1995 15 hp Johnson, top cylinder missing

[NWChris]

Hi, my 1995 15 hp Johnson has a high speed miss on the top cylinder, I think. The boat runs fine at low speeds, but will miss off and on every few seconds at high speeds. Backing off on the throttle will actually make the motor run a bit better, but nothing like what it did years ago.

If I stop the engine and check the plugs, the top one is always wetter, the bottom grey and dry. I have switched coils by reversing coil and spark leads, still the top cylinder. I have done a warm compression test: 102 psi top, 100 psi bottom. I have checked spark gap by hand cranking: 1/2 inch blue/white spark on both cylinders. I have removed power pack and coil assembly and cleaned all grounding terminals. Head temperature seems fine, I can touch it for about 2 seconds, then ouch. Problem is there when warm or cold, but does seem to get a bit worse after about 20 seconds of running. I have tried: carb rebuild kit, fuel pump rebuild kit, new fuel line and tank, decarbed with sea foam three times (concentrated mixture), new plugs, and a new top seal. Nothing seems to help and the problem seems to have slowly gotten worse over the last year.

I think my next step is to buy a new power pack, unless someone has a better idea?

Thanks for all the great infromation I've found here.
-Chris in NW Ontario

 

[HighTrim]

Re: 1995 15 hp Johnson, top cylinder missing

What makes you mention the top cylinder?

 

[Daviet]

Re: 1995 15 hp Johnson, top cylinder missing

Is your fuel pump mounted on the top cylinder?
If so, maybe you have a fuel pump leaking fuel into the cylinder.

 

[NWChris]

Re: 1995 15 hp Johnson, top cylinder missing

Thanks for the quick response guys.

I believe the fuel pump is lined up more with the bottom cylinder, but I will check this tomorrow. Plus I changed the fuel pump diaphragm already.

I think it's the top cylinder missing because that's the plug that comes out wet, especially if I shut it down quickly while its missing.

I just hope its not rings, though the 102/100 psi compression test indicates probably not?
-Chris

 

[HighTrim]

Re: 1995 15 hp Johnson, top cylinder missing

I would bring an inductive timing light with you for the next test run.

You may have spark while testing, but lose it at WOT while underway.

Hook the light up to each cylinder, one at a time, to test the frequency/strength of the spark while underway.

If spark still checks out, use a spray bottle with premixed fuel in it and spray it into the carb throats while underway and it is acting up. If this straightens it out, you have a fuel delivery issue. You will obviously need to remove the face plate for this test.

 

[HighTrim]

Re: 1995 15 hp Johnson, top cylinder missing

....also be sure that the coil wire is not seperating while underway where it enters the timer plate

 

[NWChris]

Re: 1995 15 hp Johnson, top cylinder missing

I will try to borrow a timing light tomorrow, try these tests and report back. Thanks!
-Chris

 

[NWChris]

Re: 1995 15 hp Johnson, top cylinder missing

I hooked up an inductive timing light today, saw no change in either plug wire as the engine was missing.

I then removed the air box from the carb inlet to allow spraying in gas from a spray bottle. What was interesting was that with the free air coming into the carb throat, the engine ran significantly better!

Spraying in even the smallest amount of gas while the engine was missing caused the symptoms to worsen.

So I'm thinking it's an excess fuel issue now. As was suggested, I also noted that the fuel pump is more in line with the problem top cylinder than the bottom. Hmmmm.

So tonight I diassembled the fuel pump. I'm not sure, but I may have installed the diaphragm a little off center late last season, possibly causing gas to leak into the crankcase. I have corrected this, I hope, and will try the engine tomorrow and report the results.
-Chris

 

[NWChris]

Re: 1995 15 hp Johnson, top cylinder missing

Solved, but I'm not sure why.

I ran the motor yesterday, still not getting smooth WOT. I then took the engine off the boat and removed the fuel pump from the crankcase, and pressurized the primer bulb. No fuel dribbled out the hole that gets the pulses from the crankcase. I also pressure tested it by sucking air out that hole with my mouth, then sealed it with my tongue, and the vacuum held for quite a while. No rude comments about this test, please. So, the fuel pump was off my list, next stop the carb.

I disassembled the carb, it looked spotless. I blew out the passages with a can of compressed air anyhow. I then put back the OEM needle and seat that I took out last season as they looked fine. I also made a small adjustment to the carb float angle to make it level when upside down.

So I reassembled and attached the carb and fuel pump, and before testing I decided to replace the impeller and thermostat. I know doing too much at once is not wise as you never know what was the problem, but we are a family of three that live on an island on Lake of the Woods during the summer, and being down to just one boat is a pain.

So, off for a test drive, and it runs like normal now. It always had occasional slowing pauses when lightly loaded (perhaps over rev limiting in the power pack) but loaded up with three people it did not seem to have that missing sound on and off like it did yesterday.

When I got back to the dock I checked the plugs, the were both white tipped, though the upper on had a bit of oil residue around the rim, more so than the bottom one. This may always have been the case, I never looked that closely at them before. I may try the new iridium ones someday, NGK's BR7HIX as opposed to the BR7HS, I believe.

The motor temperature was noticeably cooler, I could hold my fingers on the upper surface of the block for 8 or 10 seconds, maybe too cool? I'll monitor the plug colour for the next couple of weeks.

Anyhow, thanks again for all the help. A great resource here, and helped me troubleshoot without throwing too much money at it.
-Chris

 

[Daviet]

Re: 1995 15 hp Johnson, top cylinder missing

Glad you got your problem fixed, but when you change spark plugs why don't you install the proper plug, champions?

 

[NWChris]

Re: 1995 15 hp Johnson, top cylinder missing

Thanks daviet, I did some reading about plugs after your post. I did not realize the resistor vs suppressor issue. Champion QL82C's I believe are my plugs, and will get some shortly!
-Chris