1998 Promax 225 rough idle

[Bassman53]

I have a 1998 Mercury Promax 225 EFI on my Champion 203, I am trying to sell the boat but some guy that looked at it didnt buy it because he said the idle is too rough and has too much smoke. It seems to run fine with no load but at around 900 RPM under load it seems to be a little rough as in you can feel it in the boat itself. Is this normal for the performance motor or is there something i should address?
Same guy also told him that the compression on this motor should be around 140-145lbs, that seems a bit too high, mine are between 121-114.

[JaySmithRacing]

There are many things that can cause rough idle on a motor .
1. Bad sink and link on the throttle butterflys
2. TPI is set wrong ( needs to be set at .250 volts with DDT with butterflys closed )
3. Leaking reeds ( I use only TDR REEDS )
4. Bad leak down ( motor ring /bore health bad )
5. Injectors fowled ( I use Brucato for injector services )
6. Using an oil with a high ASH content ( go to pen. blends at 40:1 )
7. If your still using VRO the oiling system may be too rich ( replace and pre mix )


Just a few causes and fixes.

PS: a stock Pro max will NOT show 140-145 lbs cranking compression at sea level more like 130 ish on a healthy motor..

Good luck,
Jay

[Bassman53]

Excellent thank you greatly for sharing your knowledge, I will get started on figuring it out. Probably the fact that I was adding a little oil to the gas in the tank as well as the auto oiler isn't helping (recommended by a friend but have since heard not good). I will run this gas out first and then start on that trouble shooting list.

[flipper]

Sounds normal for a PM to me.

Interpretation of a rough idle could be a result of the solid mounts. Mine have always resonated at that RPM range. They all smoke and they all idle rough compared to a 4 stroke Yammi. Find another buyer.

[wrechin2]

I have also had props that would cause a vibration at low RPM's that could be felt throughout the boat. If you have a spare prop try it and see if it make a difference. Just a thought.

[Bassman53]

Yeah i took it down to my local dealer had everything checked out, and it is all good, no issues and with 120-125lb compression on all cylinders.
Still cant seem to pull in $9000 for the whole boat even with this motor on it. Not sure whats wrong with people, well only 1 person has come to look at it too though.

[JaySmithRacing]

The economy is in the toilet and everyone wants something for free...
Its not just your rig its everything used..
Jay