Going Pre-Mix 1994 MERC 225

[flounderpounder225]

I know my OMC's and Yamahas, but never played much with MERCs.  Got a friend who is going to go pre-mix on a new rebuild 94 model 225 Carb'd Merc.  I said plug the oil Uptake line from the external tank, plug the vaccum pulse line to the oil pump (I am assuming MERCs are vaccum operated Oil Pumps?) and your done?? Yes, or No??

[slvrlng]

Not that I know but, I wonder if it has a low oil alarm warning system that you would need to disconnect.
I'm sure Fitz will know.

[fitz73222]

Hey Marc,

Mercs are mechanical driven oil pumps right off of the crankshaft via a worm gear configuration; there is a rotational motion sensor that detects if there has been a mechanical failure of the pump and sounds the alarm. A very reliable system that I would simply replace the components for a new rebuild and forget the pre-mix PITA. V6 Mercs don't like 50-1 at idle and will smoke like no tomorrow. If it was me; I would replace the oil pump for about $200 for a new rebuild engine and forget about it. My $.02

[fitz73222]

I guess I didn't answer your question. If he insists on pre-mix-
Disconnect the inlet and outlet oil lines from the oil pump; cap them off with vacuum plugs and zip tie in place and the motion sensor lead.
Disconnect the low oil sensor leads at the small pony tank, tape them off separately. Do not disconnect the warning module leads because you may lose overheat warning.
That should be it...

[flounderpounder225]

Thanks Fitz, I agree, I like my oil injection, didn't realize MERC was mechanically driven like Yamahas..

[seabob4]

Farley, isn't the gear driven off the crank plastic?  And ain't it a beetch to get to?  My old buddy at PL had a 200 carbed that the gear took a dump on...

[fitz73222]

Farley, isn't the gear driven off the crank plastic?  And ain't it a beetch to get to?  My old buddy at PL had a 200 carbed that the gear took a dump on...

Hey Bob,

The driven gear is a plastic material but I've seen very few fail (this is since 1984). It is protected by the rotational motion sensor if it did fail. You can service the oil pump and driven gear without removing the powerhead. Now the drive gear is another story. You have to remove the powerhead and the lower roller bearing cap assembly to pull it off of the crank. I guess the point I was trying to make earlier is that in the long term you are going to do more powerhead damage from over oiling than the potential risk of an oil pump failure.

[wingtime]

I've done this before.  Merc sells a plug that takes the place of the oil pump drive and caps off the block.  This is important for two reasons.  One it holds the pump shaft bushing in place inside the block so it doesn't fall inside the powerhead.  Two it takes up the volume of space that the pump drive used to occupy.  With out it the block chamber for that cylinder would have a larger volume in it that will change the fuel air mixture for that cylinder.  On the starbord side of the block is a check valve that provides crank case pressure pules to pressurize the oil tank.  Remove it and replace it with a plug.  (Fitz may have covered this).  

Personally I'd trust the oil system.  Those gears usually only fail in two cases.  One is just after a rebuild if the two halves of the gear are not aligned properly.  And two: in motors that have sat for a long time and the oil pump is stuck from sitting and it strips the gear on start up.

[GoneFission]

Ditto Fitz and Wing's comments.  The Merc oil injection system has proven to be very reliable and the motor will run better with oil injection than it will on premix.