1972 Johnson Carb Question

[James Akers]

I just bought a pontoon with a 1972 Johnson 25 hp motor. The darned guy had it set up to die when the throttle was about half way down, so I would like to readjust. When I take of the hood, there is an adjustment on the front of the carburator that I am not familiar with. It looks like it used to have a knob on it, but it is broken off. The only two adjustments that I can find is this darned shaft where a knob used to be, and the low speed jet adjustment. What is the purpose of this adjustment? How do I adjust the idle? Is the high speed jet fixed, or is it adjustable? Oh by the way, I just found out tonight that I got water seeping out the side of the head gasket -- looks like I need a new head gasket. Checked compression, and had 118 on the top and 98 on the bottom. Can the head gasket be responsible for this? The bottom piston had alot of carbon buildup compared to the top. Can the head gasket be responsible for this too? One last word... When I fired it up in the barrel, it will run at an idle for about 5 seconds -- then it will cough an die. Could this also be caused by the head gasket? It has new points plugs and condenser, new water pump, and the coils were checked at the marina. Got the bill from the previous owner. Sorry for the lack of organization with all these questions... I guess they have been building up inside my brain, and they happened to spew forth at random.<br /><br />Thanks in advance

 

[Xcusme]

Re: 1972 Johnson Carb Question

James,<br />Yup, sure sounds like you're going to need a head gasket. When replacing, be sure mating surfaces are clean and check for flatness using a steel ruler on both the powerhead and head itself. After replacing head gasket, do a decarb and re-check compression figures.<br />Sounds like you have 2 mixture screws, one for high speed, one for low (idle) speed.<br /><br />Carb Adjustments are courtesy of Joe Reeves, and are as follows.<br />--------------------------------------------------<br />(Carburetor Adjustments - Two Adjustable N/Vs)<br /><br />Initial settings are: Bottom high speed = seat gently, then open 1 turn out. Top slow<br />speed = seat gently, then open 1-1/2 turns.<br /><br />Setting the high and low needle valves properly:<br /><br />NOTE: For engines that DO NOT have a shift selection, obviously there is no<br />NEUTRAL position. Simply lower the rpms to the lowest setting to obtain the low<br />speed needle valve adjustment.<br /><br />(High Speed) Start engine (it will run pretty rough), shift into forward gear, take up to<br />full throttle. In segments of 1/8 turn, waiting for the engine to respond between turns,<br />start turning in the bottom high speed needle valve. You'll reach a point whereas the<br />engine will either start to die out or spit back (sounds like a mild backfire). At that<br />point, back out the needle valve 1/4 turn. Within that 1/4 turn, you'll find the<br />smoothest setting.<br /><br />(Low Speed) Slow the engine down to where it just stays running. Shift into neutral.<br />Again in segments of 1/8 turns, start to turn the top needle valve in. Wait a few<br />seconds for the engine to respond. As you turn the valve in, the rpms will increase.<br />Lower the rpms again to where the engine will just stay running. Eventually you'll hit<br />the point where the engine wants to die out or it will spit back. Again, at that point,<br />back out the valve 1/4 turn. Within that 1/4 turn, you'll find the smoothest slow speed<br />setting.<br /><br />When you have finished the above adjustments, you will have no reason to move<br />them again unless the carburetor fouls/gums up from sitting, in which case you<br />would be required to remove, clean, and rebuild the carburetor anyway.<br /><br />--------------------<br />Joe (30+ Years With OMC)

 

[James Akers]

Re: 1972 Johnson Carb Question

Thanks for the Input! I replaced the head gasket, and now have exactly 132 lbs on both cylinders -- I thought that was pretty unreal for a 32 year old motor!<br />Did the best I could in the driveway with the carburator, and put her on the lake last Friday. I thought the gas looked a little funny, since the it was red and smelled bad. Sure enough, my boy was out on her Saturday night and it started spittin an sputterin. Changed the plugs, took the carb off, disassembled it, sprayed it out with two cans of carb cleaner, blew it out with the air compressor, and put it back on. After about 20 minutes running in gear on the lake, with my boy at the throttle (adjusting the idle)and me at the low speed adjustment, I got her down to where it will idle along in gear forever without missin. I used to have a big 135 hp Evinrude with surface gap plugs, and it just wouldn't idle without fouling plugs. Ya think this motor will idle for extended periods without problems?

 

[James Akers]

Re: 1972 Johnson Carb Question

OOps...<br />Forgot to mention that as soon as I saw and smelled the gas, I dumped it, rinsed the tank out and filled her up with a fresh mix. Thought I had better throw this in or I would look pretty stupid.