Underpowered Johnson 25hp

[aussie_ricardo]

Hi all,<br />I'm a newbie to boats and this forum,<br />Last weekend a brought my first boat.<br />A 12'fibreglass runabout with a 25hp Johnson outboard motor. The model number of the motor is 25RL75B so I guess it's around 25 years old.<br />The motor started, idled and run no problems, but the maximum spped I could get out of the boat was about 6 kph or around 3.5 mph, and it used alot of fuel. A boat mechanic told me that a 25hp motor should make the boat plane.<br />He suggested that 1 cyclinder might not be working.<br />My questions are:<br />1) Is it possible for the motor to run ok only on 1 cyclinder?<br />2) Is there anything else it could be?<br /><br />Thanks<br /><br />Ricardo

 

[SeaDawg]

Re: Underpowered Johnson 25hp

Yea, it could be running on one cylinder, but it is not OK.<br /><br />I've had it happen to me, and you are correct, you will just putt around and suck a lot of fuel. Eventually you will foul the plug, and it will die. And you will be forced to fix it.<br /><br />Some of the other guys can give you advice, there is also a lot of good posts earlier if you search this site too.

 

[omctech]

Re: Underpowered Johnson 25hp

Sounds like the prop to me. Were you able to reach high rpm's?

 

[skopje]

Re: Underpowered Johnson 25hp

It sounds like it is in bad need of a tune-up. Could be that the ignition on one cylinder is not working. Have the carb cleaned and a carb kit put into it, synchronized, and timed. Then let us know what happens.<br /><br />Skopje

 

[alcan]

Re: Underpowered Johnson 25hp

Hello aussie ricardo<br /> Yes it's possable to run on one cylinder.Yes it could be a lot of other things also. How is your motor rigged, tiller or remote control? Have you tested for compresion? Have you checked for spark at both plugs? What do the spark plugs look like? Did you check the vent on the gas tank?

 

[Fouled Plug]

Re: Underpowered Johnson 25hp

Aussie_ricardo, try this: With the motor idling, pull one spark plug wire at a time. If the motor slows down, that cylinder is firing. If nothing changes, it's not firing. Could be as simple as a bum spark plug.

 

[Stream_fisherKY]

Re: Underpowered Johnson 25hp

I've had the same problems as you previously. In my case it was a combination of cracked coils, untuned carb, and worn piston rings. I'd replace the plugs and try again and if that fails, look in these three areas.

 

[aussie_ricardo]

Re: Underpowered Johnson 25hp

thanks all for the response. Will check spark and do a compression test on the weekend. BTW what sort of pressure reading should I get.<br /><br />regards<br /><br />Ricardo

 

[alcan]

Re: Underpowered Johnson 25hp

The compression in each cylinder should be no less then 100 psi. There should not be more then 15 psi difference between holes. Hope that helps...<br /><br />Alcan

 

[G DANE]

Re: Underpowered Johnson 25hp

Hi<br /><br />I am about to finish my second rebild of engines exactly like yours. It has point/condenser ignition. The ignition coils sometimes shortcircuit, or the breaker pionts burn. Both are easy to fix. reply again when you have checked ignition.

 

[G DANE]

Re: Underpowered Johnson 25hp

By the way,<br /><br />When my first motor was rebuild - a 1974 25HP Evinrude, i did 20 MHP with a 15" glasfibre dighny. Had a lot of fun. Hope you will too. Its a really relieable engine.

 

[aussie_ricardo]

Re: Underpowered Johnson 25hp

OK,<br />Last night I check a few things.<br /><br />Firstly I remove both spark plugs, both looked black and damp, but not too oiled up.<br />Then I held the top cylinder plug against the motor and turned the motor over, gota blue spark.<br />Tried the same on the bottom cylinder and got a shock from the spark plug lead and no spark, tried again same result.<br />My question is can I just replace the spark plug lead from the ignition coil or do I have to place the whole coil and lead?<br /><br />Secondly after inspecting the motor I notice black oil had run dowm the starboard side of the exhaust housing. After closer inspection I noticed a fine crack about and inch and a half long running from the upper side mount accross the exhaust housing.<br /><br />Is this bad?<br />Can it be fixed,(ie. welded)?<br />How easy would it be to find a replacement exhaust housing?<br /><br />Thanks<br />Ricardo

 

[alcan]

Re: Underpowered Johnson 25hp

Hi aussie<br /> I would try new plugs first. Twist up a small piece of wire, form a loop, about the size of a spark plug, fasten tag end to an alligator clip. This will make testing a lot more pleasent. If your coil is leaking, replace as unit (coil and wire). As far as cracks go, if they are not suposed to be there, they are bad. It sounds as your's is not to serious. Yes they can be welded. I've had some older motors with small cracks and just left them be. Good luck.

 

[DJ]

Re: Underpowered Johnson 25hp

Aussie,<br /><br />If you're getting "nailed" by the spark on the one cylinder-that spark is looking for a ground. Unfortunately, you were it.<br /><br />Take a good look at that spark plug wire (secondary) and look for cracks, etc. <br /><br />I do not recall if those wires were replaceable, seperate from the coil.<br /><br />If they are, you can substitute an automotive plug wire for diagnostic purposes.

 

[aussie_ricardo]

Re: Underpowered Johnson 25hp

UPDATE:<br />Checked compression, both cylinder ~100-105,<br />but notice that I was only getting 3/4 full throttle. Adjusted cable, now getting full throttle. Took the boat out Saturday and it got it up to about 12 mph. Sunday morning I changed the spark plugs. Check the catalog at the auto store I noticed that my motor should be running champion J4C plugs, but had J6C plug in it. The store didn't have any J4C's, so I got new J6C. The guy said they were a hotter plug and would burn the oil better???? Took the boat out on Sunday, it was hard to start, started travelling at full throttle and the motor went bang and started revving its head off. Started again no throttle. Checked and a linkage had come off. Fixed that started again and the engine was running poorly, sound like one of the cylinders was missing. Stopped and decided to go back to the ramp. On the way back about 1 mile the motor would run fine for about 40 secs then start to die. I would throttle back, wait 10 seconds then go again, this kept repeating itself up the river. Any ideas on the cause of this?<br />I noticed the top coil pack is a lot newer than the bottom. The bottom one is still giving me abit of a thump then I test the plug. I'm going to try and get a new one today.<br /><br />Any help would be most welcomed.<br /><br />Sorry for the long post.<br /><br />Regards<br />Ricardo

 

[alcan]

Re: Underpowered Johnson 25hp

Hi aussie<br /> Not trying to sound like your mother but, I would like to suggest that you obtain a repair manual. This will save you a lot of head aches, a ton of money and some time. If you don't want to buy one you can find one at the public library. Of corse a model specific manual is best, but one covering several model will do fine. It sounds to me that you may have several little problems. <br />I think you need to give your motor a good over all servicing and tune up. As you probably need some other routine maint. parts also, ask your dealer\parts guy to test your coil for you. Remember he's got to make hay while the sun shines. Be sure your remote control is adjusted and working properly. Let us know how your makin out. Good luck.

 

[almost retired again]

Re: Underpowered Johnson 25hp

Hi Aussie-Ricardo I always tell my students to do these things in this order always. Take the engine cover off and do a thorough visual inspection, correct any problems found by this. Next preform a compression test on all cylnders. If you get bad readings from this test you will have to de-carboinize, or overhaul. If everthing up to now is ok, then do a spark test on all cylinders, if spark is good then check for fuel. I believe these steps will save you a lot of time and trouble when attempting any new repair. As for your present problem I think that you have a bad plug wire or coil. I would also do a sync and link procedre on this. Just my opinion.