Re: Merc 7.5 water leak at spark plug hole
Nothing is likely to be cracked; that cover only seals the water circulating area of the block - it's not a head gasket.
2 choices; if you can get the cyl block cover bolts out without breaking them, replacing the gasket is an easy job. You can still get the part from a Merc dlr.
If the bolts won't give even with application of heat from a propane or MAPP gas torch, they're going to break if you try and force them further and it's probably not worth proceeding without removing the powerhead and drilling out the bolts (if it's only top bolts you can probably drill but if I recall the bottom bolts aren't accessible without the powerhead pulled).
You can remove the spark plug, clean the hole with a suitable degreasing solvent, and use some non-copper-containing RTV to seal the leak from the outside, without having to remove the cover. If it still leaks after that, try installing the spark plug and RTV-ing around it too. That should have a thick enough bead of sealer to hold.
Re the rich running, you may need to adjust the idle mixture screw for smooth running. Do this in gear in the water, with the motor warmed up to normal operating temp. Turning the idle mix adjustment screw clockwise leans the mixture out, counterclockwise richens. These motors are pretty fussy about idle mix and the range of adjustment for smooth running is quite narrow.
This may be all that's wrong with it. Also check to make sure the choke is fully open once running. If you also have any fuel pouring out the carb, you probably have a leaking float needle/seat which will require removal and disassembly of the carb to fix. Last on my list would be a bad fuel pump diaphragm, if the motor stays running without having to pump the priming bulb it's probably not that.
It might be a good idea to run some carbon-cleaning additive in the fuel. Seafoam is good, or get the Merc or OMC equivalent additives. Make sure the spark plugs are the right heat range, L78V Champs if I recall.
Use a good quality 2-stroke marine oil and it may smoke less. Synthetics will smoke less, Merc has a Quicksilver Premium oil that's good; I find that Penzoil Synthetic Blend (sold at most Wal-Marts for less than $12/gal) does well too.
One last thought, if you don't know when the impeller was replaced, it's a good thing to do, not much over $10; cheap insurance to prevent meltdowns! If you search this forum you'll find lots of How-to posts to help you D-I-Y.
HTH...........ed
