Johnson 15 hp, 1975 bottom plug fouling

[Alby1]

Scratching my head on this one. Need to know where to look.
Here is the problem (not me of course). Brought the motor in for tune up because it kept fouling the bottom plug. When on the lake I kept changing the plug right after fouling, with a new one, and would run ok for some time. Thing about it is the motor runs perfect at mid throttle to WOT, not missing a stroke. Motor didn't get hot, (normal temp). As soon as I come down to low throttle or idle, the bottom cylinder goes dead. But, if I can get the rpm back up to mid range and up, the bottom cylinder kicks back in.

In the shop I changed points, condenser with new ones. Checked all wires for loose or broke connection, including mag plate to engine grounding wire. Checked ignition coil's for resistance, both checked ok. Even swapped positions of coils top to bottom. Timed engine with timing fixture. Checked mag plate timing on cam follower. Checked compression; Top @119 psi, bottom @120 psi. Checked Ohms of Driver coil (2Ω). Used Coil Locating ring to set gap on the Driver Coil.

Put the muffs on, fired right up. Idled a little rough and adjusted Idle screw to the lean side a bit and seemed fine. Throttled up good. Put it on the boat and took it for a test run. Started good at marina, idles good, idled 300 yards though slow zone, opened it up and run WOT to the other end of the lake and back (about 40 minutes running @ WOT) . Throttled down for the slow zone and the problem was back, running on one cylinder. Immediately checked engine temp and it was normal. Ran on one cylinder all the way through the slow zone to the trailer.

Back at the shop. Checked my work and things look ok. Bottom plug looked wet and definitely fouled. Put spark checker on it @ 1/4" gap and the spark was a nice pretty blue. BTW, I tried different spark plugs in previous trouble shooting and the fuel (50/1) is new., made no difference.

Where do I look next? Do I have a reed valve problem I wonder?

 

[Bonaventure]

I think your low speed mixture could be too rich. It is possible that at low speeds , one cylinder may get more fuel than the other, but I am not an expert on this. The 50 to one mixture is mainly for high speed lubrication. At lower speeds the engine doesn't need as much oil. 2 stroke motorcycles that have oil pumps are designed to give a varied mixture, the higher the RPM, the more oil is injected. With premix, the oil is calculated at the top of the RPM so there is no chance of getting insufficient lubrication.

I would go through the steps of adjusting the low speed settings to see if you can improve the engine's performance.

 

[flyingscott]

Have you looked at the carb? I would put a rebuild kit in that especially if it has the original cork float.

 

[Tim Frank]

What plugs are you using? I've had that problem with NGK plugs in a 6 HP of that vintage....switched to the OEM champions and problem went away.
(in fairness to NGK....tried Champions "equivalents" in my Yamaha M/C and had frequent fouling. went back to the NGK OEMs and problem solved.)

 

[oldboat1]

Might be a chance it is running too cold, and loading up. It could be worth just pullng the head, taking off the t.stat cover and checking the stat (if in place). The location is tough, and the bottom head bolts have to be loose but partly inserted to remove and replace. Will want to resurface both the head and t.stat covers ( r & r the t.stat and cover on the workbench.)

 

[kbait]

NGK B7HS @ .030"

Is there any lateral play in the armature plate?? That'll mess w/point gap/timing. If there's play, post back..

 

[thumbnut]

Kbait is on to something...I've got a '76 with loose armature plate. I just installed a pair of Atom point eliminators, NOS off ebay. The motor runs so much smoother and throws a much better spark. When one adds the running clearance of the upper needle bearing and the running clearance of the plate, which can be huge when worn, there is no way to get a consistent gap when running.

There is still a pair on ebay for sale...search "magnatune"

Also there are unequal intake tract lengths necessitating that baffle on the intake. That doesn't help...

That bottom condensate pump needs to be beefed up a little. Mine accumulates a lot of liquid on the bottom cylinder and causes funky low speed running.

I run a b6 ngk on the bottom and the b7 on the top.

Thats all I got...

Oh...if one takes a look at the 93 and up intake there is a 3/8 passage between the end of the intake runners. Something like an equalization passage..probably to mitigate the unequal length runners. Last winter I made that passage on my '76 intake with epoxy putty and a burr bit. I am glad I did...

 

[flyingscott]

I don't think your stator is your problem. Sloppy stators affect both cylinders and if it is your stator that is an easy fix. Either stake it or get shim stock and make shims and put them under the stator to stop the slop. If you have the original cork float it could be allowing the carb to leak through the needle valve. Before I would put atoms on one of those motors I would buy the OMC later CDI ignition for it an easy retrofit. The 93 motor is different as it is the 15 CID block that is a different design than you 13 CID block

 

[thumbnut]

You will need the 77 or 78 cdi, coils, stator and wheel. $108.00 for the pair of Atoms. My loose stator affected the bottom cyl the most according to my timing light. When I set it up static it was dead on but not while running. I think the only real difference on the blocks are the funky intake ports on the 9.9's, the ones I've seen anyways...besides displacement and that intake manifold. If there are other differences I'd love to know. I used a center punch to kind of knurl the pivot boss but it did not last.

 

[flyingscott]

You can use the stator assembly/coils and flywheel off of any 9.9/15 up to 87. I have made that conversion many times and the advantage is you are not stuck with parts no longer made. I actually put the magflash ignition onto an 84 block because the guy was to cheap to buythe CDI ignition. On Ebay you can do the whole conversion for $150 or less.

 

[thumbnut]

I think the 75 uses that smaller white throttle roller with the metal throttle cam. The later 79 and up uses the larger black roller on the plastic cam. I don't think the 79 and up stators will screw on to the 75 swivel mounting plate. There was also talk of a keyway clocking issue as I remember.. If you say it does great..when the Atoms puke I'll go that way.

To the op..those motors run funky on the bottom cyl anyway. You have probably checked out leeroysramblings web site?