Mercury 15 hp 4 stroke - running right?

I know running a long shaft motor on a short transom will yield no performance. Having said that.

The motor can't get the 13' inflatable w 2 ppl (370lb) to plane. The average speed is about 5mph w a max of 7mph. I checked the compression and both cylinders are 90-100 psi. What the normal psi for this outboard?

I also noticed excessive white smoke from the exhaust at idle when I'm in the water. When I start it up at home w a hose I don't notice too much white smoke.

Theres a black hose coming out from the back of the motor. Water comes out of it. I'm guessing this is to let me know the water pump is working. I see water coming out of it. Do I just let this dangle behind the motor or should it go somewhere?

Besides that, the motor starts up every time.

The original owner told me it was a short shaft and I took his word for it.
Fuel was pouring out the carb but a rebuild kit took care of that.
I'm planning to sell this and get a short shaft but don't want to sell it if its not running right.

My guess would be the "white smoke" is water getting into the cylinders.

Most likely culprit would be a blown head gasket. A 15 horse on a 13' RIB should make upwards of 20 mph without working too hard.

Sounds like the original owner dumped a motor that never saw a day of maintenance and was mistreated at best.

You didn't mention the model year but the one service manual I have for 9.9/15 horse 4 strokes lists the factory spec compression at 185-190, with anything less that 120 indicating a significant issue.

So it had carb issues that you corrected.

It probably never had it's valves adjusted (which should have been done at the end of the first or second season of use - absolutely necessary to prevent later issues).

Did the owner have maint records? Did it have regular oil changes? Waterpump service?

The tell-tale (pee stream) hose should be connected to either a fitting or a grommet in the lower cowl (shouldn't simply be flopping around). So either the powerhead was pulled at some point and the tell tale wasn't replaced properly OR someone got in there messing around.

You want to hope that the poor compression is simply a head gasket (bad enough), anything more could mean you have yourself a 120 pound paperweight.

Unfortunately it sounds like you are experiencing, the hard way, why buying a 4 stroke in a private sale is a very bad idea. Too many guys/gals treat them like 2 strokes and you simply can't do that. They require significant care and maintenance or you turn them into a pile of scrap in short order (most dealers fail to mention that when they are flogging their virtues).

If you are mechanically inclined get ahold of a Seloc's manual and pull the head - you really need to get a look inside. If not you could pay a shop the couple hours labour to tear it down and get a quote.

Just don't be surprised if they come back with a number higher than the cost of a new motor.

Sorry if I sound negative, have seen this before on a few occasions....

Thanks for the quick reply Graham.

Its a 2000 15ELH 4ST.

Owner didn't have any record but I didn't ask either. Boating is all new to me but I'm learning fast.

The pee stream is a rubber hose coming out from the lower cowl. The hose extends about 2 feet and flops around. Should I cut it down to a few inches? The stream of water looks good.

I'm fairly mechanically inclined. I've changed head gaskets on cars before and I rebuilt the carb by reading the seloc manual. So I guess I'll pull the head and change out the gasket and see what happens.

Looking at the manual, looks like rebuilding the motor shouldn't be too difficult.
A lot smaller/lighter than building a Chevy 350. LOL

Will a long shaft 15 hp still be able to get a short transom inflatable to 20 mph even w all that drag?

Because the motor starts up and idles smooth. I have confidence the motor is salvageable.
The motor does knock when I rev it up to 100%. Hopefully a new head gasket and valve adjustment will fix that.

First off, yes, the tell-tale hose should only extend maybe 1/2" below the lower cowl - it was probably replaced (for whatever reason). It's sole purpose is a "visual" for you to tell the waterpump is working.

And if you have rebuilt any motor than a outboard is no different - just lot's of parts will look "miniature".

There is no reason that a long shaft, particularly a portable (25 horse and under) will not work perfectly fine on a short shaft transom. You will hear "it creates more drag", which it does, but seriously, this is a 15 horse not a 150, so proportional to it's top speed, you would never notice. I get about 22 mph with my 15 horse, pushing close to 700 pounds, so even with that tiny extra bit of drag (an inflatable actually planes easier than a V bottom) you should get somewhat similar results.

If there is any draw back to the long shaft on a smaller boat it's the extra 5" hangin' down in the water. Just have to be extra vigilant in very shallow water - you have twice as much "junk" underneath as compared with a short shaft.

On a runabout or a utility you could put in a 5" spacer above the top of the transom - on a RIB I would simply live with it - tilt it up if piddling around in the shallows.

And yes, there may be nothing more wrong with than a head gasket. Hey, when you pull the head you will get a good look at the pistons/cylinders anyhow (at least the top of the cylinders and the condition of the rings - kinda).

The "white smoke" really is indicative of a blown head gasket...

Good to hear the long shaft will work with my short transom.
I'll chop off the length of hose.
I'm using it in the ocean so shallow is not really a problem. I just have to remember to raise it when at the boat launch.

Almost tempted to change rings n bearings while I'm at it but more time fixing motor means less time fishing.

Yes, besides, everyone needs an "off season" project

Just took a look at cylinder head cover. Bolts are to be torqued to 6ft-lbs. but the lower left bolt is real close to the lower cowl. I won't be able to use a torque wrench there. From what I can tell the lower cowl is one piece and it doesn't seem like it can be taken off without removing the power head first.

Any suggestions?

According to the seloc manual the cylinder head should lift right up after all the bolts are off.
My problem is the collar that keeps the head gasket in place on the bottom right is preventing the head from coming straight up. The oil pump is recessed in a hole on the bottom and the hole does not allow the head to move back enough to clear the collar. I tried pushin the collar all the way in the head and all the way in the block. Neither way gave me enough clearance to pull straight up.

Should I carefully grind the hole bigger where oil pump is located? Looks like hole is aluminum.
should I carefully use a hacksaw to cut collar?
can I get more clearance by taking off the upper case? Or is that just a decorative cover?



Thanks

So I ended up taking the powerhead off.
Cylinder walls looks good. Still has cross hatching.
Cleaned up piston top w some parts wash.
Changed exhaust gasket and head gasket.

Found all the valves were tightened all the way down.
Loosened them back to spec. Could be why I was only getting 5mph.

Reassembled everything and compression is still at 90-100.
Does this mean I have to change out the piston rings after all?

Can you get ahold of another compression gauge?

Make sure you are getting the correct reading before going any further. That's only about half of what you should be reading..

Used my other compression gauge and its reading 90 on both.

Thinking of getting a leak down tester from harbor freight.

Leak down tester came in.

Air coming out of oil dipstick and oil fill hole.

Guess I'll change the rings after all.

Before you do anything do a leak down test. That will tell you if the rings are bad or you have leakage past the valves. I wonder if the motor was over heated at one time which is the kiss out death for an outboard. The previous reply is absolutely correct in that the black hose fits on a fitting on the lower case housing. Another thing you can do is after a fast run on the engine, shut it down asap and after it has cooled, check the plug to see if they are the proper color and they should be the same. Light grey.