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Loss of acceleration with 2000 model year 40 hp merc.

N

newt22

New member
I have a 40hp 2000 model year merc Ser. 0T098567. Starts easy idles good. At 1000 rpm in gear a little rough (pushing a 2000lb boat) but acceleration is clean up to 2200 rpm and runs very smooth. When I push up the throttle to 3000 rpm it bogs down like missing a cylinder firing, then just past 3000 it runs very smooth up to any rpm after that. Changed plugs and gas and compression is between 115 and 120 on all 3 cylinders. Not sure what to look at next. Any help appreciated.
 
try pumping primer bulb when issue arises, Place a timing light on all plug wires while running to see if they are firing evenly
 
Thanks for the reply. Problem doesn't happen when out of gear so I need to try the bulb pumping you recommended under load.
I'm thinking that not enough fuel supply might be the issue. As soon as I can take it out I will find out and post again.
 
I got to the bottom of it. Tried the bulb and the bogging down went away. Short answer, I changed the anti-siphon valve at the tank and the problem went away. It looked like it was sticking. I noticed though that at the same time something new started. The vent hole just below the "Pee Hole" started exhausting a very light smoke or mist. I also noted that it only will exhaust out that vent when the motor is in the water and does not do it with the Muffs on. Could this be related to the low fuel flow that has now improved?
 
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