Erractic rpm's 220 special

[Bill St]

Anyone who can lend me a hand would be greatly aprecieated. I have a 1986 220 special and I recently began to have a problem with the motor. It's attached to a 23 ft wellcraft walkaround if that helps any. It seems the motor will ramp up to about 2500 rpms and then suddenly die out and kick down to 1000 rpms. Then suddenly and very qiuckly go back to 2500. It will do this until I go back and pump the priming ball and the problem will go away. The motor will then bring the boat up on plane and run perfect for a while. I usually run at about 3800 rpm. Then after some time the problem will come back. I checked all the filters and changed them but this did not help at all. The tank looks clean. The engine runs very well at idle and starts very easily also. Any idea's?? maybe a fuel pump?

Thanks
Bill

[Ken]

Bill,
Could be fuel pump(s) or vent or anti-siphon valve on main tank .
If your main tank has an anti-siphon valve, remove and dissassemble and inspect/clean/replace.
Also, check the vent for the main tank - could have a kink in hose and main tank is developing a vacuum. Check all hoses for proper connection - fuel pump may be sucking air from somewhere.
If your fuel pump (your model may have more than one) has not been replaced in last five years, you might want to do it - or at least test pump(s) per the Yamaha service manual.
If your motor is carbed, and it has been at least five years since carbs were rebuilt with kits, I would do it.
Good luck [img]smile.gif[/img] ,
Ken K

[jb123]

age and year and symtoms, such a wide drop in rpm sounds like youre losin a plug, grounding then coming back, but definately check the fuel system too.