johnson 20 runs good but will not open up at full throttle

[jwilkey84]

I just got this motor running good, except for one thing. It is a 1972 20hp johnson. When I throttle all the way up with the motor in gear, it won't get up on plane but one in 10 times probably. It is on a 12 ft lund fishing boat, so it isn't a weight issue. There is no water in the bottom of the boat. Just me, a couple of gal. of gas, and a battery. The choke is in the normal position. The fuel bulb stays hard and squeezing it had no effect. The armature plate is turning all the way. compression on this motor is great. I need help figuring out what to do next.

 

[Xcusme]

Re: johnson 20 runs good but will not open up at full throttle

I just got this motor running good, ....

What have you done so far to get it "running good"? Good is a relative term, you missed something....

Questions that need answers:

Compression numbers?
Carb clean?
Sync n' Link settings?
Verify Spark at both cylinders?
Under flywheel inspection--coils, point gap etc.

 

[jbjennings]

Re: johnson 20 runs good but will not open up at full throttle

Jwilkey,
Sounds like you're only getting one cylinder firing. I'd almost bet on it. You might want to check to see that the throttle plate is opening up all the way when you have it wide open. You can check for both cylinders firing by pulling a plug wire when it's running and see if it dies. If so, try the other one. If pulling a plug wire has no effect on it running, that wire goes to the bad coil or whatever. As you already know, if it's getting power from both cylinders, that 20 should roar on a 12 foot boat.
Just my opinion,
JBJ

 

[jwilkey84]

Re: johnson 20 runs good but will not open up at full throttle

spark is good, but I guess I can check it under load by pulling the plug wires. Under the flywheel, everything looks new. Carb has been cleaned, rebuilt, and cleaned and cleaned again. All fuel line is new. All wiring for the key switch and electric choke is new. Fuel pump has been disassembled, and looks great. compression was like 125 in both cylinders. water pump is new. I just had the lower unit gears out last night and looked good, new oil. Fresh gas as of 3 days ago. I think the only thing I havent done is to take off the plug wire while it is running. But is idles and starts so good, could it be running on one cylinder? Every once in a great while it will act like you just gave it a shot of nos and it will take off but then I get scared, because it is on a 12' boat and it was windy today and the boat is very responsive so I let off. I havent done a linc & sync, mainly because I never have and don't know how. I have the seloc manual, but as far as I can tell it doent tell me how to do that, and in my honest opinion seloc manuals are junk and I'll never buy another one. What a waste!

 

[jbjennings]

Re: johnson 20 runs good but will not open up at full throttle

JWilkey,
You said "every once in a while it will take off like you gave it a shot of NOS".
I'd now bet my last dollar after that quote that you have one cylinder with intermittent spark. Somehow you've got a spark problem. check the ends of your wires for corrosion inside where the coils go into them if you have the old universal magneto setup. Also check in the plug wire boot of the faulty cylinder. I'll bet when you pull one of the plugs under load it will have no effect when you pull the wire. These spark problems can be pesky varmints sometimes and just because a coil looks good it doesn't mean it is. Remember, if there' just one break in those secondary windings inside the coil, it will ruin it. And the secondary windings are made of tiny wire. You might even try using a cheapo meter and testing the spark plug wires for an internal break by checking the resistance while flexing the wire. You can bet you have a short in the insulation, bad connection, or faulty coil/condenser/pts. or whatever.
Good luck finding the gremlin,
JBJ
BTW, if it scares you when it surges it will REALLY scare you when it gets both cylinders hitting and goes wide open. I'll bet you it will be squirrely as heck and you're going to want/need a 14 foot boat for it. You can always not use full throttle, though.

 

[jwilkey84]

Re: johnson 20 runs good but will not open up at full throttle

yeah, I am just borrowing the boat for something to test the motor on.

 

[samo_ott]

Re: johnson 20 runs good but will not open up at full throttle

"JWilkey,
You said "every once in a while it will take off like you gave it a shot of NOS".
I'd now bet my last dollar after that quote that you have one cylinder with intermittent spark. Somehow you've got a spark problem."

I agree 100% with JBJ. It's an intermittent spark based on the NOS comment. Look closely, the problem is there somewhere.

And a 20hp on a 12' Jon boat should fly! I find an 18 on my 14' aluminum is friggin' fast!

 

[jwilkey84]

Re: johnson 20 runs good but will not open up at full throttle

this sounds a little redneck, but I have 40hp on my 14' aluminum semi-v and we pull skiiers!! And yes it's a tiller also.

 

[samo_ott]

Re: johnson 20 runs good but will not open up at full throttle

this sounds a little redneck, but I have 40hp on my 14' aluminum semi-v and we pull skiiers!! And yes it's a tiller also.

I think I hear the Deliverence theme somewhere!

 

[jwilkey84]

Re: johnson 20 runs good but will not open up at full throttle

Ok, I took the boat out this morning. Same things....but I pulled off the plug wires while at idle; it affected the motor equally. I also put the motor under full load and pulled off each wire individually, it also affected the motor equally. I then tested the spark under load and at idle, with a spark tester, and it will jump a blue 1/4" spark all day long. The only other thing I can think of is the fact that I have this long shaft motor on a boat that is designed for a short shaft motor. with the cavitation plate being that low in the water, and me a gas can and a battery all in the back of the boat with no weight in the front, is it possible that it just isnt getting up on plane? I think that may be the issue, because it is just like I am pulling an anchor around. Ideas, thoughts? I was once told that you can use a long shaft motor on a boat designed for a short shaft, but I am beginning to wonder. Currently my cavitation plate is about 8" below my hull and the prop is about 13" below the the bottom of the hull, could this be my problem all along?

 

[itstippy]

Re: johnson 20 runs good but will not open up at full throttle

Take it out again and try adjusting the vertical angle of the outboard to different settings. Straight up and down, angle in, angle out. Could be you're pushing too much water and the few times it "takes off" is when it manages to pop up on plane. WEAR a life vest. That rig sounds real dangerous - improper setup on an overpowered rig. Recipe for bad things happening very quickly.

 

[jwilkey84]

Re: johnson 20 runs good but will not open up at full throttle

that still doesn't answer my question. Is the fact that I am running a long shaft motor on a boat that is designed for a short shaft motor my main problem, in your opinion? Has anyone else ever put a long shaft motor on a short transom boat? If so, how did it perform?

 

[itstippy]

Re: johnson 20 runs good but will not open up at full throttle

I don't know for sure, since I have never attempted what you describe. But it is entirely possible that that is your problem. You cannot run a longshaft 20HP motor full throttle on a 12' jon boat. You are violating a bunch of laws of physics. You are ripping the transom apart because the long shaft gives much more leverage to the stress on the transom mounts. The anti-cav plate running 8" below the hull means you've got tremendous water resistance from the lower unit. As you say, just like your dragging an anchor around. You are not getting a good on-the-water test of the outboard.

 

[Benny1963]

Re: johnson 20 runs good but will not open up at full throttle

i run a 50 hydr elec drive 1972 on a14 by 48 welded bateaux
it is 20 inch you have to pick your motor up cav plate level with bottom of boat i used a braket from iboats it was cheap and worked great
picked it up six inches boat came on plane easily and ran 46 mph on gps
moved my trim adjust up 2 holes and ran 48 mph on gps at 5500
before moving up 3200 was the best i could get and it pushed water up and over transom on boat