Before you start pulling the engine apart look at this first. I have a '93 150 and had a similar issue- smoke would come from the front of the engine at idle and I thought base gasket or some other gasket was leaking, allowing exhaust into the engine compartment. However, I found the culprit and it was a cheap and easy fix.
Remove the cowling. The lower cowling is in two pieces- right half and left half. If you look at the rear of the lowers right in the center you will see a small square port that is the above water exhaust tube. It looks like a square rubber grommet from the outside. Look in the engine compartment and it looks like a z shaped rubber boot. Mine was torn where it went through the cowlings and it allowed exhaust gas into the engine area. That left an oily film in the lower cowlings and made a mess. It also caused the smoke from the front of the engine while idling. I found the piece on eBay of all places, replaced it an POOF- no more smoke or mess. I think it cost me $15.
Here's a picture of the piece I bought- It's #38
http://www.usboatsupply.com/Mercury_Mercruiser_Parts_Catalog3a.php?mc=01&ms=7240&si=573
Find your engine and then click on "driveshaft housing and exhaust tube". It says mine has been discontinued but I found one anyway. Got lucky I guess.
UFM82