Help please - 2002 Mercury 9.9 idle problems.

[Gilera]

I realise that this is a bit of a smaller motor than most here are used to but I guess 2 stroke outboards are much the same when it comes to troubleshooting regardless of size. I have limited experience with outboards but understand basic troubleshooting for 2 stroke motors.

Symptoms are as follows:
Prime bulb, primer in and out 3 times and turn to fast idle position. Starts with literally half a lazy pull even if been sitting for weeks. Runs a fast idle smoothly.
The motor has an unknown number of hours on it but runs very well at WOT with lots of power.

The issue is with idling. Even when its started back up when warm, it often cuts out. Sometimes within a few seconds after starting and sometimes it will take about 30 seconds. Not enough time for me to even quickly pull up the anchor. Only way to get it started is to pull the primer out and turn to fast idle. Then it idles fast and smoky of course. Once you push the primer in and put in gear it will often feel like it is going to die unless you can get it past a certain point in the throttle and then it will burst into life if it hasn't stalled.

Weird thing is that when I have pulled up on the beach where I store it, it seems to idle ok for a few minutes when flushing with freshwater. The motor is in the highest tilt position when I do this. Before storage, I usually pull the fuel line and let it use most of the remaining fuel in the system until it start to run a little rough and then kill it.

The other issue is that after a run and then slowing down for a 6 knot zone (fast idle) it runs for a couple of minutes and then starts missing (?backfiring) occasionally and you get the occasional "boof" with a puff of smoke coming out. You have to give it a twist to get the revs up for a second to keep it running.

I have tried a couple of different kinds of oil without any change. 50:1 ratio with fresh 98 octane unleaded. Vent open on tank. Plugs look at bit gunky but ok - see attached photo. Both plugs look the same.

Running with cowl off shows nothing usual like fuel leaking or arcing.

I have read that these engines never did idle well when properly tuned, but obviously it should be better than it is.

Any ideas on where to start? Clean the carbs and check float?

[perfmarine1]

Clean carb.

[Gilera]

Only one carb on it. I'll start by cleaning it then. Thanks.

[LakeFever]

Check the quick connects on the fuel line from the tank to motor, the o rings can wear out. Check the rubber lines inside the cowl. Install fresh spark plug. Clean the carb. It’s most likely the carb gunked with ethanol splooge. Green foamy crap. Make sure you can see clearly through all the little holes in the brass tube and the float is functioning properly as well as the needle and seat. If it’s bad gas it’s varnish dark yellowy stuff

[Gilera]

Thanks.

[powerabout]

cleaning the carb means doing it in a ultra sonic cleaner....

[staylor]

I've owned 4 of the 15 hp version of this engine and have had similar issues. First, replace your plugs and verify at a dealer that you are buying the current Merc suggested plug- which is not necessarily the plug that came with the engine when you bought it. The dealer can put in your S/N and tell you what the current plug is supposed to be. Next you need to find and clean out the exhaust relief holes at the top of the tower housing. If you've never cleaned these out they may be hard to find since they are choked with carbon. Usually one hole is larger then the other. Get a drill set and carefully clean out the holes- start with a small drill and work up until you start to see aluminum color- meaning you have got the carbon out and are thru the factory paint. Don't drill too deep- just enough to clean up the hole itself. On some of these motors the holes may have a box-like cover added over them which has to be removed before you can find the holes. Now try running the motor- and make sure that the motor is running with the holes in the air and not buried under water- if you run with the holes buried in water the motor will not idle worth a darn and the holes will quickly plug up again. If the motor still runs poorly then you need to put a full carb kit in which includes the carb gaskets, float needle, and most importantly- a new primer push rod and diaphram. Check the primer push rod iin the kit to make sure its the updated version with a little pin-like protrusion on the bottom of the primer push rod to force the primer ball off its seat mechanically. Early motors and many dealer parts in stock are of the early design without the little protrusion. If the primer diaphram starts to leak the motor will run like crap. After all of this, run the motor and when warmed up set to a slow idle and turn the idle screw very slowly until you hear about a 50 rpm drop in idle speed- and open the needle 1/4 turn. This should get your mixture about right. The "tell" which caused me to give you these tips was the fact that you need 3 cycles of the primer knob for a cold start. Usually the motor should start just fine with 1-2 push-pulls of the primer- and the primer knob should be "out" for a cold start, pushing it back in when the motor is running smoothly. Primer knob should be rotated to full clockwise position for a cold start. When you've got everything working, make sure that all slack is out of the fast idle linkage with a warm motor at slowest idle speed- this is a simple screw adjustment on the plastic part that holds the fast idle wire in place.
Doug

[LakeFever]

Scream and fly is awesome! The knowledge base here never ceases to amaze me

[97mirage]

Before you do too much, check the integrity of the small hose (bleed line) that comes off the bottom of the carb. It will never idle properly if you have a hole in this line.

I had the same issue with the same motor and I found a small tear in that bleed line where it connects to the nipple on the bottom of the carb. Replaced that two-inch piece of bleed line and was perfect afterwards!

Also, my OEM Mercury manual says BP8H-N-10 plugs for the 9.9. Not sure if that is what you’re running.

[25two.stroke]

Plugs in that motor are based on which coil is in it. Bolt-through or bolt around. Only 2 different plugs.
Bleed line at the bottom of the carb mentioned above is the issue with 40% of those 6/8/9.9/15 mercs I work on. The other 30% of the time its a ripped primer diaphragm or someone left out the check BB in the primer (it'll run real rich). The last 30% is a bad switch box. Those are the three primary issues with that engine.

Link, sync, clean carbs, replace reeds, set timing...mentioned above.... Reeds NEVER fail in those motors, don't mess with the timing/throttle plate adjustments or timing. It is right. Just turn the one screw that adjusts the throttle plate stop.

[Dave S]

I am working on one of these motors........Want to change from 20 inch to 15.......But all the 15 inch drive shafts are thicker below the water pump......so I can't use a 15 inch in the 20 inch case....... The DS just pull out on these motors.... the 15 has a washer on the shaft under the pump plate......Any one?.....I was thinking of changing the bushing under the pump......but the cases are different.......There is a screw by the bushing that seems to do nuthing.........

[Capt.Insane-o]

gen 1 vs 2 gear case.

[Dave S]

Thats not good news.....I was just thinking it was a long/ short thing........Thanks........

[Gilera]

Thanks for the advice. Just thought I'd post an update.

Cleaned the carb and reassembled. There was a bit of black debris in certain areas. Looked a bit like broken down gasket material. I attempted to reuse gaskets as the local dealer didn't have the kit in stock and wait time was about a week. Motor ran a lot better and was not cutting out when idling in gear, however fuel was leaking from the bowl gasket.

Ordered the kit, stripped it back down and replaced gaskets, primer diaphragm etc.

Replaced plugs with new BP8HS-15. According to the Mercury catalog these or BPZ8HS-15 are the correct ones for the 9.9/15hp. BP8H-N-10 is listed as being for the 6/8hp models as far as I can see.

Started back up and idling well.

Noticed after running again and playing with the idle adjustment that the engine was still quite cool to touch. Realized that there was no thermostat in it. I then read that unlike some outboards, it is quite important to keep thermostats in these. Bought a thermostat and put it in ($80AUD!).

I then read the updates to this thread and indeed the smaller of the two exhaust relief holes was totally blocked. I cleaned it out with some small drill bits as suggested. Would this have had a significant impact on the idle? I don't really understand what role these holes play.

I haven't had a chance to start it since putting in the thermostat or clearing out the exhaust hole. Will be interesting to see if it idles smoother as it was still kicking around a bit regardless of mixture adjustment. I figured it was normal for these engines.

One thing the service mechanic at the dealership said was that he finds that the correct idle mixture setting is at least 2 turns out on these. Sometimes 2 1/4 and even 2 1/2 turns out. I would love some feedback on whether others have found this. I will be adjusting it again next weekend.

I did have a look at the bleed line on the bottom of the carb and also trimmed off a few mm to ensure a good seal. Perhaps should have checked it more carefully though. Will see how I go because I can't check it now without taking carb off again.

I was playing around in the 1 1/4 - 1 3/4 range and felt that this was where the smoothest running was. I was adjusting while in gear at idle once engine was warmed up (except it prob never was warming up without the thermostat in there). He said that they adjust it warmed up in neutral and look for a clean crisp opening of the throttle. I can't see how you could do this with my motor as the gear selector is built into the tiller throttle control.

[Capt.Insane-o]

The idle circuit is very finicky on these motors. My 15's it's a day to day thing, often the adjustment is a tweak the width of the screw driver blade. The race reeds that Chris Carson sells are a huge improvement in the overall running quality of these motors.