Anybody here know 1980s Johnson outboards?

So now that I have a 4.5 HP outboard that runs well I would like to hop it up a little. My last outboard was a 10 HP Merc. I liked the power. I'm pretty sure the 4.5 HP Johnson will leave me a little underwhelmed. So I'm thinking about hopping it up a little. If I could get another 2 HP out of it I would be really happy. So is there anything fairly simple I can do to up the HP? I don't want spend tons of time or money but I am willing to do some work this winter. I have looked on the web but have found precious little about hopping up outboards except for the really dramatic increases in HP and RPM. I'm hoping that maybe a larger carb or a different throttle cam or a thinner head gasket or larger exhaust ports might get me the extra HP I want. I don't think I have the time to change all the porting. The exhaust ports are easy to get at but the intake and transfer ports are not. I'm not looking for much of an increase in RPM, I am instead looking for an increase in torque so I can use a prop with more pitch. I have found that a 5 HP motor is fine when it is just me in the boat but when I have a passenger 10 HP works much better. Thanks, Eric

Reply to
etpm
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It was a good deal, but underpowered? Sounds like life.

A port and polish would be the ideal solution, I'll bet. Too bad there's not time/money for it. Check with local engine builders for their suggestions on carbs for it. That and an exhaust porting would probably give you more low end torque. Some older repairman who works on outboards exclusively will have the knowledge you seek, Eric.

So I guess this is out of the question?

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Reply to
Larry Jaques

The small 1.something to 3 hp engines from many manufacturers vary only in the higher horsepower models having a larger carburetor. You might look at that as a start.

Reply to
John B. Slocomb

On Mon, 25 Aug 2014 10:56:11 +0700, John B. Slocomb wrote as underneath :

John B. Slocomb - Quote an engine (series) that this is true for please, you say many ???

Reply to
Charlie+

Not outboards but this is common practice with many power equipment makers. Kohler, Briggs and Honda have done it for years.

A company will go to them and say "we need a XX HP engine" They take a larger unit and stick a different carb or main jet in and get the requested hp.

Reply to
Steve W.

Do you know of a good on-line resource for the carbs and jet sizes for small engines? I couldn't find anything useful concerning the appropriate main jet sizes for straight versus E10 gas.

Dialup, XP and IE8 are beginning to limit my searching ability, though the net was never rich in hard technical data beyond what the manufacturers provide.

-jsw

Reply to
Jim Wilkins

Mercury's 2 stroke line 1.5 and 3 HP, and I believe, all of the other sort of generic 1.4, 1.5, 2,3 HP 2 strokes made by God only knows who but sold by about every Japanese outboard maker.

Take the cowling off and they are all the same and the parts manuals show that the only difference between the high power model and the low power model is the carburetor.

Reply to
John B. Slocomb

I can because I've been working on one for my neighbor , in the 1984 version of the OMC 9.9/15 hp motors the only major difference is the carb . His 9.9 motor is now running a gearset from a 15 .

Reply to
Terry Coombs

Never really looked online. Kohler and Briggs both issued tech sheets on the changes required to older engines to get them to run on E10/15. Those may be on their sites.

The basic rule of thumb is the starting point is to drill out fixed jets by 5-10% over the percentage of ethanol. This is usually still a bit lean but it gets you close.

Reply to
Steve W.

Put it on Craigslist: "4.5HP Johnson, sell or part trade for 10HP Mercury".

Then buy a 10HP Mercury, and your problems are solved.

Reply to
Tim Wescott

Several years ago one of the boating magazines I read tested the myth about the Mercury 9.9 and 15 being the same. I think it was Bass & Walleye Boats. Anyway, after they did the parts swap the 9.9 developed close to but clearly less than the same power as the 15 they tested against. That could just be variances among engines or there could be something else, but it did produce more power. It did not produce as much power as the engine they tested it against.

I do know there are variances in motors though. I've got a 16' flat bottom bass/jon with a "bone stock" Merc 50 ELPTO that goes about 15 MPH faster than everybody says it should. I've had to special order higher pitch props to keep it from over reving at WOT.

Reply to
Bob La Londe

We've got a 4hp 4 stroke Yamaha on a 14' jon boat for a small lake near home, Georgetown Lake. It can't unload with the stock prop when I'm sitting in the stern. Yeah, I'm 265. Goes about 5.5 mph with two aboard. Plenty on 1300 acres.

Once when I had it out by myself, I locked the tiller, went to 100%, and got on the middle seat, steering by leaning. You could hear it unload, went about 9.2 mph. It let the nearly square transom rise to near level with the water. So a more graceful displacement hull works a lot better with low power. If I wanted to plane I'd swap for a 15.

What's silly to me are the 250 hp bass rigs roaring around that lake. They can get on plane for about 30 sec. Mostly, you see that on weekends, and I don't go on weekends, being retired.

Pete Keillor

Reply to
Pete Keillor

============== see

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Reply to
F. George McDuffee

You may find the installation of adjustable trim tabs helpful to get your hull to plane or at least get the nose down.

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Shouldn't be to hard for a RCMer to cumshaw a set for a test/evaluation. Gunner????

Reply to
F. George McDuffee

Dad wasn't even an RCMer , and he made a pair for an old boat we had . Two pairs butt hinges , flathead machine screws , and two pieces of aluminum plate . Adjustment was with a turnbuckle with an added locknut .

Reply to
Terry Coombs

There is another technique that perhaps works. A large trim tab that clamps on the "anti-cavitation" (I think it is called) plate, the cast in plate that is just above the propeller. The ones I've seen for a small engine are, say 10 inches wide and perhaps the same in length with a turned down trailing edge.

To be honest, some people rave about how well they work and others sort grimace and say "maybe". But it is an easily made temporary trial sort of thing and you can buy a commercial one if they work for you.

Reply to
John B. Slocomb

It's not important. I really don't expect a 4hp to plane anything I'm riding in. Most of the time I run at half throttle. Before I used trim tabs, I'd put one of these in.

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But that'd screw up fishing, so probably won't. Any number of small stitch and glue designs would make better use of the motor, but when I build a boat, it'll be this one.

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Pete Keillor

Reply to
Pete Keillor

I guess I could polish the exhaust ports. I do not want to go to the trouble of roughing up the intake and transfer ports. And changing the size of the ports is out too. Probably there is nothing I can do beyond a good tune up and using good fuel to get significant power increase. And bolting a V8 Hemi on would make the boat a little too heavy on the back. But thanks for the suggestion. Eric

Reply to
etpm

This motor looks like it won't benefit from a larger carb. But I am looking into it. Eric

Reply to
etpm

Yeah, wouldn't that be nice, someone wanting to trade down in HP. I am not wanting to spend much because I'm already out the cost of the merc. I have been looking for someone who wants to trade a 7 to 10 HP motor for a 12 foot aluminum boat, no interest so far. Eric

Reply to
etpm

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