some silicone to cover up those cracks if you can’t buy a new pair of them.

You should test them with a merc-tronic to see how much power they are outputting, etc. Otherwise you’ll go nuts using trial and error, which is tedious, time consuming, and annoying, to say the least.

[fleetwin]

quote Enrico-Italy:

Talking about the “low tension system”…

I had several engines with this magneto version (now I have in my garage two late ’70 22cu 25 hp and a ’76 6 hp.)

I had no problem for more than 20 years but I used them just a bit in the last 10 years… now, all the external coils I have appears to lost their power… I’m unable to combine a decent set of two for ONE motor.

Is it a known issue for these coils, as the well known ” age cracks” in the older internal coils?

Yes, it is quite common for these coils to crack. But, unlike the universal mag coils, most seem to work just fine with what looks like large enough cracks to create external leaks. I guess there must be another layer of some sort of insulation between the windings and the outer plastic casing.

[fleetwin]

quote fleetwin:

quote Enrico-Italy:

Talking about the “low tension system”…

I had several engines with this magneto version (now I have in my garage two late ’70 22cu 25 hp and a ’76 6 hp.)

I had no problem for more than 20 years but I used them just a bit in the last 10 years… now, all the external coils I have appears to lost their power… I’m unable to combine a decent set of two for ONE motor.

Is it a known issue for these coils, as the well known ” age cracks” in the older internal coils?

Yes, it is quite common for these coils to crack. But, unlike the universal mag coils, most seem to work just fine with what looks like large enough cracks to create external leaks. I guess there must be another layer of some sort of insulation between the windings and the outer plastic casing. But again, it is best to leak test them with a mercotronic. The nice thing about the external coils is that leaks are easily heard and usually easy to spot visually while the engine is running/misfiring.

[Mumbles]

Here’s an external coil on a ’72 20 horse Evinrude I had. The coil was badly cracked and leaking but the motor still ran!

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