1984 Johnson outboard 140 HP bogs down and stalls at wide open throttle

[arch526]

Hello. I have a 1984 Johnson V4 outboard with VROiling system. When I start it and run at WOT it will run good (about 30-35mph) for a brief period but will eventually bog down (engine loses RPMs and eventually shuts down despite the throttle lever being still full forward). Bought the boat used just last summer so did some work over winter to hopefully fix problem. Changed plugs, pulled gas tank and washed it and let dry (no corrosion in tank but there was varnish in bottom--- all cleaned out now), replaced fuel primer bulb and put new inline see-through fuel filter on. All fuel lines are in good shape. Started boat and still same problem. Runs real smooth at WOT but bogs down. Re-pulled plugs and left 2 plugs are dry with light tan deposits. Right 2 plugs are moist with black deposits. Next step in my head is to check fuel pump diaphragm for holes and remove air cleaner box and clean carbs. Was also going to check the color of spark when engine turned over. Dont' know if anyone else has any suggestions other than this??? Very frustrating as I live about an hour and half from lake so its frequent, expensive guesses at problems and then break down in the middle of lake... luckily my buddy has been with me recently to tow me back. Thanks for any input.

 

[multimech]

Re: 1984 Johnson outboard 140 HP bogs down and stalls at wide open throttle

Have you done a compression check recently? I would. It sounds as if it is bogging down because a piston is seizing. If it is okay, then check your carbs. Check the fuel pump pressure is 5-7psi. Check the vacuum on the inlet side of the pump, it should be no more than 4hg. After all of these checks, take a timing light in turn on each cylinder and watch for ignition break down on any of the cylinders. Make sure the W.O.T timing is to spec.

 

[arch526]

Re: 1984 Johnson outboard 140 HP bogs down and stalls at wide open throttle

Have you done a compression check recently? I would. It sounds as if it is bogging down because a piston is seizing. If it is okay, then check your carbs. Check the fuel pump pressure is 5-7psi. Check the vacuum on the inlet side of the pump, it should be no more than 4hg. After all of these checks, take a timing light in turn on each cylinder and watch for ignition break down on any of the cylinders. Make sure the W.O.T timing is to spec.

Thank you for all of the suggestions... i suppose the second major question i have is what is the best way to check the engine at wot? I have the service manual for the boat however they do not discuss the use of test tanks. I have read various sources that say it is not a good idea to use the muffs to test the engine over idle speed... so i have considered using a 55 gallon tank with the side cut out to submerge the intake and exhaust but some ppl say the cavitation is too bad and that damage can actually occur with the use of these techniques... does anyone have any suggestions? Or is the only safe way to rev the engine with the engine on the lake?
Thanks again!