1971 Johnson 100hp hard start, stalls on accel

I've got a 71 hp Johnson 100 hp that normally starts fine when sitting a while but after several minutes of running on the water and then stopping, it will stall and not want to start. It turns over and tries to start (spits and sputters) but then just won't go. If we wait about 15 minutes or so, then it will take off. Sometimes I notice it is on the verge of stalling if we are going no wake speed and have to give it the gas to keep it going. It runs great at planed out speeds. I have carb kits on order so I hope that fixes it. Any insights to why this happens?

Re: 71 Johnson 100hp hard start, stalls on accel

you need to check and see if you have spark to all the plugs when it stalls out and will not restart. It could be the powerpack heating up going to ground and then after it cools down it will work again until it heats up again and goes back to ground. just something to check. hopefully the carb kits fix it.

Re: 71 Johnson 100hp hard start, stalls on accel

I converted this engine to an MSD ignition spark box last year and its worked fine. The old amplifier units are known to burnout after a few years. It doesnt seem to be spark since it works at high speed and will idle and even troll in gear at idle, but when you give it throttle it dies and may not start again.

Re: 71 Johnson 100hp hard start, stalls on accel

I believe the carb kits will solve the problem. Just be sure you clean them thoroughly, I prefer to soak them, & blow them out with compressed air.

Re: 71 Johnson 100hp hard start, stalls on accel

Another possibliity I just learned about is the fuel pump. That thing has a vacuum operated fuel pump that uses the #4 cylinder as the vacuum pump. Diagram is attached. From the manual;


"Many times,

No. 4 (bottom port bank) cylinder. During highspeed

operation, gas quantity is limited, the
plug is not foul an8 wid1 therefore fire in a ​

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satisfactory manner."

The test I am going to do tonight;​

"If the spark plug of the cylinder to which
the vacuum line is connected becomes wet
fouled, the cause may very well be ​

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a ruptured

fuel pump diaphragm. This reasoning
is sound for both type of fuel pumps. Both
type of pumps have an inlet hose from the
fuel source, an outlet hose to the carburetors,
and a vacuum hose from the powerhead. A​

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good test for the pump is to disconnect

the vacuum line from the engine, operate
the squeeze bulb in the fuel line until i t
is firm, and then to carefully observe the
end of the vacuum hose to detect any fuel
leakage. The smallest amount of fuel from
the hose indicates a damaged diaphragm.
The pump must be rebuilt to restore satisfactory
service of the pump. If the pump is
a non-rebuildable type, the unit must be
replaced."​

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