Hi folks. It looks like both 75 and 90 hp models have the same displacement. Is it possible to up the power of my 75 by using a 90 carb?
I was hauling a bunch of peeps at the lake recently, and the poor old 75 was just wimpy. Nothing like the merc800 it replaced. I dont know if its worn out, but it starts easy and now it even idles down real nice. Recently rebuilt carb, and idle jets adjusted to perfection!
Its running a real small prop like 10" diameter? Im not sure, but small,, just no hole shot, unless its just me in the boat. At higher altitude, no hole shot at all. Over the week end, it struggled to wind out with 3 passengers, but would eventually get there.
Part of it is balance, as the evinrude is a real lump, and I also removed the forward fuel cell, replacing it with a rearward plastic tank. Very tail heavy.
Anyway, Ide like to bump up the power even a little bit if possible.
Im currently replacing the floor, and moving the batteries up front. Going to leave the fuel tank in back. I might also remove the 6 horse merc kicker,, but I hate to, because its been a real day saver some times...Just got to get the boat balanced again, and more power!
The 90/100hp 4-bbl sidedraft carb certainly flows better than the old-style 2-bbl downdraft setup on the 75hp. We used to say that half the gas went out the bottom of the motor as it went around the 90-degree bend before getting to the crankcase!
You can't just bolt a 90hp carb onto a 2-bbl block, it won't fit. You would have to take the entire front half of a 90-hp block, and swap that onto the 75hp block to effect the change.
Quite a lot of work, since the front half of the block is line-bored with the main block assy, then the mounting flanges are drilled for tapered dowel pins to hold a precise alignment.
My buddy Thom (aka Dr. Frankenmerc) has swapped a lot of Merc front covers in his day, and he said that generally they matched up pretty well but sometimes had to either machine the top or bottom, depending on whether there was a misalignment of the 2 halves.
I'd assume the same would apply to a V4 block. Except even more critical 'cause there are sealing rings on the crankshaft that have to seal properly against both halves of the block.
And more than likely, the drilled alignment holes wouldn't match up and you'd have to drill new holes for the tapered pins, once the halves are bolted-up and machined as required.
Far as the crankshaft goes, if you bolted on the "foreign" cover and the crank still turned, probably gonna work.
An awful lot of work, though. Personally I'd be looking for a 90hp Starflite or a 100hp Meteor, etc. They are some pretty strong running motors, the 90hp would almost keep up with my old 850-Six with 100-hp carbs, on boats that were essentially the same hull.
That was back in the day when gas was cheap enough that my good boating buddy & I could afford to race down the lake. He never could beat me, but that old 90hp Evinrude V4 was The Motor That Would Not Die! It took all sorts of abuse including overheating, getting sunk in the lake, lost head bolts, and kept on running!
Anyway, Craigslist is your friend! Or perhaps a classic Tower of Power Inline Six may be in your future!!!
Any of the older V4's up to 1972 use the separately-mounted motor brackets. The way these are designed you can pull the stainless pins on each bracket and the entire motor will slide upwards and out of the bracket. Then the next motor just drops right in.
In 1973 they went to a more modern integrated bracket but I couldn't tell you without looking if the holes match up. I wouldn't bet that they did. Not a bit deal, really, just plug the old holes with 3M 5200 sealer and they're sealed permanently. Drill new holes and install the bolts with a coating of 5200 and they'll never leak.
If you want a more "modern" motor with reliable ignition, a '73-and-up 85hp or 88 Special would be a good choice.
Far as older ones go, later years would see an 80hp version of your 75hp, still with a downdraft 2-bbl carb. Not much of an improvement.
IMHO the best V4 of the pre-hydroelectric 60's motors would be the '66-'67 100hp. They had a larger gearcase than the older Selectric, much more durable than the wimpy springs in the smaller L/U.
The '67 models had an unreliable electronic ign system, which can be easily converted back to points by swapping-in the points breaker plate along with a different coil & ballast resistor.
My good boating buddy threw his older 90hp powerhead on a '66 Johnson Golden Metor with blown 100hp powerhead. It even had power tilt, which was pretty handy. The 90 ran real well, and the larger electric-shift L/U was reliable. He had a lot of trouble with springs on the older Selectric.
Another guy I know that used to race Mercs a lot has built a 100hp JohnRude for his boat, but transplanted the powerhead onto a mechanical-shift 65hp body.
Makes for a very fast motor! Lighter than the electric models and using the streamlined smaller L/U. I'd imagine you'd have to make sure the L/U clutch and gears were in perfect condition and shift linkage slop-free, properly adjusted. That 100hp is gonna put a lot of stress on the lighter-duty gearcase!
V4 models starting with 1965 used a spring-loaded shift clutch which helped hold the clutch into the gear, cutting down on clutch/gear wear and consequent jumping-out-of-gear (which is a problem with these).
So if you were to build a performance FrankenRude with 100hp powerhead/manual gearshift lower, you'd want to use the propshaft, balls, spring & clutch from '65-up.
Or just use the heavy-duty L/U from the old Fat Fifty, these continued to be an option in the mid-60's, besides the light-duty manual and the Selectric.
Depends on whether you're looking for ultimate speed (with the light-duty L/U), or wanting to push a lot of load while swinging a prop with a lot of surface area (heavy-duty).
BTW the motors with the light-duty gearcase (manual or electric) had quite the overdrive gears and that's why you see them with small-diameter, fairly-small pitch props.
Merc had a good idea, by going with 2.3:1 gears on their 65, 80, 85, and 75hp Fours, which allowed the motor to carry a larger, more efficient prop and get the rpm up into the usable range faster.
But a V4 JohnRude does have much different torque curve than an Inline Merc, so I'd imagine that's the reason for the steep "overdrive" gear ratio.
At any rate, there's a lot of mix-n-matching that can be done with these, if you're determined to stay with a "classic" 60's motor.
But quite frankly, you'd have a lot less trouble with and find a lot more of the '73-up V4's.
Lots of good info, you might poke around there. Meanwhile the below chart shows what propeller sizes were available. You'd need to hook up a tach to your engine to figure out if the prop is too small or too large. A car tach set on "V8" will work on a 2-stroke 4-cyl V4 with one coil (unless you have a magneto, that's a different story).
Muchos Gracias Ed. Ima leave the 75 alone, possibly optimize the prop for what I want, and start shopping for a newer motor.
As far as I can tell this 75 is all good. Its mechanical shift. With some luck, it will be worth a couple hundred when it comes time to upgrade.
Sure is reliable and quiet.
I kinda thought perhaps it was a gearing thing why this motor swings a tiny prop, and the merc800 swings a big prop. Coming from aircraft design, a bigger prop is always more efficient than a smaller 1. its just the nature of the fluidity of air, reynolds numbers... too much math.
So, it makes perfect sense now why the merc was better in this speed range, guessing around 30-35mph. The glasspar has a tach. I will research hooking it up to the evinrude(mercury tach), and see where this old motor really is at condition wize.
Its one thing after another here lately. Soggy floor, soggy motor.
I still need to get the alumacraft with15hp out to tune it in. I like that boat. It works well, and is easy to handle at the dock.
However,, family likes the bigger boat for joy riding and eventually pulling tubes.
Even a change to a mid 70s 65, 70, 75, or 85 will put you in a 13" to 14" prop, which would give you a much stronger hole shot. My 73 Evinrude 65HP is running a 13.75" x 17 OMC Viper prop. Takes my 15.5 foot 1973 Crestliner open bow 35MPH with 4 or 5 on board.
Sounds like it might be a good idea to test compression on yours, probably getting low.
A good running 69 or newer 85+ hp would be an animal on that boat compared to an older 75. They're stump pullers. I would personally stay away from any of the 3 cyls. I've seen a lot of mechanical problems with them.
Heres a vid of my boat. Just me and my 18yr old kid on board, both of us up front for best balance.
With only 2 of us on board, its quick, but that wasnt from a dead stop either. Ran real well today. Took pics and vids for selling. Started first try no problems. Unfortunately the 6 hp merc has been very prone to flooding lately. I changed plugs recently. Not sure if that was any kind of improvement.
Sounds pretty healthy to me. Struggling with a greater load and running OK with light load is a pretty good indication that it needs a smaller-pitched prop when running the heavy loads. It doesn't sound "sick" at all.
I really didnt think it was sick, just a bit weak compared to the merc800. Sounds very nice on the video. Runs out real well at 4500 feet and using smaller of 2 props I have, with only 1 passenger.
I did find a 4 blade michigan wheel too. Tempted to get it if it isnt already gone.
I only paid 300 bucks for this speedifour. Had to go get it, and install myself, but Its worked well all summer besides the rusty fuel issue which was my fault. Carb linkage was a bit off in the beginning also, but with you guys's help easily solved.