1994 28 marin yamaha problem

[neverenoughx2]

I am baffled. I have twin 1994 250hp yamaha outboards. I keep losing rmps at 3900rpms. It will fluctuated down to 3700-3600 then back up. It happens quick to where you can't miss it. When I run at 4200 rpms it doesn't happen. I tested all the coils and found one bad one and went for a ride and still happened just not as bad. So I swapped out the coils between the port and starboard motor. The problem remained the same on the same motor (the port). I changed all the fuel pumps same problem, rebuild all the carbs, same problem. swapped out the fuel filters same problem. I guess the cdi unit is next on the list. Any one have any ideas? Oh compression is 105 on every cylinder. will the pulser coil do this? Is there a way to test it?

thanks Tom

 

[gw204]

Try running the problematic motor off of a separate fuel tank.

 

[Another Distraction]

Run the boat and when the engine slows hit the choke. If it revs up it's fuel related, if not it's electrical.

See if you can get to WOT for a short spell too.

OHM out the stator and compare port to starboard results. This may be your issue if the choke test points electrical. Go to CDI electronics and read the yamaha trouble shooting guide. It'll help.

May not be plugs but change them anyhow.

Starting at the battery, clean all of your ground connections well