1973 Johnson 25 HP fouling top plug. Keeps Stalling. Stumped!!

[mwp71467]

I am working on a '74 Johnson 25 HP electric/pull start. Here's the issue. The outboard bucks and hesitates then dies intermittently. Will idle and rev fine in a barrel for a few minutes then will buck and stumble and die. Also it sounds to me like there is an exhaust back fire but when I squeeze the primer bulb it works fine again for a bit. Top spark plug is black wet carbon fouled. I have tested and replaced many things and here is the list:

Replaced spark plugs ( NGK BR7HS .030 gap ) New fuel pump and tested by cranking electric starter and it pulses fuel great. New coils and I switched from top to bottom to rule out a defective coil. They are new but you never know. Tested compression- 123 psi in both holes. Tested spark using tester and with plugs outside of holes-great spark and jumps 1/4 inch. New points and condensers .020 gap. New coil laminate ( magneto ) and again tested spark with same results. New primer bulb, new fuel tank, hoses ( not the cheap Walmart stuff ) and tested the primer bulb and pumps up firm when priming. Checked for any vacuum leaks.

I removed the intake manifold to inspect the reed valves and found a very small ( 2 inch ) dead tree frog in the manifold against the reed valves. ( yeah, I know ).
I put a light behind the reed valves to see if they were seated ( I read they can be tested that way ) and some were and some were not seated and appx. .011-.012 inches off seat. The carburetor was rebuilt with new parts by myself and float was set even with the body of the carb when inverted. I made sure the needle was seating by blowing through the inlet with it turned right side up then upside down. Slow idle needle final adjustment was 1 1/4 turns out. Reassembled everything and same problem. I noticed a little bit of gas coming out of the front of carb when I held my hand in front of it while motor was running.

I am leaning towards the reed valves which may seem obvious but I wanted to make sure there isn't something I've missed. The person this motor belongs to has spent a small fortune on this old motor. Perhaps a leak down test ? I don't feel it's electrical as squeezing the primer bulb eliminates the problem for a few minutes. I'm sorry for the long read but I wanted to be thorough. Any help would be greatly appreciated. I can upload pictures of anything needed to help solve this problem. I will continue to update this thread til resolved to help others

 

[nwcove]

strong spark and a fouled plug would lead me towards a fuel pump leak back through the bypass cover.....but isnt that pump mounted to the lower bypass cover? pumping the bulb and it works for a bit still points toward the fuel pump, but not towards possible flooding of one cyl. could be a few issues acting together . jmo

 

[adominique0313]

sorry I hit the wrong button.

 

[F_R]

That motor pumps the crankcase drain through the hose to the upper cylinder. That makes the upper one run richer (same thing as lower one leaner). So if you set the slow speed needle rich enough to prevent the lower cylinder from sneezing and backfiring at idle, the upper one will be a tad rich. That's the way they were made in response to emissions regulations. Older motors just dumped the drains into the lake. But they ran better.

All this affects mostly idle or slow speeds. So, it is not the best motor for a lot of idleing or trolling. If mostly used at higher speeds, it should be tolerable.

Needless to say, everything else has to be right or a bad situation becomes worse. Can we assume you have the carb/spark sync dead-on?

 

[mwp71467]

F_R, I will check the carb/spark sync. That is one thing I didn't even check. I was unaware off how the crank case drain worked but now it makes sense. And nwcove, I did move the inlet cap of the fuel pump to swivel it in a direction that would be better for house routing. Maybe I did something when reinstalling it. Great things to look at that I didn't think of. I will check it out today and post back my findings. Thank you both for your help.

 

[Tim Frank]

Spark plug matching is a contentious topic in here, but I've always been a proponent of using the OEM recommended plug.
That is a Champion J4C.
NGK make great plugs, but just because there is a x-reference number from the Champion, does not mean it is a direct match.

 

[mwp71467]

Okay all, here is an update. As F_R mentioned, the carb/spark was off so I adjusted that to the specs shown in the manual. Seemed to help but still every once in a while it would hesitate/backfire but a big improvement. I turned my attention to the fuel pump and removed it and inspected the gasket which looked like I might have initially put it on unevenly (if that makes sense) as there appeared to be a slight leak Very small but never the less it was a leak. Fixed that and the problem got better but still kicking a a very little bit.I reset the air/fuel needle which further helped. The kicking and hesitation was almost not there. I then realized that I put one of those new glass fuel filters in line between the tank and the motor connector. I have always used a plastic/paper filter but thought the glass one would be better. Not so. I removed the glass one and installed the cheap paper one and finally it runs great. I know there is a rating for fuel filters but I'm not sure what is the best for this motor but problem solved. Thank you to all of you for your input. As nwcove said, " could be a few issues working together" was correct. One more thing. I'm changing spark plugs to champion today. Thanks Tim Frank.

 

[Tim Frank]

Gotta love the successful outcomes!