Mercury 300xs Optimax Loss of RPM and Extra Fuel burn

pipo6007

Recruit
Joined
Feb 2, 2012
Messages
3
Hi all,
My port motor is burning significantly more fuel between 4,000rpms and 5,000rpms. @ 4,000rpms my port is at 16gph and my starboard is at 8.5gph. Under 3,800rpms both motors are within 1 or 2 gallons per hour difference.

Now the other thing is that at Wide open throttle the starboard motor shoots up to 6,000rpms and the port stays stuck at 5,150. To reach these rpms they are both tucked in and not trimmed up. If I trim the port to the sky it eventually reaches 5,600rpms. All things being equal when I got the boat both motors were burning the same fuel and the boat was faster at WOT by 4mph.

My mechanic can't seem to find the problem and we tried cleaning all the fuel and air injectors plus changing all the thermostats because one was stuck open on the port.

Does anyone know what this could be? :confused:
 

woody66912

Senior Chief Petty Officer
Joined
Jul 12, 2008
Messages
690
Re: Mercury 300xs Optimax Loss of RPM and Extra Fuel burn

Not sure what the tech has done already. Ask him if he has done a visual inspection on the fuel pressure regulator valve, and tracker valve, on the port engine. Even though he can hook up a dump gauge to the air side of the fuel rail and see if fuel is crossing over, it can be such a small amount that this test may not confirm a tear in either of the valves.
 

Faztbullet

Supreme Mariner
Joined
Mar 2, 2008
Messages
15,589
Re: Mercury 300xs Optimax Loss of RPM and Extra Fuel burn

Also a broken reed pedal will cause this and is a common problem on the Opti engines
 

woody66912

Senior Chief Petty Officer
Joined
Jul 12, 2008
Messages
690
Re: Mercury 300xs Optimax Loss of RPM and Extra Fuel burn

Also a broken reed pedal will cause this and is a common problem on the Opti engines

Yes this is very common, and when he call tech service, that is one of the first things they will ask him to check, simple job to do. By removing the intake air handler.
 

pipo6007

Recruit
Joined
Feb 2, 2012
Messages
3
Re: Mercury 300xs Optimax Loss of RPM and Extra Fuel burn

I decided to do some work myself this morning. Started by removing the plugs on the port motor. When I got to cylinder 5 the plug was clean and brand new. Did a spark test on the coil and there was no spark. Changed the coil and ran the boat. Guess what the problem was fixed. Everything is working just like new again. :)
 

woody66912

Senior Chief Petty Officer
Joined
Jul 12, 2008
Messages
690
Re: Mercury 300xs Optimax Loss of RPM and Extra Fuel burn

I decided to do some work myself this morning. Started by removing the plugs on the port motor. When I got to cylinder 5 the plug was clean and brand new. Did a spark test on the coil and there was no spark. Changed the coil and ran the boat. Guess what the problem was fixed. Everything is working just like new again. :)

Great, I guess all we can say now is continue boating.
 
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