1972 20 hp Johnson seahorse

[Jonboat23]

I have a 72 20 hp seahorse it seems that it doesn't want to get up to full throttle while in forward. But in neutral it revs all the way up. But when I get it in forward it barely seems able to plane out my 12 ft jonboat. Any idea about what is wrong

 

[kbait]

Re: 1972 20 hp Johnson seahorse

It's likely only running on one cylinder. Check for 1/4" blue bolt from each plug lead w/spark tester to confirm.

 

[ONERCBOATER]

Re: 1972 20 hp Johnson seahorse

12ft jon boat and 20hp.... umm well ok.... err mine is rated for 10hp.... I have a 15 on her... and with self and my 3 1/2 yr old son we moving out between 20 and 25mph. might want to check your transom and look for CG rating plate... I know the 20hp merc I stuffed on her 9 yrs or so ago was just flat out way too much.

That said i thinking that you likely running on one cylinder as well.... in fwd the prop is far more effecient than in rev... and therefor takes more power to get it to spin at a given speed.

Sean

 

[JB]

Re: 1972 20 hp Johnson seahorse

Howdy, Jonboat.

Welcome to iboats.

Revving the engine in Neutral (no load) is an invitation to thermal runaway and exploded engine. Never rev over about 1500rpm in a no load situation.

I agree with the above. . .dropped a cylinder.

 

[Jonboat23]

Re: 1972 20 hp Johnson seahorse

The jonboat I got is rated for a 15 hp. It's wider than normal so it can handle it. I checked the spark and the bottom cylinder is not firing. Seems like I have to take the flywheel off to check all the electrical out. Would it be the plug wire or the coil?

 

[Sandor27]

Re: 1972 20 hp Johnson seahorse

@JB :

"Revving the engine in Neutral (no load) is an invitation to thermal runaway and exploded engine. Never rev over about 1500rpm in a no load situation."

I have done this during a SeaFoam decarb. I hope I did not cause any damage to my motor. Since the impeller is directly connected to the driveshaft it will pump water regardless of being in neutral or in gear, right?

My motor ('70 6hp Johnson) did not seem to mind WOT (kept RPMs respectable) in neutral for the 15 minutes or so during decarb.

What leads to the thermal runaway and ultimately the possibility of an exploded engine?

Thanks,

Sandor27

 

[JB]

Re: 1972 20 hp Johnson seahorse

Sandor:There is not much agreement about what actually causes thermal runaway. The theory that makes the most sense to me is that residual combustion chamber heat puts the engine into a Diesel (or glo-plug) cycle on idle amounts of fuel.

When this happens (if that is actually it) closing the throttle has no effect, nor does removing spark. The engine runs away. If it reaches high enough rpm (way beyond normal top rpm) something breaks. Worst case breakage is an explosion, with shrapnel all over the place.

It doesn't always happen and when it does it doesn't always destroy the engine, but when it does destroy the engine it makes you very paranoid.

Jonboat23: You are 33% overpowered. The rating takes the width of the hull into account. You really need to go to a 15HP or smaller engine.

 

[Jonboat23]

Re: 1972 20 hp Johnson seahorse

Dont have a chance to go for a smaller motor. I may try and find a 14ft jonboat but cant afford to sell or buy another motor. What is more likely to go out first JB the coils or spark plug wires?

 

[ONERCBOATER]

Re: 1972 20 hp Johnson seahorse

i think 72 would be points style ign correct.... I would start 1st checking points if it is a points style engine. you can check the wires and coils by swapping them... using them both one at a time on the closer cylinder if you must.... that should allow you to eliminate coils,wires,plugs from equation. then you can focus on points next. If this model has the coils under the flywheel... it will be a lil more trouble.. and you can check the points while you are in there. my guess would be make sure they are clean and contact correctly... also would guess. about .020 but i don't have my manual with me as i write this.
http://www.boats.net/parts/search/BRP/JOHNSON/1972/20R72R 1972/MAGNETO/parts.html will give you a blow up diagram of your ignition system... please note it is points and condensor.... so you will have to check them as well.

Sean

 

[dew2]

Re: 1972 20 hp Johnson seahorse

Dont have a chance to go for a smaller motor. I may try and find a 14ft jonboat but cant afford to sell or buy another motor. What is more likely to go out first JB the coils or spark plug wires?

Do you have a electrical tester (ohm meter?) go to secret files and check the coils as instructed there,A spark gapped tester will let you see spark they cost about 6 bucks.For that six dollars I would start there.If you know there is no spark switch the coils top coil to bottom and viseaversa if the bad cylinder follows a certain coil you most likly have a bad coil.If low on dollars test test test then you wont replace a working part

 

[JB]

Re: 1972 20 hp Johnson seahorse

Top Secret Files is the first thread (read only) in this forum. That is what dew2 is referring to.

 

[1946Zephyr]

Re: 1972 20 hp Johnson seahorse

You may not have to go through the time of swapping coils, if the coils look good. If the insulation is cracked, they're likely arcing out on the mag plate. If your coils look good, then I would just start by polishing and regapping your points. If that doesn't work, then swap condensors. It's far easier than swapping coils. Still no sparky?? Then move on to the spark plug leads. Chances are one has lost connection either at the coil or the spark plug boot. Often times, people change coils and keep the old spark plug wires. Unfortunately, connection often times goes bad as a result. If you keep your old spark plug wires, then trim off 3/8" on the coil end and 1/4" on the spark plug end and you'll have fresh new connections on each end.

 

[Jonboat23]

Re: 1972 20 hp Johnson seahorse

Well managed to take off the flywheel and the bottom cylinder coil was cracked to pieces so i replaced it and put it all together and she runs like a champ. But when i tried to restart it she would fire hoping its just flooded will let yall know soon

 

[asdasc]

Re: 1972 20 hp Johnson seahorse

I hope you replaced BOTH coils while in there! they are pretty cheap, and it is a pain in the tail to keep pulling the flywheel. I have a few motors, and whenever I pull the flywheel, I just bite the bullet and replace everything under there - coils, points, condensors, and wires. It wasn't always that way, but my time is worth more than the $35 or takes to replace it all. Especially when I want to be on the water.