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2003 Johnson 50HP - Running on 1 cylinder

J

jmc99

New member
Lots of great info in this forum which I’ve used to try andsort out my problem but I’m still stuck.
I have a 2003 Johnson 50HP (J50PLSTD), a couple weeks agomaking headway speed through the no wake zone, throttled up to close to 5K, ranat that speed for maybe 2 minutes and there was a sudden drop in RPMs, and itcontinued rev up and down, while trending downwards. I could run ok under 2500,though it still sounded rougher than normal, trying to throttle up to anythingover that resulted in the same sudden RPM drop.
Back on land with the ear muffs on I fired it up and pulledeach plug wire individually. When I pulled the top wire, there was little to nochange in RPMs or how it sounded, when I pulled the bottom wire it stoppedrunning immediately. The engine is running primarily on the 1 cylinder.
So far I have:

  • Done a compression test – results were good,140ish in both cylinders.
  • Put in new spark plugs, gaped to .03
  • Tested the spark using an adjustable tester, don’tknow the exact gap I tested at, but it was significantly further than the .03 recommendedgap. To my eye the spark looked strong and consistent from both wires.
  • Took off, inspected and cleaned the carbs, theyboth looked immaculate and all passages appeared clear.
  • Ensured there were no blockages in the fuelsystem prior to the carbs by pumping the bulb with the carbs off, gas camethrough the system fine.

I’ve also observed a lot of oily sludge coming out theexhaust, my assumption is this is the excess oil not being burnt off in the topcylinder.
Given the result of compression and spark test, am I rightin eliminating those as possible problems? Should I focus on the fuel system? Whatshould I inspect next?
Thanks in advance

 
yes, vro is still hooked up. Just read up on the S.L.O.W. mode, from another post:

The SLOW mode limits rpm to about 2500 and the motor bucks and shakes when you advance the throttle to try to increase that speed. Only overheating puts the motor into SLOW unless there is a blown diode in the engine harness.

What I experienced initially does sound like S.L.O.W. mode kicked in based on the above, but does it explainrunning on one cylinder? When I run at idle in the driveway, it is running onthe one cylinder. You can hear it, especially with the air silencer off, and itbecomes more pronounced if you get it any throttle.

 
Thanks for the link. I tested the ignition on the spark plug end again, both with an inline tester and a gap tester, both looked good. Swapped the plugs and wires, and it was still the top cylinder that wasn't firing.

However, I noticed when I partially choked the top carb with my hand, it would start firing and RPMs would pick up. Am I right thinking this indicates it is running lean (too much air, not enough fuel)?

Took the carbs back off, with rebuild kits on hand, rebuilt and cleaned both, putting a piece of wire through all jets, set the idle screws out the recommended 3.5 turns, mounted them back up, fired it up and same problem, top cylinder was not igniting, but would if I choked it out some.

Started turning the idle screw out and at around an additional 1.5 turns out (5 total) it started running consistently, however now it is idling at 1200-1300, compared to the 800ish when it was running well.

Could the float level in the carb be off? Vacuum leak in the fuel system?

Also, at this high RPM speed it seemed to be running hot, the temp gauge was indicating green bordering on yellow (never ran that high before) and the top of the block and thermostat cover were very hot to the touch, could only keep a finger on there for a second. There was a strong stream coming out of the tell tail, and the water coming out was still pretty cool. I just replaced the thermostat, but only the thermostat and not the spring or diaphragm.
 
Wow! Yamaha makes a pretty good 50 horse.

No, seriously, was there ever a mouse nest in there?
 
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Sounds like S.L.O.W. troubles to me, but gotta figure out why. Choking carb really should make it worse, since you already have unburnt fuel coming out exhaust. The SLOW mode interrupts/adjusts spark to keep the motor running richer as well as cooler. So you probably have a cooling problem. How long since you did an impeller?
 
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