1957 Johnson 7.5 hp problems and maintenance

[Alex57johnson]

Hey everybody,<br /><br />I have a 1957 Johnson 7.5 hp on my little 14 footer right now. Lately I've been having a few problems with it. The problems are that: it locks up quite frequently, it is hard to start without giving it tons of gas, and it doesn't seem to go nearly as fast on full throttle as it used to(it doesn't even sound like it's work hard at all). I'm not real great with motors and what not but I would really like some tips on how to solve these. Also since this motor is so old, what would you recomend to replace or do to hlp maintain the motor and keep it running as fast as it can. <br /><br />-Alex

 

[Paul Moir]

Re: 1957 Johnson 7.5 hp problems and maintenance

First, welcome to Iboats, Alex!<br /><br />Although I'm not exactly sure what you mean by 'locks up', I'm thinking your problem is probably a simple fix.<br /><br />First, make sure your compression is good. If it's not then unless you enjoy restoring old engines, it's not worth it. Get/borrow a compression gauge, and check both cylinders. You'll probably need to give it a couple pulls before the guage 'tops out'. Report what you find here. Good is ~90psi+, but far more important is that there be no more than 10% variation between the cylinders. <br /><br />Sometimes badly carboned up engines test bad, and decarbing helps restore them. If you've been running 10w30 rather than TCW-3 through the outboard, that's a really good possibility.<br /><br />If you've got good compression, then fixing the engine should be pretty painless. It's either fuel or ignition. <br /><br />Ignition coils go bad on these old outboards - they dry out and one cracks, leaving you running on one cylinder. If pulling one sparkplug lead (not with bare hands!) has no effect on how the engine runs at throttle, you've found your bad coil. Checking spark is sometimes misleading: I've had ones that produce beautiful looking sparks at idle, but at higher rpms the coils arc over internaly (rather than at the plug). New coils are cheap and available.<br /><br />The other likely possibility is that there's crud in the carburator clogging a jet. Generally, you pick up a carb kit which has new gaskets, etc, then you disassemble, clean, and reassemble the carb. Usually you clean the carburator by soaking the metal parts in carb cleaner.<br /><br />All these procedures are covered in the service manual, and you should obtain one and read it before trying to clean your carburator or replace an ignition coil. You can get excellent aftermarket ones from Iboats (I recomend the Seloc for your vintage). Your local library might have one to lend too. <br /><br />Good luck!

 

[OBJ]

Re: 1957 Johnson 7.5 hp problems and maintenance

Alex....if you want a copy of the original OEM service manual, which is preferred, click the link below. Which ever way you go, as Paul posted, you should get one. It'll pay for itself in no time. <br /><br />Good Luck.

 

[Chinewalker]

Re: 1957 Johnson 7.5 hp problems and maintenance

Hi Alex,<br /> First and foremost - what are you running for mixed fuel? That motor NEEDS a 24:1 mix to stay healthy. If you're running modern 50:1 mix, it'll certainly lock up, or worse.<br /> Second, when was the last time the waterpump impeller was serviced? If the motor is used regularly it should be replaced every 2-3 seasons. If it hasn't been used in a long while, it should be replaced as a matter of course.<br /> Third, are you running on both cylinders? If the coils are original style, then they are likely cracked. It's also possible that the points are corroded or oxidized. If the condensers have the original fiber caps, then they should be replaced with the newer rubber cap style.<br /> Fourth, check the cylinder head and exhaust cover for signs of discoloration. If the paint on them doesn't match the rest of the motor, then it's likely the motor got hot at some point in the past and there may be cylinder damage. Check compression, as suggested by Paul.<br /> Good luck!<br />- Scott

 

[alcan]

Re: 1957 Johnson 7.5 hp problems and maintenance

Follow all the good advive previously given, especialy about the manual. I will also add, check your Lower unit oil. You may have a problem in your gear case causing the lock up.

 

[Alex57johnson]

Re: 1957 Johnson 7.5 hp problems and maintenance

Thanks a lot for all the info guys,<br /><br />I looked at the motor yesterday and noticed a hole about 1 inch in diamter in the moter. I don't know the exact part but it's on the middle left side a couple inches above the fule line connection. I don't know if this is affecting the engine because I jsut knowticed it and I haven't ran the motor since Augest. Any ideas?

 

[Chinewalker]

Re: 1957 Johnson 7.5 hp problems and maintenance

Hi Alex,<br /> Hmmmm. Not good. Those old 7.5s are prone to "aluminum worms" which basically means it has tossed a rod. I'm assuming this is a jagged hole that allows you to look inside the motor's innards? Basically you've been running on one cylinder. The "locking up" is likely due to a piece of the broken metal getting bound up inside every now and then. Consider yourself lucky the motor brought you home. It was likely caused by lack of oil, over heating, or over-revving..<br /> Two courses of action remain. Either dig up a parts motor with a usable powerhead, or part yours out and get something else. It's really no good as it is. The one good piston and connecting rod in your motor is worth a fair amount, so don't throw your motor in the trash. The lower unit also has some value, particularly the lower driveshaft (2-piece driveshaft). Your dual-line fuel tank allows you a whole lot of flexibility if you enjoy running a classic outboard, as it'll hook up to any Johnrude with an external tank from about 1949 to 1959. ANY motor that requires the dual line tank also requires extra oil. They'll all run on 24:1, with some of the larger ones able to handle 32:1 at your discretion. Or, there are several posters on this site who have expressed interest in acquiring dual-line tanks. The next step is up to you...<br />- Scott

 

[oldoutboard]

Re: 1957 Johnson 7.5 hp problems and maintenance

Alex,<br /><br />I do a little work on those old motors and may be able to fix it for you, or offer a similar replacement. Please email if interested oldoutboard@rogers.com <br /><br />Steve

 

[oldboat1]

Re: 1957 Johnson 7.5 hp problems and maintenance

Just for future reference -- the Intertec manual shows a 16:1 mix for the '57 Evinrude 7 1/2 (24:1 for the 10). I have been curious about this, but try to run mine at 16:1. I use the recommended 24:1 in my 10 and 35s, although sometimes maybe a little bit leaner in the 35s (probably not much more than 30:1, though). <br /><br />Chinewalker's reference to a little history with the 7 1/2 is instructive -- presumably the reason the recommended oil mix is richer, at least according to specs shown in the Intertec manual. My 7 1/2 is the best runner I have, and fast -- think I'll want to be a little careful to avoid overrevving.

 

[Chinewalker]

Re: 1957 Johnson 7.5 hp problems and maintenance

Hi Oldboat1,<br /> I should clarify that I actually run a "fat" 24:1 in all my older motors - always adding that extra dollup of oil after the ful quart has been added. In any case the 24:1 mix is fine simply due to the newer, higher quality oils available today, but any leaner and the bronze sleeve bearings will start scorching. The oil also acts as a sealant to prevent compression loss, both in the cylinders and the crankcase - there are no rubber seals or o-rings in these 7.5 powerheads...<br />- Scott

 

[seahorse5]

Re: 1957 Johnson 7.5 hp problems and maintenance

Those 7 1/2's had a history of throwing their aluminum rods when they are over-revved. Most of the time it happens when a prop is worn down from running in sand, or a shear pin breaks and the motor races. Of course, over its 46 years of use, anything is possible.

 

[oldboat1]

Re: 1957 Johnson 7.5 hp problems and maintenance

Interesting stuff, Chinewalker (and seahorse). I also tend to cheat a little in favor of a richer mix with these motors -- had been curious about the 7 1/2, though. It's still a favorite.

 

[flathead239]

Re: 1957 Johnson 7.5 hp problems and maintenance

Hi Alex,<br /> First and foremost - what are you running for mixed fuel? That motor NEEDS a 24:1 mix to stay healthy. If you're running modern 50:1 mix, it'll certainly lock up, or worse.<br /> Second, when was the last time the waterpump impeller was serviced? If the motor is used regularly it should be replaced every 2-3 seasons. If it hasn't been used in a long while, it should be replaced as a matter of course.<br /> Third, are you running on both cylinders? If the coils are original style, then they are likely cracked. It's also possible that the points are corroded or oxidized. If the condensers have the original fiber caps, then they should be replaced with the newer rubber cap style.<br /> Fourth, check the cylinder head and exhaust cover for signs of discoloration. If the paint on them doesn't match the rest of the motor, then it's likely the motor got hot at some point in the past and there may be cylinder damage. Check compression, as suggested by Paul.<br /> Good luck!<br />- Scott

In 1957 the oil to fuel ( leaded gasoline) was 1 quart of number 30 non detergent motor oil to 4 gallons gas. With the 2 cycle oils today I use 1 quart of 2 cycle oil for snowmobiles, weed eater, ect. and 6 gallons gas has worked great! The only problem I have had is an ignition coil failed OEM cracked replaced with a new coil from Napa and still going strong.

 

[Crosbyman]

19 year old post....He must have figured it out by now.