Merc 800 bottoms out at full RPM

simrey

Cadet
Joined
Aug 24, 2014
Messages
7
Hello, have a 78 Merc 800 with throttle problems, starts just fine, idles just fine, high idle; just fine. The problem I'm having is when applying fully throttle in gear, it runs as expected for 20 sec and then bogs down, almost to an idle RPM. Never stalls. If I leave it in idle and slowly bring the throttle back up does the same thing. Runs at full RPM, 20 seconds back to bogging down and idle. It's not over heating, at first I thought the fuel line was kinking; not the case. Thought the tank wasn't venting correctly so I pulled the cap; same thing. It's very strange, any insight would be greatly appreciated.

Thank you!
 

MH Hawker

Vice Admiral
Joined
Jul 13, 2011
Messages
5,516
Sounds like a fuel supply problem, try pumping the ball it it picks up and runs its the fuel pump.
 

inthedirtagain

Petty Officer 1st Class
Joined
Aug 10, 2011
Messages
321
You can also try gravity feeding a smaller fuel tank and completely bypass the on-board fuel system. If it runs good with gravity, you know the issue is isolated to your tank/bulb/pump.
 

DJ Jaws

Seaman Apprentice
Joined
Nov 6, 2008
Messages
43
You could have a partially blocked fuel jet. Try to CAREFULLY pour a few drops of gas in the carb while it's bogging down. If it lights back up, you either aren't getting enough fuel to the carb or the carb isn't letting enough fuel into the intake. It doesn't take much to block a jet, so any build-up or piece of junk can get stuck. It could be as simple a fix as spraying some carb cleaner into it.
 

simrey

Cadet
Joined
Aug 24, 2014
Messages
7
Thank you for your replies, I think I've found something has to be causing issues. Upon removing the cowling there was a very evident smell of gas. Looks like the lower gasket is blown on the bottom throttle. I think I'll start there...
 
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