7.5 hp merc will not come to operating speed

[jmayo]

I have a 7.5 hp merc that idels fine runs great at half throttle but will not come up to full throttle when on the boat. It runs 100% perfect in a barrel of water but not on the boat I am getting very discouraged with this and am in need of some help figuring this out
Any help would be greatly appreciated
thanks
John

 

[steam_mill]

Re: 7.5 hp merc will not come to operating speed

Over pitched prop?

 

[jmayo]

Re: 7.5 hp merc will not come to operating speed

no It is the original factory prop
John

 

[Laddies]

Re: 7.5 hp merc will not come to operating speed

Is it a long shaft motor (20") on a short shaft boat (15")

 

[jmayo]

Re: 7.5 hp merc will not come to operating speed

no thatis not the problem either

 

[Laddies]

Re: 7.5 hp merc will not come to operating speed

If the engine is runnlng on both cyls and the timing is alright then you may have a bad high speed winding on the stator

 

[jmayo]

Re: 7.5 hp merc will not come to operating speed

that still does not explain why it comes up to speed while running in a barrel of water does it?

 

[Laddies]

Re: 7.5 hp merc will not come to operating speed

I guess that I never ran one in a barrel but it is quite common in a test tank for a engine to cavitate and rev up, I could be more help if I could test it myself, sorry

 

[Firestar]

Re: 7.5 hp merc will not come to operating speed

Did the engine run fine on the boat before and if it did what were the circumstances that led to the present problem?

 

[tmcalavy]

Re: 7.5 hp merc will not come to operating speed

The back pressure isn't the same on the boat/lake as in the barrel. When it does this on the boat/lake, pull the top cowl and pull a plug wire, each in turn. You should notice a difference in the way it runs with one plug wire disconnected. If you a pull a plug wire and it doesn't affect the way it runs, that cylinder and its coil are dropping out under load. Probably. Pull the flywheel and inspect the coils/points/condensors...just guess you have these cause you didn't post the year or model of the outboard. If the coils are cracked...time for a tuneup. Replace coils/points/condensors. Don't know what to tell you if its new enough to have a solid-state ignition...not familiar with those.