1974 Johnson 25 armature/idle issue

Folks, I recently purchased my 1st Johnson outboard, a model 25R74M, and have an idle issue that I could use some help with. When I got the engine, about a month ago, it was in barely running condition. A shop had replaced the points and condensers just before I took delivery. Compression is 125 psi top cylinder and 115 bottom. I rebuilt the carburator using a Sierra kit after soaking the metal parts overnight in Gunk carb cleaner with load plugs removed, cleaned it up, replaced the driver coil, leads, ignition coils, and plugs, blew the fuel lines clear, replaced the fuel pump, and changed the gear oil. I followed the procedures in the service manual for sync'ing the cam follower/armature/throttle-valve and adjusting the low speed needle.

The engine now runs strong across the rev range, but has trouble at low idle. If I close the throttle slowly the revs drop and the engine continues to run (although a little roughly; I don't think I have the low speed needle quite dialed in yet), but if I chop the throttle, even from mid-range, the engine dies. With the cover off, when I close the throttle quickly it looks like momentum causes the armature to rotate clockwise a couple of inches past the point where it stops when I close the throttle slowly. The armature does not wobble, it just rotates too far, which I think (and this is speculation on my part, grain of salt here) retards the timing to the point where the engine cannot continue to run. The idle stop screw at the base of the tiller handle does not prevent this extra armature rotation when the throttle is sharply closed.

I looked in the service and parts manuals to see if I'm missing a spring or some other stop screw, but didn't see anything obvious. If one of you could tell me what is supposed to check the clockwise rotation of the armature, I would really appreciate it.

Re: 1974 Johnson 25 armature/idle issue

There is nothing preventing what you are describing. The magnetic pull of the flywheel magnets against the driver coil poles is always trying to pull the armature plate toward the slow position anyway. And now you are adding to it with the momentum issue. I give you credit for diagnosing the problem.

Can we assume you have the driver coil set back for correct clearance with the magnets?

Re: 1974 Johnson 25 armature/idle issue

I think I have close to the right clearance between coil and magnets, but I do not have the coil locating ring special tool. I could see that the original driver coil had scored the inside of the flywheel, so when I replaced it I positioned the new one a hair more towards the center, and did not hear any scraping when I rotated the parts by hand after reassembly.

Thanks for the info. It's good to know this is normal and that I just need to use a steady hand with the throttle. This is a great forum, BTW. I found lots of valuable info here while doing the work described above.

Re: 1974 Johnson 25 armature/idle issue

Pull the plugs and hand-turn the flywheel clockwise. If the arm. plate moves in the same direction, you either have the driver coil mounted too close to the flywheel, and it's physically rubbing, or you have excessive play in the armature plate itself. If the latter is the case, search forum for 'armature plate wobble' for fix. If that plate can move laterally at all, it WILL change the point gap, and thus the timing, and it won't idle well. Good luck!

Re: 1974 Johnson 25 armature/idle issue

That is good advice. But it should be known that those particular model of motors had sloppy armature plate mounting from the git-go. It's a matter of how much can be tolerated.