1976 25hp Johnson ignition question

[gmarshall43]

Hello all

I normally work on the smaller hp motors but I was asked to work on a 1976 25hp Johnson that has sat for many years and won't start. Needless to say the ignition is a little different then a smaller hp motor. There are two blue coils on the side of the motor that go to the plugs and there is one coil with two sets of points and two condensors under the flywheel. There isn't any spark on either plug. Can someone explain to me how this all works?

Gary

 

[F_R]

That is the beloved OMC Lo-Tension Magneto.

Do you want the electrical engineer's answer or the simple answer?

Simple answer is the driver coil generates an electrical current as the magnets pass by. The current is from driver coil, through one set of (closed) points, to armature plate, through other set of (closed) points, and back to other end of driver coil, completing the primary circuit. When one set of points opens, primary circuit is interrupted and a surge of electricity is directed to the spark coil, which increases the voltage to fire a spark plug. Note that BOTH sets of points must be closed as the current is generated. The points in your picture look cruddy to me.

 

[gmarshall43]

That is the beloved OMC Lo-Tension Magneto.

Do you want the electrical engineer's answer or the simple answer?

Simple answer is the driver coil generates an electrical current as the magnets pass by. The current is from driver coil, through one set of (closed) points, to armature plate, through other set of (closed) points, and back to other end of driver coil, completing the primary circuit. When one set of points opens, primary circuit is interrupted and a surge of electricity is directed to the spark coil, which increases the voltage to fire a spark plug. Note that BOTH sets of points must be closed as the current is generated. The points in your picture look cruddy to me.

Thanks for the information. I will assume that if that driver coil on the plate goes bad, you won't have spark on either coil.

What should the compression be on each cylinder?

 

[jimmbo]

Whole Armature plate looks a little oily. Could need the upper crankshaft seal replaced.

If the either of the points are dirty, no spark will happen either

 

[gmarshall43]

I'm getting things put back together and I was wondering what that white plastic piece is at the top left of the motor is and what it does? I ask because it looks like the tip of it is broken off.

 

[F_R]

It is the recoil starter lockout. Prevents starting in gear.

 

[gmarshall43]

So I cleaned up the points and set them them to .020, compression is about 90 in both cylinders, clean and rebuilt the carb and the fuel pump feeds fuel to carb but it will not start. There was a bit of oil on the coil plate. I cleaned that up and I am getting good spark but won't try to turn over at all. If that upper crank seal is bad, would that cause the motor not to turn over? Anything else to check for?

Thanks,
Gary

 

[racerone]

Bad upper seal , it would still start and run on bottom cylinder.

 

[jimmbo]

If the engine doesn't turn, how did you manage to gap both sets of Points?
A bad Crankshaft Seal will not prevent the motor from rotating. It will allow Air in and oil-fuel out.
What are the compression Numbers? Spray a bit of Premix in the carb and see if it starts

 

[saltchuckmatt]

So I cleaned up the points and set them them to .020, compression is about 90 in both cylinders, clean and rebuilt the carb and the fuel pump feeds fuel to carb but it will not start. There was a bit of oil on the coil plate. I cleaned that up and I am getting good spark but won't try to turn over at all. If that upper crank seal is bad, would that cause the motor not to turn over? Anything else to check for?

Thanks,
Gary

Your description of turn over is confusing. I'm assuming you mean it won't run. Obviously it's not stuck and will rotate. Like some someone else try spraying fuel directly in the carb or some starting fluid. If it farts or runs for a little bit then you know it's a fuel problem. Also, spark wise you need a tool that tells you how far it will jump. (Spark strength) as far as 90 lbs of compression, there at about 140 new and I've seen them run at 90 but never all that well. Probably better to toss it but I've been in your shoes so good luck.

 

[Kawaiikeno]

Also make sure the wire from the driver coil to the bottom set of points isn't grounding on the points it's hard to see from the picture.
Note I have replaced 2 driver coins on my 18hp that has the same setup in 2 years

 

[gmarshall43]

Sorry for the confusion but the motor is not stuck and does turn when pulled but it won't start. I mentioned the compression was about 90 and that was from pull starting it by hand. It does have electric start so I got a battery to run it and the compression is now 120lbs. I need try spraying starting fluid into the carb when I first try to start it and it did fart. I tried it a few more times but it didn't fart anymore. Maybe I flooded the carb? When you refer to the bottom set of points, would that be to the bottom cylinder? And if it is grounded, would I still see spark?

 

[racerone]

Both set of breaker points must be clean, clean.

 

[jimmbo]

Starting Fluid, a great way to ruin a 2 Stroke. JFC...

 

[gmarshall43]

So I got this motor running but it doesn't run for long. The problem now is that woodruf key on the crank shaft keeps breaking. I've replaced it 3 times. It seems to fit snug in both the crank and the flywheel. Any ideas why it keeps breaking?

 

[saltchuckmatt]

So I got this motor running but it doesn't run for long. The problem now is that woodruf key on the crank shaft keeps breaking. I've replaced it 3 times. It seems to fit snug in both the crank and the flywheel. Any ideas why it keeps breaking?

Others have said before because your not using a stock omc key. I'm sure people will pipe in shortly.

You might have to lap the flywheel and are you using a torque wrench to tighten the nut?

 

[racerone]

Stop-----You need to use a factory key.-----And the key is there to locate / time the flywheel for proper spark.-----The flywheel is driven by the locking tapers.------So get some valve grinding compound and lap the tapers for a proper fit.----Clean the tapers.----Assemble clean and dry.----Torque to specification.

 

[gmarshall43]

I bought OMC 120395 keys and have 1 left. Hope lapping the tapers works. Those keys are pricey. Where can I find the torque specs?

 

[lindy46]

Torque flywheel to 40-45 ft.lbs

 

[gmarshall43]

I’m going to lap the flywheel this week but I also would like to know which set of points goes to which coil. This motor has the 2 coils on the side of the motor and the 2 set of points and another coil under the flywheel. How do I tell which points goes to the top and bottom coils? I would like to be sure before I put it all back together