1993 9.9 hp question for Graham

"Graham,
How low do the compression numbers have to be before a rebuild is in order? I have a 93 9.9 with 85 and 90 psi. I have gotten mixed answers as to rebuild it or not. It runs ok but drinks the fuel, smokes a little and leaves a ring on the back of my boat. The carb and fuel pump have been rebuilt by a shop. What are your thoughts?"

"Richard,
Merc's publishe

"Richard,
Merc's published "normal" compression for your model is 115-125.

In practice I have never found them to be much above 100 (but that could well be my gauges which are not calibrated beyond the way they came from the mfg).

Merc recommends anything below 100 psi be "investigated".

Having said all that, your numbers are close (to the 100) and in general I wouldn't be concerned with anything 90 or better, so it's kinda a coin toss.

At most, because of the engine design and the parts available, you can only hone and re-ring. There is no allowance to bore these models, nor does anyone (that I have found) make an oversize piston for them - so there really is only so much you can do.

Additionally, I have never seen one of these eat either a piston or rings unless it was run with straight gas.

What is infinately more common is carbon build-up.

So first thing I would try is the 5 dollar fix.

Get a can of Merc's "Powertune" - comes in a spray can - just follow the directions.

After all the cooties clear (visible as a goopy mess that resembles melted McDonalds chocolate milkshake coming from the prop and exhaust relief holes), test the compression again and see if it clears up some of the smoke and excess oil/gas.

If it doesn't help at all, get back to me and we will move on to more drastic steps..."