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1990 Johnson 90 hp won't stay at wot

R

riopga

Regular Contributor
Compression between 120 and 128. Shows good spark. Will start perfectly every time. Jumps out of the hole, but will only stay at wot for 10 to 20 seconds, then slows but runs steady. Heads are not hot to touch.
 
Boobie said:
What happens if you pump the primer bulb ??
Click to expand...

The bulb is not flattened, and pumping it does not change rpms. Also, I tried removing fuel tank cap. No change. Does this engine have the SLOW system? And if so, what can activate it?
 
Boobie said:
What happens if you pump the primer bulb ??
Click to expand...

The bulb is not flattened, and pumping it does not change rpms. Also, I tried removing fuel tank cap. No change. Does this engine have the SLOW system? And if so, what can activate it?
 
racerone said:
I do not believe it has S.L.O.W on that model.
Click to expand...

Racer is correct.......... I checked the book, S.L.O.W. is not featured on that 1990 90hp model.

After 10/20 seconds, you say the engine slows down and you indicate that the engine is not overheating which is logical as it would take a longer period of time for the engine to overheat. You also indicate that there is no fuel restriction as you are able to pump the fuel primer bulb while the engine is running and the rpm does not change.

This slowing down....... Is it a gradual slowing down of the engine ... OR... a sudden instant drop of rpms?
 
Last edited:
joereeves said:
Racer is correct.......... I checked the book, S.L.O.W. is not featured on that 1990 90hp model.

After 10/20 seconds, you say the engine slows down and you indicate that the engine is not overheating which is logical as it would take a longer period of time for the engine to overheat. You also indicate that there is no fuel restriction as you are able to pump the fuel primer bulb while the engine is running and the rpm does not change.

This slowing down....... Is it a gradual slowing down of the engine ... OR... a sudden instant drop of rpms?
Click to expand...

It is a quick drop-off of power. I haven't gotten a tach for it yet, I just got this motor recently.
 
riopga said:
It is a quick drop-off of power. I haven't gotten a tach for it yet, I just got this motor recently.
Click to expand...

A quick, instant, fast change of rpms indicates ignition trouble.

When you have time, double check the compression and let us know exactly what the compression PSI is per cylinder. The numbering of the cylinders is... as when standing in back of the engine, facing the spark plugs:

2.....1
4.....3

Then with all spark plugs removed, and using a spark tester that you can set a 7/16" gap (important!).... check the spark which should be a strong blue lightning like flame... a real SNAP!

Let us know what you find exactly... not in generalities such as "The spark and compression is great" or some such statement. We get that a lot and it doesn't really tell us anything.
 
Thank you for the suggestions. I will try to implement them and post back here in a few days. I appreciate the help.
 
riopga said:
I have the service manual ordered. Should be here next week.
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not to derail this thread, but where did you order the service manual from? I've looked for a while and found nothing... other than ebay vintage manuals that want too much $$. I'm going to be replacing the powerhead on a '91 90hp and i'd like to have torque specs and proper procedure, etc.
 
avionator said:
not to derail this thread, but where did you order the service manual from? I've looked for a while and found nothing... other than ebay vintage manuals that want too much $$. I'm going to be replacing the powerhead on a '91 90hp and i'd like to have torque specs and proper procedure, etc.
Click to expand...

Kencook.com
 
Pull the fuel line going to the carbs and pump the promer nulb. Do you have a free flow of fuel to the carbs? Just sounds like a restriction down stream from the bulb.
 
Closure! It turned out that it was the throttle cable. It would move the throttle lever, but then it would let it slide back in. Replaced the cables and she is running strong.
 
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