Logo

1972 johnson 100hp not going full speed..?

mrtjoec

New member
I've had this motor for several years and generally it runs good for me, this past year it started to not run so well. occasionally it would not take off it feels like it is not going full, maybe half speed, and then after a minute or two it kicks in and runs great and can go full speed. that would happen off and on all day long. well this year it never kicks in and it seems to not be missing, but just won't kick to the next level and really run. it runs just at about 15 mph instead of the normal 30mph. what could cause this and where should I start and move to next? thanks!!
 
Re: 72 johnson 100hp not going full speed..?

What is the compression of the individual cylinders?

With the spark plugs removed, the spark should jump a 3/8" gap with a strong blue lightning like flame..... a real SNAP! Does it?

Plugs should be Champion L77JC4 plugs with a gap setting of .030 .

Let us know what you find.
 
Re: 72 johnson 100hp not going full speed..?

I ran the tests;

compression was right at 100psi on all cylinders
I have a good strong spark,
and the plugs are the correct plugs and looked to be in good shape.

where to go next?
 
Re: 72 johnson 100hp not going full speed..?

With proper compression, spark, and plugs, the most likely cause of your problem would be fouled carburetors.

The cure would be to remove, clean, and rebuild both of them.

You may or may not have the type carburetors that have slow speed needle valves, if you do, the adjustment procedure follows.

(Dual Carb V/4 With 4 (2 each carb) Slow Speed Adjustable N/Valves)
(J. Reeves)
The adjustment procedure of the carburetor slow speed needle valves follows. NOTE... if the needle valves turn too freely, replace the nylon bearing retainer (the nylon item at the front of the carb that the needle valve goes through first) with the newer type RED retainer #315232. If your engine has that weird linkage that connects all four of those needle valves, the newer RED retainers will enable you to discard that linkage. The RED retainers make it impossible for the needle valves to vibrate out of adjustment.

(Carburetor Adjustment - 2 Slow Speed Adjustable Needle Valves, each carburetor)

Initial setting is: All (4) Slow speed valves = seat gently, then open 1-1/2 turns.

Facing the carburetors, start with the top left needle valve, then the top right valve, then the bottom let valve, then the bottom right valve. It may be necessary to redo these steps to get the adjustments ideally set

Start engine and set the rpms to where it just stays running. In segments of 1/8 turns, start to turn the S/S needle valve in. Wait a few seconds for the engine to respond. As you turn the valve in, the rpms will increase. Lower the rpms again to where the engine will just stay running.

Eventually you'll hit the point where the engine wants to die out or it will spit back (sounds like a mild backfire). At that point, back out the valve 1/4 turn. Within that 1/4 turn, you'll find the smoothest slow speed setting.

Do not attempt to gradually adjust all four of the valves at the same time. Do one at a time until you hit the above response (die out or spit back), then go on to the next valve.

When you have finished the above adjustment, you will have no reason to move them again unless the carburetor fouls/gums up from sitting, in which case you would be required to remove, clean, and rebuild the carburetor anyway.
 
Re: 72 johnson 100hp not going full speed..?

I need a little more advice

I have Good news and bad news.

First the Good, after the carb cleaning and rebuild it cranked up and hits top speeds, seems to run really good!

Bad news, at idle or slow speeds it will sometime instantly die, no sputter, just bam its off, but it will restart right away just fine. at higher speeds it doesn't seem to have this issue....???

by the way it does not have the slow speed adjustable valves.

Thanks!!
 
Last edited:
Re: 72 johnson 100hp not going full speed..?

anyone out there have an idea on what might be causing this? my guess is something went wrong with the carb rebuild, but I don't want to take them apart without an idea of what might be wrong..... or if it is unrelated.

Thanks!!
 
Re: 72 johnson 100hp not going full speed..?

Try bumping up the idle speed. On my 77 it is supposed to idle at 650RPM in gear. Hard to measure this on muffs, by the way, the lack of backpressure causes it to idle faster than it does when it's in the water.
 
Back
Top