Strangest problem I've ever seen......

jbjennings

jbjennings

Captain
Joined
Jul 18, 2007
Messages
3,903
My '72 50hp johnson is rigged, ready, and running good...........
EXCEPT, if I take the spark tester off the bottom cylinder and put the plug boot back directly on the plug as it should be. Then I lose the bottom cylinder after a few seconds.
It's like it needs the resistance of the tester making it jump a 7/16" gap (which it has NO problem doing) to keep making good spark. Brand new coils.

Could it be a bad spark plug? The plug gap looked fine. I am totally baffled and have no idea how to proceed at a fix.
If someone could give me some quick advice I'd appreciate it, as I'm wanting to take it for the maiden voyage and do a little fishing.
I don't understand the problem here.
Thanks,
JBJennnings
 
oldcatamount

oldcatamount

Lieutenant Commander
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Apr 4, 2010
Messages
1,740
Re: Strangest problem I've ever seen......

Jb, that sounds more like a connection issue than resistance issue. The plug may be cracked or the top isn't screwed down tight. Have you tried swapping plugs to see if the problem follows the plug?
 
HighTrim

HighTrim

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Joined
Jun 21, 2007
Messages
10,486
Re: Strangest problem I've ever seen......

I would also start by swapping plugs, ensuring all the connections in the boot through to the coil are proper.

How are you verifying losing the cylinder while connected, timing light?
 
jbjennings

jbjennings

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Joined
Jul 18, 2007
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3,903
Re: Strangest problem I've ever seen......

Well, it was a darned bad spark plug. Weird....
I took it on its maiden voyage. It planed out and ran decent top end, but has consistent small surging and never really got up the full amount of steam. I think one problem is the prop is the wrong pitch, but will check later. I'm going to rebuild both carbs---AGAIN. I'm also having trouble getting this thing synchronized with the timing correctly and think this is probably the problem. This motor is really hard to get just right. :(
I wish I had more knowledge of twin carbed outboards. I've had good luck getting my twin carbed 65 mercurys running, but this thing is more particular.
Thanks for the suggestions,
JBJ
 
Faztbullet

Faztbullet

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Mar 2, 2008
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15,937
Re: Strangest problem I've ever seen......

Dont worry about getting it just right, just get it close and running. The 71-73 models had problems with crankcase leakage (flanges) and excess piston skirt wear(crankcase pressure) I have had customers bring em in for no idle or runs out of fuel and engine only have 1-3 psi crankcase pressure and 130psi compression. Hard to explain to a fellow enine is worn out and it have great compression. Most of them like a liitle more gap between roller and carb pickup to get a hotter spark before fuel is dumped. Did you get your gears?
 
jbjennings

jbjennings

Captain
Joined
Jul 18, 2007
Messages
3,903
Re: Strangest problem I've ever seen......

Hey Faztbullet,
Yes I did get the gears!! Thanks! I've been working day and night on this johnson and haven't even gotten to my merc yet. I made progress on the johnson about 11pm last night. I discovered that I had the throttle cable positioned wrong in the motor with the adjuster on the wrong side of the adjuster mount, causing me to have wide open throttle timing all the time. THe timing wouldn't budge because the arm couldn't go all the way back. I am just learning about bigger motors and throttle cables and such, so even though it's incredibly embarrassing, at least I'm learning. I am finishing my cable adjustment to proper specs, and am going to clean my high speed jets on both carbs, after which I'm going to retest the motor and see how she runs.
I sure do appreciate the merc manual pages you sent! No doubt I will need them. If you don't mind, I'd like to post some pics of those gears so others can see that it's possible to fix them. Will also post some pics of the 25xd when it's finished.

My johnson has brand new rings, and good pistons. I hope I've got the rod holes oriented properly, and that my ring gap is correct. I've ran the motor about 2 miles yesterday, so hopefully all is well.
Thanks for all your help, guys! If anyone wants to go fishing, I've got plenty of boats now!
:)
JBJ
 
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