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Johnson AD-10 won't start

R

RickR90s

Contributing Member
I'm helping a buddy with his 50's era 10hp that won't start.

He bought it from a guy who had it running a couple years ago.

  • Motor spins and appears to have good compression. (Thumb Test)
  • Spark plugs fire with healthy blue spark. Installed new plugs
  • Carburetor looks very clean and fills with fuel, float working as fuel flow stops as float rises
  • He sprayed a small amount of fuel into each cylinder and the motor fired and sounded healthy for a few seconds
  • I had him check the reed valves for sticking and he says they're clean and functioning

So at this point, the engine won't run and my gut tells me something in the carb is the culprit.

Is there a circuit that we should be looking at?
Can we remove the reeds and attempt to start?
What do the reeds actually do?
I'm assuming the high speed circuit (needle that threads into the float bowl) wouldn't be affected on initial startup

It's a sweet motor so doing my best to help him out. His 10 year old son is anxiously waiting to go out and do some fishing...:)

I'm familiar with this era of motors, having owned a 1960 5 1/2 and currently a 1967 3hp folding motor.

Thanks so much,

RickR90s
 
Before you spend another minute or dollar on it, pull the flywheel and check the coils. If they haven't already been replaced, they need to be. ALL 1950's Johnson twins have bad (cracked) coils if the originals are still in there.

The reeds almost never are a problem, unless some critter built a nest in there.
 
AD--10 is a 7 1/2 HP motor.-----The reeds must be in there to get crankcase compression.---The reeds allow fuel / air in when piston is on compression stroke.-----Reeds close when piston is on the down stroke.--Without crankcase compression the motor will NOT RUN.
 
Note, racerone said crankcase compression That's not to be confused with cylinder compression. There are tons of info out there on how two-stroke engines work. And a lot of mis-info too.
 
fdrgator said:
Before you spend another minute or dollar on it, pull the flywheel and check the coils. If they haven't already been replaced, they need to be. ALL 1950's Johnson twins have bad (cracked) coils if the originals are still in there.

The reeds almost never are a problem, unless some critter built a nest in there.
Click to expand...

We haven't looked at the coils but spark test showed strong, crisp blue spark. i agree with your assessment however, on cracked coils. Mine were on my '59 5 1/2.
 
quick update.

Got the engine running last night. it appears to have been the gas tank not sealing well. We replaced the gasket for the black bracket (that contains the button, fuel gauge window, etc.) and now the engine starts and runs. New issue is that it won't idle well once the throttle is at or below the SHIFT setting.

We tried adjusting the low speed mixture from 1-3 turns out and it didn't help.

Thanks,
 
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