1979 johnson 100 horse problems please help!!!!

Grizzboss

Recruit
Joined
Jul 17, 2012
Messages
5
Hey all,

I have a 1979 johnson 100hp v4 that is having some issues. Heres the stroy. Last summer it was running however it would only max out at about half throttle and any more throttle it would eat gas and bog and stutter really bad. Sometimes it would have a hard time starting cold and while up to opperating temp. When running on the muffs it always started good and reved up with no problem. When it was under load it would still bog real bad.

Just pulled out of stoarge last weekend and it wont fire. It turns over just fine however no fire. Cleaned the carbs replaced all the vacuum lines and fuel pump and plugs and still no go. Sometimes when I crank it over it back fires and smokes. There is raw fuel in the exhaust so Im guessing its getting fuel. When I was checking for spark I got zapped while holding the plug boot so Im think new boots, wires and coils are needed in the near future. Checked compression and all range between 90-120.

Anyone have any thoughts on why its not running?
 

emdsapmgr

Supreme Mariner
Joined
Dec 9, 2005
Messages
11,551
Re: 1979 johnson 100 horse problems please help!!!!

Your compression is questionable. 120-125 is a good figure for that engine when it was new. All cyls should be within 10% of that figure, or no less than 108. If you've got some in the low 90's you might want to pull the heads and check the head gasket seal ring to see if they are intact around each cyl. (esp. the low ones.) Do a spark test. Possible not all cyls are firing. That engine will seem to run just fine when running on 2 or 3 cyls when on the trailer. It's when it's in the water that you need all 4 cyls firing. Best done after the engine is fully warmed up. Weak ignition components may run ok when cold, and fail once the engine reaches normal operating temps.
 

boobie

Supreme Mariner
Joined
Nov 5, 2009
Messages
20,826
Re: 1979 johnson 100 horse problems please help!!!!

Agree +1, your compression is very questionable. The OMC Manual I have says no more than 15 psi/ cyl.
 

Grizzboss

Recruit
Joined
Jul 17, 2012
Messages
5
Re: 1979 johnson 100 horse problems please help!!!!

Thanks for the response. I will double check compression and spark in the near future that is if I can get the thing to run. I'm thinking timeing could be out of wack because it turns over just no go
 

emdsapmgr

Supreme Mariner
Joined
Dec 9, 2005
Messages
11,551
Re: 1979 johnson 100 horse problems please help!!!!

Check the flywheel key. If it shears, the engine gets out of time and will backfire.
 

71Windsor

Petty Officer 1st Class
Joined
Jun 25, 2008
Messages
286
Re: 1979 johnson 100 horse problems please help!!!!

I'm with emdsapmgr it may even look like its intact but once you get a closer look you might find it to be the culprit
 
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