Johnson 120 Looper Idle and Spark Plugs

Repair or modification of Boston Whaler boats, their engines, trailers, and gear
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Johnson 120 Looper Idle and Spark Plugs

Postby klcunningham » Tue Sep 06, 2016 6:12 pm

A 1985 Johnson 120 Looper long shaft (J120TXCO) is mounted on a 18-foot boat. The last time the boat was out on the water was 9 months (November) ago but I run it on the muffs every six weeks. In May I re-did the carburetorss since the motor died a lot more than usual at idle on the November trip. I also replaced the spark plugs, changed lower unit lube, re-greased, etc.

When I took it out this last weekend I could not get the motor to run at idle so I was not able to get it off the trailer. I could open up the throttle to start it easily after manually choking it (key choke stopped working some time ago) but once I dropped it to idle (out of gear) it would die. Behavior is the same with the old stabilized fuel and with fresh fuel bought that day in a separate tank. I verified no water in the old main tank. This is 50:1 premix (VRO plugged).

I was at the ramp for an hour and half, checking jets on the carburetors. I have the idle timing advance screw backed all the way out so I'm as far out as I can go (though since I got the motor in 2006 I have slowly had to back this out a little more each year).

After pulling the new spark plugs (QL77JC4 gapped to 0.4) I notice fluid on two of them (Bottom left cylinder and one from the right head, maybe the bottom one). One is badly gunked up and the other had droplets. If you have seen this before let me know. I don't want to assume water even though that is what I think it might be.

I have verified spark on all four plugs via grounding to the block. I have not used spark plug "jump the gap distance" test.

I checked compression and it is approximately the same as when I bought it in 2006. There is lower compression in the bottom left cylinder but it has not changed. Bottom left registers 110 while the three others register 125.

Another observation I noticed is the carb/intake suction. When I worked on the motor at the ramp I noted the top intake for the upper right carb pulls a lot more air than the other three such that when I covered it with my hand for a split second the motor would drop in rpm. The other three carbs did not do this to the extent that this one did.

Repair History: Regular water pump, lower unit oil changes. Replaced with new Fuel pump in 2007. All fuel lines from the tank to the motor including all branch lines to carburators replaced in 2007. Replaced Voltage regulator in 2007.

The local mechanic here in Spring, TX won't even look at it since the engine is from 1985, so that is why I'm here. I hope someone has seen this.

I realize this could be the head gasket but compression is consistent in terms of its previous behavior. I heard others mention on the forum the exhaust gasket but I want to be sure of some experience here before I run off on block mechanical when it could lie in the carb or ignition category. My obvious fear is the shearing off the bolts that connect the exhaust housing to the powerhead.

I have never seen or worked on the reeds so I'm not sure if they are involved.

Any help would be greatly appreciated. I've had a run of bad luck this summer which costly band aid replacement of central A/C so my will power is sapped right now. Fun times.
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Location: Michigan, Lower Peninsula

Re: Johnson 120 Looper Idle and Spark Plugs

Postby jimh » Thu Sep 08, 2016 6:02 am

If your engine has a fuel-enrichment valve on the fuel pump--usually a red handle valve--check that this valve is completely closed. You may have inadvertently opened the fuel-enrichment valve, giving the engine too much fuel. This will particularly cause problems at idle speeds.

Also, the spark plug gap you mentioned must be 0.040-inch, not 0.4-inch. However, that sounds like a wide gap. Check the factory manual for specifications. As I recall, on my carburetor V6 OMC engine the spark plug gap was 0.030-inch. If the plug gap eroded to a wider gap there were often running problems.

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Re: Johnson 120 Looper Idle and Spark Plugs

Postby Seahorse » Thu Sep 08, 2016 7:35 am

Yes, .030" is the correct gap but the biggest problem is water on the sparkplugs. How did it get there?

Is the fuel supply suspect or are there internal engine problems?

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Re: Johnson 120 Looper Idle and Spark Plugs

Postby dg22 » Thu Sep 08, 2016 2:48 pm

Last summer, I ran into the same problem when having difficulty starting my 1973 50 HP Evinrude one very humid morning. I pulled the plugs and noticed water droplets on the plugs. At first, I also thought I had a worn head gasket but the compression was fine. I cleaned out my tank and put fresh gas in the tank and everything was good.

This year I switched to a slightly hotter burning plug (from a UL-77V to a L77JC4). I find it starts a little easier and it seems to burn more efficiently. The UL-77V was the recommended plug from 1973 but I noticed BRP now recommends the L77JC4 for my motor.

Regarding your idle, possibly your low-speed needle is not set right in each carb. With my motor they say to carefully turn the slow speed needle in until it is seated and then back off 5/8 of a turn. After setting for each carb, I tested the idle. I then made a slight adjustment to the idle timing advance screw to get the correct RPM for going into gear.

All the best.