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Johnson Seahorse 7.5

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Sean352

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Seeking some help diagnoising my little 7.5 2 stroke outboard. Its a 80's model #J8RCTA. It is attached to my gheenoe classic. Hopefully with some help I can identify or narrow down on the issue rather than start changing parts.

The issue presents itself after I have run full throttle for about 10 minutes. The motor will start to slow down in RPM suddenly and will then stall out with the throttle all the way open. Once it starts acting up I can no longer go WOT or it will stall. I can start the motor right back up and will have to slow speed part throttle back home. If I try to open up the throttle it just smothers the motor and stalls. Once I let it cool for 30 minutes it will operate like normal again for about 10 minutes then the issue comes back.

It runs very well up until the 10 or so minutes of WOT operation.

Things I have tried so far are:

-Making sure the gas tank is venting properly
-New fuel
-Checked to make sure the ball is pumped up (it always is)
-Checked the fuel pump and screen - nice and clean and was replaced
-Motor is not overheating - steady pee stream and the head is not to hot to touch
-New plugs
-Seafoamed
-Fuel lines are good
-Lower unit oil change

I was orignally suspecting this to be a issue with the fuel and or carb. However I can't understand how it works perfect after cooling down then the issues comes back however the motor does not seem to be overheating.
 
I will tap in with the standard questions:
1) What have you for compression numbers?
2) What are you able to get for spark? Both before and after the issue. Spark should jump 3/8 to 1/2 inch, or there abouts. When you get the issue, try go pull one plug wire to see if it stays running, then do the other wire.
3) If you give it choke when the issue presents itself will that help? If it makes it worse, then you might rule out lack of fuel. Fuel bulb should feel that there is fuel in it, but not hard, nor colapsed tight.

Sounds like ignition issues IE: bad coils. Had a friends old ski boat do the same, cutout when warm. But don't rule out fuel altogether. My 9.9 had a crack fuel pump that wouldn't show until it heated up.
Others will drop in, but check these basics first. Keep in touch..
 
Welcome to the forum. So it's a 1983. Brupp covered about everything. Vic's test will detect a fuel supply issue. My thought is with Boobie. Sometimes when electronic spark components heat up, they fail. This could also be the trigger/timer/exciter under the flywheel too. Also, could be one external coil dropping spark to it's cylinder when hot. You can isolate the effected cylinder by aforementioned suggestions, or even an infrared gun.....everyone should get one, they are helpful for cooking, frying, you name it. Sometimes one external coil will fail from internal arcing, but only when hot. As previously mentioned the spark control module will fail and can direct spark to only one cylinder as well, then later can fail on both cylinders. So you have five different spark components to produce spark on this motor beginning with an Exciter coil under the flywheel then going to a timer coil which sets the timing and from there to a power pack and that splits to a couple of final coils that bump the voltage up for the spark plugs.
 
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Firstly thank you for the replies and suggestions. It won't be until this weekend that I have a chance to try anything else. Pumping the bulb does not help the issue. I will let you all know what I find this weekend. I have a infrared temp gun I have for work I can use. I will report back. Thanks
 
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