1972 Johnson 125 problem

[timmer66]

I have been trying to get this motor running properly for two seasons now, and I've come to a wall.

When I first take the boat out, the motor revs up and pushes the boat just great. I can run WOT all day without a hitch. When I slow it down to 3/4 it runs just great there, too. When I slow down to an idle, while in gear, it runs pretty good.

But when I want to "hit it" again, it starts to rev up and feels like it has no power. The boat won't plane, and the engine sounds like it's really struggling, so I back off the throttle. This will go on anywhere from 20 minutes to all day. Meanwhile, it no longer wants to idle steady, so I have to run it about 1/4 throttle.

Sometimes after doing this for 20-30 minutes, it decides it wants to run again, smooths out the idle, and I am able to cruise at full throttle again. Then the cycle repeats.

The motor has rebuilt carbs, new plugs, new power pack, new coil, new wiring, new fuel pump, and fresh 50:1 super unleaded fuel.

Any thoughts/suggestions would be greatly appreciated!

Thanks!

 

[tashasdaddy]

Re: 1972 Johnson 125 problem

it only need 87 octane, quit wasting money on premium. have you cleaned your carbs and installed a rebuild kit? have you check spark on all cylinders when it is acting up?

 

[emdsapmgr]

Re: 1972 Johnson 125 problem

Follow TAS's advice on the spark test. Electrical problems show up after the engine gets warm. Mine did that. Found the timing sensor would go bad after it warmed up.

 

[timmer66]

Re: 1972 Johnson 125 problem

Yes, carbs have been cleaned and rebuilt. I was unware that high octane fuel was not necessary - that's good news.

I will definitely check the spark next time it acts up - what/where is the timing sensor?

Thanks!

Oh - I also have a brand-new 900A battery installed.

 

[HighTrim]

Re: 1972 Johnson 125 problem

If testing in the driveway, a hairdryer can be used to heat the ignition components separately while observing the spark on the inline tester. Set the gap on the tester to 7/16 of an inch. It should jump this with a bright blue ZAP!!

Let us know what you find. Determining that a particular cylinder(s) is/are not firing will lead us to a better conclusion.

These testers can be bought at any auto parts store for about 10 bucks. Get the inline adjustable type.

 

[timmer66]

Re: 1972 Johnson 125 problem

I got a tester - the adjustable kind was not available, so I got the one with the neon strobe.

Cylinder #3 is very weak with an erratic bright pulse about every 3/4 a second, while the other three cylinders were steadily flashing bright, possibly a slight skip every now and then, but nothing like #3.

This was done in the driveway at idle, the motor sitting in the sun all day.

I double-checked the tester on my truck to make sure I was reading correctly.

 

[timmer66]

Re: 1972 Johnson 125 problem

Spark plug wire looks good - what should I try next?

 

[steelespike]

Re: 1972 Johnson 125 problem

Don't assume wire is good there could be hidden corrosion You could just replace the wire,use solid core wire.If there are seperate coils you could swap them see if problem follows.

 

[HighTrim]

Re: 1972 Johnson 125 problem

May be the ground on the coil needs to be filed and cleaned. If that does not do it, do as Steelspike stated. Swap the coil to see if the problem follows.

 

[timmer66]

Re: 1972 Johnson 125 problem

The coil (there is only one) is brand new. With the old coil, the engine performs a little weaker. There is a crack in the housing of the old coil, so I replaced it.

I think I may have bigger problems now - as I tried another set of spark plugs, the standard type with a gap, and the piston in that cylinder smacked the gap closed! I put the new ones back in, the flat marine type, and it still runs rough.

Could the rod bearings be going out? What else could cause this?

Thanks-

 

[tashasdaddy]

Re: 1972 Johnson 125 problem

now is time to check compression, to make sure that piston the smacked the plug, is not damaged.

 

[CharlieB]

Re: 1972 Johnson 125 problem

One coil?

1972?

Points?

TWO sets of points under the flywheel

Clean and adjust gap on BOTH sets, one is dirty and you have a weak spark, probably weak on two cyl's, you just noticed it on one.

While you have the flywheel off take a good look at your distributor and rotor. Look to see if there is any carbon tracking to ground from the inside post for #3

 

[emdsapmgr]

Re: 1972 Johnson 125 problem

I think you are at the point where you need a factory service manual, so you can do some component testing. You need to test output on the pulse pack, stator and the sensor. The manual should give you some ohms tests on each component. It is possible that it will also give you some peak voltage tests on the same components. I'd follow Tas' advice on the compression -first. You don't want to put a lot of $ into a block with low compression.

 

[timmer66]

Re: 1972 Johnson 125 problem

I will check the compression tonight. I just got a Johnson service manual for this motor, still hoping I don't need to tear into it. I'll run some of the tests listed, and post my results.