Hey 614bass,
I going to review a couple things, please don't get offended if I review some basic stuff, because I don't know your knowledge base regarding outboard motor.
1.) Make sure your vent plug is open plug on your tank open when your running/starting your motor if you have a plastic tank. (You may laugh, but all of us have forgotten to open that vent plug while starting up the motor in the morning for a day of fishing and find a little while later the motor is running like do do, because the air can't entering the tank. The motor's and fuel pump is now trying to suck gas from a tank that has a negative pressure in it.
2.) Check your fuel line going from your tank to the motor. Make sure the primer bulb is near the tank and the fuel arrow flow image is pointing to the motor. Make sure your gas line is in good shape. If in doubt, go to Walmart or Academy and buy a new setup. Because if air is leaking into the line from small crack it could affect your fuel delivery.
3.) Gas/oil mixture 50:1 = equal 6gallons of gas to 16ounces of TWC-3 2 cycle oil. ( don't let others tell you the stuff in for weed-eaters and chainsaws is ok)
4.)Spark plugs- Champion J4J (torque to 17 to 20 foot pounds with a torque wrench)
Spark plugs gapped at 0.030 (use a feeler gauge)
5.) Check to see if you have good spark on both cylinders using a spark plug test, not just a spark on the end of the plug. ( Please undo both plugs, so you don't accident start the motor) Should have a good strong spark at 1/4 to 5/16 inch on the spark plugs tester. (ground the tester to the motor). If you have a cylinder that has no spark or very weak then your basically running on one cylinder. Father in-laws Evinrude 6hp motor was running ok and idle a little rough but it was running on one cylinder, no spark on top cylinder and was running about half power. Had to change the coil on the motor and now it runs smooth and he is happy again. These motors are tough and will run on one cylinder but poorer than usual.
Another way to tell if you are running on one cylinder is to start up the motor(in the water) and let her warm up for a minute or two. Carefully unplug the boot to the top plug, if runs rough but maintains some then the bottom plug/connnected cylinder is firing. If it dies then you know that you have a cylinder that is not firing/spark. Replace the top spark plug boot carefully and let a run for a minute or two, then unplug the bottom plug boot, if it dies then you know that you have the top cylinder plug/wire that is not firing/spark. If runs rough but maintains some then the top cylinder is firing.
spark plug tester $7.00
https://www.autozone.com/autozone/catalog/accessories/accProductDetails.jsp?itemIdentifier=10257_0_0_&skuDescription=GreatNeck+/+Adjustableignitionsparktester&brandName=GreatNeck&displayName=Adjustableignitionsparktester&skuDisplayName=GreatNeck&categoryNValue=10199999&navValue=101404&categoryDisplayName=Adjustableignitionsparktester&parentId=01-10&itemId=1404-10&store=3106&productId=10257
feeler guage $7.00
https://www.autozone.com/autozone/catalog/accessories/accProductDetails.jsp?itemIdentifier=1929_0_0_&skuDescription=OEM+/+18bladescombinationTune-Upgaugewith6wiregaugesand1electrodeadjustingtool&brandName=OEM&displayName=18bladescombinationTune-Upgaugewith6wiregaugesand1electrodeadjustingtool&skuDisplayName=OEM&categoryNValue=10199999&navValue=101293&categoryDisplayName=18bladescombinationTune-Upgaugewith6wiregaugesand1electrodeadjustingtool&parentId=01-10&itemId=1293-10&productId=1929
torque wrench $21.00
https://www.napaautoparts.com/Search/Detail.aspx?R=BK_7760951_0006395373
It sounds like your motor runs fine for a little bit then slows down and goes back to wot(wide open throttle). This suggest a fuel or a fuel air issue. Try suggestion #1 above, if that does not solve it then try taking off the hood/cowl from the motor. If you get consistant improved performance that means you have an exhaust leak issue, which was common for this specific motor. ( We will cross that bridge, if that is the issue, later)
Another suggestion is try adjust your tilt and trim. I know this is a manual tilt. I have two 9.5hp motors years 1965 and 1971.
-When it is just me in the boat I have it tilted at position 3 hole and best performance in that slot.
-When it is 2 people in the boat I have it tilted at postion 4 hole and that is the best performance in that slot.
Trying adjusting and see how effects your ride and speed
Now I will tell you will probally at best get about 14 to 15mph with your boat a setup.
Good luck my friend,
cajuncook1
