1978 mercury thunderbolt 402 40 hp bogging under load

[younggun]

My mercury 402 thunderbolt 40 hp is bogging under load I checked the plugs and the top one was clean but the bottom one was all dirty and they were brand new never been ran when I took it out. This makes me think it is only firing on 1 cylinder. It idles fine and runs fine on muffs but not under load. What could be wrong?

 

[younggun]

Re: 1978 mercury thunderbolt 402 40 hp bogging under load

Anybody got any ideas as to what is wrong?

 

[redlight]

Re: 1978 mercury thunderbolt 402 40 hp bogging under load

I have a 1988 80hp and had the same problem. Took in and a $100 later they told me it was bad gas. Emptied my entire 17 gallons, added new gas and problem solved! Good luck!

 

[younggun]

Re: 1978 mercury thunderbolt 402 40 hp bogging under load

Thanks. Ill empty the can and start over. Good thing I only got about 3 gallons in the tank!