1972 40hp 402 merc. only half power at full throttle

[boardbuilder61]

Hi everyone I'm a newbie here so please be gentle lol I have a 2 part question #1 when i purchased my 40hp it ran superb but lately near the end of season in sept. i started having problems with half power at full throttle, sometimes it would work sometimes it wouldn't sometimes when only getting half power in full throttle you would hear a sputter like it wants to let loose but doesn't then if i shut it down for a while and re start with full choke i would pump the primer bulb before starting and again when warming up in full choke and most times it would work fine (please help)
and #2 :redface: how do i remove the top cover where the pull start is i have an idea but thought i would check first.:facepalm:

Cheers Steve

 

[oldman570]

Re: 1972 40hp 402 merc. only half power at full throttle

There should be a latch on the front or rear of the motor, after removing the wrap and front plate, that will let the top cover come off. As for the half power problem, there are several diffrent things that can cause that problem. Bad points, plugs, electrical system, fuel air leaks, and several more. Best to start with just one system and test it for any trouble and correct it before moving to the next system. I would start with all the electrical first then go to the fuel next. Always run a compression test before doing anything to make sure that the motor compression is within specs and worth all the trouble and $$$.
JMO
Oldman570

 

[Maxz695]

Re: 1972 40hp 402 merc. only half power at full throttle

Is the gas all fresh? how long scince changing the filter and spark plugs? As stated always a good idea to check the compression, but you say at times it runs fine. Being a 72 402 HP and mine was a 1979 40HP with a switchbox. Did yours have points? Check the coil wires to the plugs for cracks or bare spot shorting against a ground somewhere, or a loose fitting connection on both ends. Check that all wire connections are tight including all ground wires. Check the carburetor to make sure the butterfly is opening fully at WOT. and that all linkages to the throttle are in good condition. I had a cracked bracket for the screw to to open the cam and the screw was slipping back as a result the cam would only open the throttle half way. When someone says sputter I think of 2 things off the bat. Bad carberarion settings or leaks and #2 water intrusion. Lets look at the smal things first. Might just be as simple as the plug wires as i,ve started

 

[boardbuilder61]

Re: 1972 40hp 402 merc. only half power at full throttle

Thank you max for the quick response the wires look old will replace and i checked the carb for leaks and found no fuel leaking in the housing, also to check if the motor has this or that in it with my serial # it is impossible it doesn't come up anywhere will change filter with tuneup and after checking compression and 1 more Q How do you get that top off can i remove the nut on the top and then the housing around it?

cheerers steve

 

[Maxz695]

Re: 1972 40hp 402 merc. only half power at full throttle

being you have an earlier model I,m not firmilar with a picture can help some. The serial number would also be a big help in someone giving you info on your engine being a 1972 402 I believe. Correct?

 

[MH Hawker]

Re: 1972 40hp 402 merc. only half power at full throttle

I has a 77 402 , and its a 3 piece cover the front latch is by the fuel line connection, the second piece is the band section the the top and flip latch is in the back I believe, its been years since I owned it.

 

[boardbuilder61]

Re: 1972 40hp 402 merc. only half power at full throttle

Well thank you again max and you other guys your input will help BUT I just got a call back from mercury marine and my serial number shows it is a 1974 not 72 but i have attached some photos of the motor along with the serial number in the picture you will see the rope from the pull cord and why i cant get the top off but can i undo the center nut above the recoil and flywheel to get that 4 leg guard off then I'm set.

Serial #- 7038943

cheers Steve