1979 100hp johnson-LOW RPMS ONLY.

[lennygilbert]

I have a 79-100hp johnson on my 1979 sterncraft boat.Motor runs good at idle.I can move the warmup lever all the way forward and the motor will jump up to 5000rpms with no problem.When i put it in gear and give it full throttle,I am lucky if i see 2500rpms.It feels like i am pulling a large safe through the water.The boat is creating a huge wake and will not plane.Every so often the boat will plane if someone heavy is in the front and the motor will climb to 2800-3000 rpms for about 30 sec then the boat unplains and goes back to around 2300rpms at full throttle.I was told it could be the prop.its a 13 3/4x25.out of the water with a flush kit on the motor will run 5000rpms turning the prop.Its just when its in the water and under load...<br />When i bough the boat last month the timing arm advance spring was gone.I repalced the spring but think the previous owner could have tried to time it himself???<br />Also when priming the fuel bulb it never really gets solid.Its gets kind of hard but not what my other one did..Replaced bulb,same thing.When sqeezing the bulb it takes a min for the bulb to com to fill back up.....Any ideas??????

 

[DJ]

Re: 1979 100hp johnson-LOW RPMS ONLY.

First off, DO NOT run the engine to 5K with no load, prop engaged or not. You do not want to "ventilate" the block. Multi cylinder two strokes will wind up just fine, with no load, with only a couple of cylinders working.<br /><br />This engine needs to be propped to run 5500 rpm with a light load. I have no idea how heavy your boat is, but a 25" wheel is pretty steep.<br /><br />1. Do you have spark on all four?<br /><br />2. The timing needs to be set at wide open throttle, while under load. The safest way to do that is with a test tank and test wheel.<br /><br />3. Do all the spark plugs look the same?<br /><br />4. Get a manual, available at botom of this page. It will be the best $30.00 you spend.

 

[lennygilbert]

Re: 1979 100hp johnson-LOW RPMS ONLY.

I havent checked the compression yet,and im pretty sure its getting spark to all cyl.Its not missing at wot.It does however try and die at low rpms such as while idleing through a no wake zone.I have to move the warm up lever a little bit forward in order to keep it started.I think the idle may be set to low...

 

[lennygilbert]

Re: 1979 100hp johnson-LOW RPMS ONLY.

Also I was told that this make of boat was one of the heaviest made...79 sterncraft...

 

[lennygilbert]

Re: 1979 100hp johnson-LOW RPMS ONLY.

TTT

 

[Gypsy 15]

Re: 1979 100hp johnson-LOW RPMS ONLY.

I had the same problem with my 1973 65 I went through the ignition system all three coils hac cracks so they were replaced the engine ran more even but still had a power loss at sea I then rebuilt th fuel pump and replaced the fuel line. We went to sea with it on Sunday and the engine ran at full power OK

 

[CharlieB]

Re: 1979 100hp johnson-LOW RPMS ONLY.

Boats need substantial fuel flow to make power on the water.

You stated that the primer bulb takes a while to refill once you squeeze it, this may be a clue that you have a restricted fuel line or fitting BEFORE the bulb, pull the pickup tube from the fuel tank, check to see if it is clogged up.

Any fuel restriction will cause your engine to run lean and MELT PISTONS.

Verify fuel flow, it doesn't make much pressure, 4 - 5 psi but it should flow a strong 5/16 stream.

 

[Theoutdoorsman]

Re: 1979 100hp johnson-LOW RPMS ONLY.

This engine needs to be propped to run 5500 rpm with a light load. I have no idea how heavy your boat is, but a 25" wheel is pretty steep. Get a manual, available at botom of this page. It will be the best $30.00 you spend.

Just as a referrence, I have a 17 foot maidencraft bass boat equipt with a 1979 Evinrude 140hp. I'm only turning a 19 pitch steel prop with a double cup at 5600-5800 rpm's depending on my load. I think it'd be safe to say your waaaaay over propped for your setup. Maybe you have a friend who will let you try theirs to give you an idea as to what size you might need. Definitely check compression and spark before making ANY other adjustments. If I had to guess, you only need a prop if these check out fine.

When i bough the boat last month the timing arm advance spring was gone.I repalced the spring but think the previous owner could have tried to time it himself???Also when priming the fuel bulb it never really gets solid.Its gets kind of hard but not what my other one did..Replaced bulb,same thing.When sqeezing the bulb it takes a min for the bulb to com to fill back up.....Any ideas??????

The issue with the spring missing may be why the previous owner sold the boat in the first place. Looking at your ignition systems' exploded diagrams, I don't even see one listed??? Where is it located? When priming the bulb, try holding it vertically (pointing towards the sky) if it's an aftermarket bulb and not OEM. By the way, you do have the arrow pointing toward the motor and not the fuel cell.........correct? I agree with the above. If the problem continues, inspect and remove the anti-siphon valve in the head of the pickup tube. But do so ONLY if your fuel cell is positioned BELOW the outboard, which most are.