'75 Johnson 25hp won't plane at full throttle...

[armygirlmp]

1975 Johnson 25hp won't plane at full throttle....she's purring like a kitten at an idle and runs great but she just won't plane! Here's what has happened to date....took it out of storage the end of June and she ran like a raped ape.The next weekend I changed the (Autolite) plugs cause I thought after being in storage since Jan. it may not be a bad idea. (I also tighten the prop cause it seemed a bit loose.) Got some Champions from Auto Zone (mistake). Took it out the next day and she wouldn't plane. She ran ok at an idle but I noticed the gas blub was soft, lost its prime a couple of times so I checked the tank and it was getting air in around the valuve steem so, I got a new tank. Took it back out bulb stayed hard, she ran great at an idle but still wouldn't get enough power at full throttle to plane. So I went back in and checked the prop we thought I may have tightened it too much...and we were right, I bent the sheeer pin ...so I changed it and put the prop back on according to the manual this time! I took it back out and still the same. Only when I went out that time I broke the Starter Rope and Spring! So the next day I went to a boat dealer and got a NEW Starter Rope and Spring and got it back out on the lake and STILL the same thing...run great but won't plane. I went back to the Johnson Dealer and the shop sugested I put in some NGK Plugs because the Champion plugs that Auto Zone gave me were not cross referenced correctly and were the worng number plugs for the 1975 Johnson. They said it may run better... so I tried them! We checked to make sure it was runing on both cyl. and it has hot, blue spark on both. I took it back out yesterday and she ran even better than before at an idle and she's purring like a kitten at low speed but she still won't get enough power at full throttel to plane!!! Any Sugestions??

 

[JB]

Re: '75 Johnson 25hp won't plane at full throttle...

Howdy, ArmyGirlmp.<br /><br />What happens if you pump the bulb while the engine is refusing to plane?<br /><br />Do you have a tachometer?<br /><br />Have you checked both plugs for a hot, blue spark? (By the way, I always found those old OMCs ran best on Champions).<br /><br />If you have hot spark on both cylinders and pumping the bulb doesn't help, have someone squirt a bit of fuel mix into the carb throat while trying to plane.<br /><br />Let us know what you find.

 

[fireman 757]

Re: '75 Johnson 25hp won't plane at full throttle...

You may have spun your prop. Torn up the rubber seal in it so it "spin" when under a load. Are your rpm's as high as they were to start with?

 

[armygirlmp]

Re: '75 Johnson 25hp won't plane at full throttle...

I don't have a tach to tell me the RPM's. But thanks for the tip...I will check into that!

 

[DJ]

Re: '75 Johnson 25hp won't plane at full throttle...

Armygirlmp,<br /><br />I agree with JB on the Champions. Installing a "hotter" plug in an outboard is NOT a good idea. You run the risk of melting down the pistons. If it won't run with the recommended plugs, the problem is NOT the plugs.<br /><br />I think you are losing a cylinder at speeds over an idle.<br /><br />Unplug one spark plug at a time and see if it doesn't have the same power trying to get up on plane.

 

[armygirlmp]

Re: '75 Johnson 25hp won't plane at full throttle...

Pumping the bulb while tring to plane didn't change anything...the blub was actually too hard to pump enough to make a difference. I checked for hot, blue spark and have it on both. Couldn't check squirting in mixture...I will check that next. I don't have a tach on this old wreck, sorry. Any more sugestions?

 

[armygirlmp]

Re: '75 Johnson 25hp won't plane at full throttle...

The plugs I took out of it in June were Autolites. She ran great on those before. I then changed those to Champions because Auto Zone didn't carry Autolites. Come to find out...Auto Zone ended up giving me the wrong plug. The # on the Autolite was a 7 series and the Champion plug Auto Zone gave me was a # 8 series. The Johnson Dealer cross ref. them and told me that the number on that set of Champion plugs Auto Zone gave me were the wrong number for the 1975 Johnson in the first place. So sugested I try the NGK's in the 7 series. He assured me that the NGK's were cross ref. to match the recommended plug for this engine. (He told me the higher the # is.... weaker the spark is.) I feel pretty stupid. <br /><br />When I bought the boat 2 yrs ago the guy I bought it from had compleatly rebuilt the whole engine. He told me that he had tried to run the recommended Champions in it but she just didn't like to run very well on them so he stuck with (recommened cross ref.) Autolites or NGK's. I happen to remember that last weekend when I was trouble shooting. That's one of the reasons why I changed the plugs. Thought that may be the problem. The other reason is because the Champions seem to have a bit of oil on them when I checked them. They weren't burning it off. Now I know why...they were the wrong # plugs for the 1975 Johnson engine in the first place.<br /><br />What a mess...lol.

 

[fireman 757]

Re: '75 Johnson 25hp won't plane at full throttle...

Did you do what djohns19 suggested and check for spark? He is probably right. Sounds like you dropped a cylinder to me too. You say you are confused but took none of the advice offered. You probably have a bad ignition coil and it is not burning the fuel it is getting. That is why no matter what plug you put in will come out wet. Don't blame the plug if it is not getting fire. Check for spark and repost and we will do out best to help you.

 

[armygirlmp]

Re: '75 Johnson 25hp won't plane at full throttle...

We have checked and both have hot, blue spark.

 

[fireman 757]

Re: '75 Johnson 25hp won't plane at full throttle...

It's possible that you have some crap in your high speed jet. Any little piece in there will cause this problem. Spray it out with some compressed air or use a small wire like a bread twist tie without the coating to clean.