1978 Johnson 75hp

[skylar]

My problem with my motor is that I brought it to a mechanic and he couldn't figure it out. I was out fishing for three days and the motor was running fine and then all of sudden the next day it starts fine and runs fine in trolling speed but when I tried to run it wide open it was bogging down. I changed the fuel pump and the carbs cleaned ,compression was good and it still happening. It ran fine on the mechanics diagnostics test. Please give me some ideas!!

 

[Darbyz71]

Re: 1978 Johnson 75hp

Hello, there are a few things to check at this point. If she runs good at
idle and trolling but not at wot then u have either a fuel starvation problem or a clogged up hi speed jet. U said u cleaned carbs and replaced fuel pump but u may need to replace ur fuel hose And all under the cowling fuel lines and give ur tank a good cleaning as well. A old fuel hose gets soft and starts to break down whereas u get ur hi speed jets clogged. This ethanol in our gas now days is eating up the fuel lines and fuel pumps on numerous boats that I see lately. I would also check to make sure u have a bright blue spark on all three cylinders. If u happen to go back into the carbs again I'd suggest rebuilding them but I'm sure you've thought of that. Another quick check when the carbs are off is the reed valves. Just look in behind the carbs and make sure they all are still there and that none of them have a noticable gap. You also may want to double check carb linkages and look for proper throttle advance as well as missing are broken parts under the cowling. Just a couple of things that may help from an old school guy. That 75hp is great motor if taken care of her and if u know a few tricks you can make her scream but lifespan shortens. God bless to all

 

[Chinewalker]

Re: 1978 Johnson 75hp

It sounds to me like you might have a power pack going bad. Your motor will run on two cylinders, giving the impression that it is running "fine". I've seen packs that produce spark in open air, but under compression they break down.

How thoroughly were the carbs cleaned? Disassembled, blown out, new alcohol resistant kits installed? A clogged carburetor or an old swollen gasket could cause the same symptoms.

I concur that the 75s are great motors. I've got a 1980 on my 15-footer and it is a really nice performer - reliable, good on gas and looks sharp, too!

 

[skylar]

Re: 1978 Johnson 75hp

Thanks guys for some of the ideas, but I changed the fuel line and checked the spark and that was nice and blue on all three plugs. I'm down to rebuilding the carbs or selling the boat!! I've never rebuilt carbs before are they hard to do? Would it be expensive to take it to a good mechanic and have them do it?!! Totally confused!!!

 

[Darbyz71]

Re: 1978 Johnson 75hp

Hi again, let's ask ourselves some questions. Does the motor run fine at idle?Does it have good spark at all three cylinders? You said you have bright blue fire. Ok is there any linkages broken? Look closely at them as swell as try movi g them around with ur fingers. Under the flywheel is a timing advance arm with a plastic ball n it. Make sure it advances with the throttle. Now ask your self again, when you put it in gear does it still ideal fine? Now ease the throttle forward with it in gear and keep easing it and see if the boat will slowly pick up rpms. If it does keep going until ur on plane or almost then spits and spudders then dies then ur fuel supply " pump" isn't suppling e ough fuel. Now if it dies as soon as u put it in gear you need to watch with a spark tester to determine if u kill the spark to her and then it dies or does it just die? If the spark stops then u have a faulty controller thus a other forum. If your spark is still there and she just drops out then it could be several things. As far as ur carb rebuild, take a picture of the linkages just in case u forget. Now make them top middle and bottom. Pull them off and rebuild them one at a time. Slowly take them apArt taking great notice of all your
o- rings and gaskets. Once completely apart, clean it thoroughly!!!! Taking compressed air and blowing every port and hole out after a generous amount of carb cleaner is applied. Now just rebuild slowly with all the new parts and following the 1/8 - 3/4 float rule" instructions in kit. If u forget where something goes just open up another one and take
a look at what ur missing. There will be leftover parts from the kit u purchase. Oh kits are about 20 bucks each last I looked. Now that's the quick easy way to do it. The correct factory taught method is a tear done a d 24 hr soak of all parts is a carb cleaning fluid then re building. Sorry for the type o's but I'm on my iPhone. Don't ever give up, the longer you fight it and the more knowlege you gain the easy it will become. In 25 yrs I've never found a unfixable one yet. Keep us posted and good luck. Gob bless

 

[jpwade_bsu]

Re: 1978 Johnson 75hp

... I'm down to rebuilding the carbs or selling the boat!! I've never rebuilt carbs before are they hard to do?

If you are curious about fuel flow into the intake from the carbs.

pull the covers off the face of the carbs, pull the drain plugs on the carbs to verify the bowls are filling up. Use a cup to catch the drain off as you pump the fuel line bulb --- flush the bowls.

After flushing the bowls run the motor, if you dont have a cooling water boot for a garden hose, put the boat on a step then goto WOT and note inside each carb the fuel flow...... if a carb is not dumping fuel into the intake it is not hard to tell


Watch where u are going, throttle off as you start to plane off and restart observations. if you feel uneasy get someone else to drive while you visually monitor the fuel flow into the intake from the carbs

The hardest part with cleaning carbs is taking the time and getting started