86 Yamaha 25

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RevDStan

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I just picked up an 86 Yamaha 25 for my Waco 14' jon boat. I can't find any info online for the 25's Yamaha made prior to 1988. It came with a new prop on it, but it's only a 10.5p. Assuming the max rpm, and gear ratio on the 86 is the same as the 88 (5500rpm and 2.08), I calculated a top speed on 22.3mph with 15% slip. This ,to me, seems a little underwhelming for a 25. Does anyone on here have experience with one of these older Yamahas and could recommend a prop that works well for you? I'm currently leaning towards 13, since I don't want to over rev it with the 10.5.
Thank you for any responses.
 
What model number is it? Is it the 2 cylinder/single carb motor? If so, I know those single carb 25s were a detuned 30hp where as the 2 carb'd models were the same platform as a 20hp, which I would assume would have different gear ratios.
 
I can get the model number after work. I had the top off when I bought it, and am 90% certain it was 2 cylinder, single carb.
 
I tinkered with one exactly like that last year. It's a single carb motor and a detuned 30hp, only difference being the reed plates and carburetor I believe. Never got it running right though so wasn't able to test it successfully on the water to learn anything about good prop size choices. I would have dug into it more but it was my wife's uncle's motor and I was helping him work on it and he just didn't want to mess with it anymore or spend any more $ on it.

The later 2 carb models are 395 cu in blocks and had the 2.08 gear ratio like you mentioned and had a max WOT rating of 6k rpms. I would assume a 25SJ would be a larger displacement and prob have a slightly different gear ratio too since I don't believe the lower units are interchangeable between the two models.
 
The 30's have 1.85 gears instead of 2.08, so that will help if it is a detuned 30. I still may need to step up to a 12p to reach 30mph (my self set goal). It was honestly more than I wanted to pay for an outboard of that age, but it was the only 25hp within 3 hours of me listed, and he had receipts for last falls maintenance ($850), so It'll have to make due.
 
Whoops, meant to say 395 cc's above lol. That would equal 24.1 cu in haha.

FWIW, a random google search showed a 25SJ to have a displacement of 30 cu in.
 
It sounds as though you're right in the neighborhood of where you can expect to end up with that motor. I topped out with my 1436 at 25mph running a 13P prop. A friend running a new 4 stroke with the tilt & trim was able to squeeze 28mph out of his on a 1448, but without that adjustment ability you'll probably end up about the same as what I was getting.
 
RevDStan said:
The 30's have 1.85 gears instead of 2.08, so that will help if it is a detuned 30. I still may need to step up to a 12p to reach 30mph (my self set goal). It was honestly more than I wanted to pay for an outboard of that age, but it was the only 25hp within 3 hours of me listed, and he had receipts for last falls maintenance ($850), so It'll have to make due.

I wish we had gotten that one running as I really liked it. Was easy to work on and lots of parts available, both new and used. Think He bought it for $300, then we spent a couple hundred on parts, and then he sold it for about $1000 which kinda surprised me. In this ones case, it would run well at low speed on muffs or in a barrel but had random lean sneezes and then on the water it wouldn't open up past about 1/3 throttle so it wouldn't plane. Had excellent compression but some of the ignition components were out of spec so we replaced the coil and maybe put on a used CDI to no effect. I'm 90% certain it just had a bad lower main crank seal as it would puff smoke from that area when it began to misfire and sneeze hard.

If I remember right, the parts to convert it to a 30hp only totaled about $150 on Ebay. I guess if it was a 30hp the rpms could be taken up to 6k and allow you to run an even bigger prop.
 
From the Yamaha owners Facebook group:

“Carb & reeds are the only difference. The 30hp uses the 689-13610-00-00 Reed Valve Assy. and the 689-14301-73-00 Carburetor...”

Just an FYI in case you ever choose to go the 30hp mod route.
 
them old C25's and 30's were pretty rough running at idle. NOTHING like the 395cc twin carb motors. Those idled pretty darn good in comparison. A sneeze at idle wasn't uncommon on the old 25/30 single carb motors, had to run them lean so they'd idle somewhat clean (not foul plugs), and until they were at operating temp they'd sneeze, fart, generally ran like poo. Replace the thermostat if you're restoring one. Makes a big difference in idle. If the thermostat won't open it'll never get warm enough to let a lean idle "work" like it should. Even then, on those motors, they were kinda noisy and rough at idle. Similar to Mercury/Mariner/Evinrude of the same or similar hp and year models. Also at full tilt they get kinda noisy too as some of the exhaust exits above the propeller and there ain't no muffler to quiet it down, just some water splashed around the tuner and exhaust to "muffle" it a little (very little).

yes you can make a 25 into a 30. Look at parts diagrams and compare part numbers. Carb, reed plate, and I want to say the tuner was different too but it's been so long I have forgotten all of the details. Make sure to check the timing too.

in the 1990's as the twin carb 25 yamaha came to be popular, the old 25 single carb--then called a C25--kinda fell off the map. I mean after all, it was heavier, a little slower, a LOT noisier, thirstier, rougher running at low speeds, and really smoky in comparison. They were the cheaper of the 4 25hp motors that the late 1990's were graced with (C25, 25 twin carb, F25, and 25 3 cylinder). Really the only folks that bought them were the ones who wanted the cheapest yamaha 25 they could get. Dealers--including the one I was working for at the time--had to give them away.
 
FWIW, I learned these ignition systems use a wasted spark so both plugs fire at both their respective TDC and BTC. So if you hook up an inductive tach keep that in mind as it will show double rpms at the the standard 2 stroke setting.
 

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