Motor won't do high rpms, tried most things.....need HELP

[Wiggy]

I brought a Mercury 1983 25hp about 6 months ago. The guy I brought it from said that it hadn't been used for a couple of years and very little before that. It worked fine from the start, with good compression on both cylinders.

After 4 months use, mostly trolling with some WOT in between it not reaching high revs at all. It started about 2 months ago, which was then an intermittant problem, but now it does it all the time. From the start it has had problems with oil build up due to sitting at idle (trolling), and after a while had the incapacity to burn it out of the cylinders.

I've overhauled the carb (plus new fuel lines and checking for any leakages), new plugs Champ L76V (gapped properly), compression now is 124 psi (top cy) and 142 psi (bottom cy), good strong blue sparks on both circuits and still has the inability to reach high revs. It just shudders (not violently) and is clearly not happy when I give it more than half throttle.

When I overhauled the carb, the float was a good 2-3mm too low, so that was adjusted as shown in my manual.

I need ideas/help, as I haven't the money to anything major nor just delivering into the workshop.

 

[Mike722]

Re: Motor won't do high rpms, tried most things.....need HELP

Most of the time your to compression readings should be within 10% of each other. These appear to be at or just over the edge. Also, they should be done after the engine is warm. I would recheck those numbers. You may have a cylinder going bad.

Hope this helps
Mike S

 

[Wiggy]

Re: Motor won't do high rpms, tried most things.....need HELP

I know about the 10% rule, and compressions readings were taken whilst warm.

Readings when I got the motor were 120/125psi.

I've been also using a cheap TC-W3 oil that leaves a big blue smoke cloud behind the motor.

 

[jtexas]

Re: Motor won't do high rpms, tried most things.....need HELP

From 125 to 142...hmmmm...anyway, go to the Engine FAQs forum and read the thread on "Decarb".

Are you using fresh fuel?
Does the fuel line hold pressure?

Try pumping the primer bulb while throttling up. If it helps any, it's an indication of weak fuel pump.

 

[Wiggy]

Re: Motor won't do high rpms, tried most things.....need HELP

The fuel pump has also been overhauled. The fuel line is holding pressure and new fuel is nearly always being used.

 

[roger3645]

Re: Motor won't do high rpms, tried most things.....need HELP

I'm confused here, did you run the motor for 4 months before overhauling the carbs?

 

[Wiggy]

Re: Motor won't do high rpms, tried most things.....need HELP

yes

 

[Bob_VT]

Re: Motor won't do high rpms, tried most things.....need HELP

Get a can of Mercury tuner and follow the directions on the can. This will accomplish the decarb. This will be a good place to start.

Additionally, with alot of blue smoke and vapors you might have to clean up your ignition. You might have some oil build up under the flywheel along with collected dirt and crud.

 

[Wiggy]

Re: Motor won't do high rpms, tried most things.....need HELP

My gut feeling tells me that the cylinders are full with oil or a carbon build up due to the rich fuel mixture, trolling and the inability to burn up the oil build up.

If there was something wrong with the cylinders, why would the compressions ratings increase? I always thought that a decrease in psi is a logical sign of this?

Regarding the blue smoke, this was out of the exhaust and not any place in the motor.

 

[tashasdaddy]

Re: Motor won't do high rpms, tried most things.....need HELP

Wiggy, did you do a link and sinc after you did the carbs? this resets the electrical timing, with the opening of the carbs. this makes them work together properly. also a good decarb, with seafoam, if you can get it in Norway. if you can get it also add it to your gas/oil mixture, since you troll alot, as a preventative of carbon build up.

 

[Wiggy]

Re: Motor won't do high rpms, tried most things.....need HELP

Wiggy, did you do a link and sinc after you did the carbs? this resets the electrical timing, with the opening of the carbs. this makes them work together properly. also a good decarb, with seafoam, if you can get it in Norway. if you can get it also add it to your gas/oil mixture, since you troll alot, as a preventative of carbon build up.

Now you've lost me!

I re-calibrated the fuel/air mixture, that'd be about it.

I'm going to go into the local workshop tomorrow and get either some Quicksilver Powertune or they've got another product that does kinda the same thing, and hear with them to see what they think.

 

[Plainsman]

Re: Motor won't do high rpms, tried most things.....need HELP

For the link and sync:
http://forums.iboats.com/bbBoard.cgi?a=viewthread;fid=36;gtid=1114512

For the decarb:
http://forums.iboats.com/bbBoard.cgi?a=viewthread;fid=36;gtid=1035563

 

[Wiggy]

Re: Motor won't do high rpms, tried most things.....need HELP

Just wanted to post the result......

I contacted my local Mercury dealers/workshop and they recommended this carb cleaner you put in the fuel, it also gets rid of all the gunk in the cylinders. They said this would not be as aggressive as power tune for older motors?

Well it's worked, the motor is almost working as it should (just needs one more treatment) and the piston heads have gone from a black, dirty state to nice shiny silver ones!