Merc 20 2 stroke FIXED!!!!!!!

I could really use some help on this one! My motor started to run rough today and wouldn't start at times. Got it home, pulled the plugs and the bottom plug had milky oil on the plug. Both cylinders have 125 psi compression.

Pulled the jug cover, no possible way water could have come from there.. The eater only goes around the outside of the jugs.

Pulled the exhaust cover and only the bottom cylinder had water in it. Could my whole issue be a failed exhaust cover gasket? I am not sure if water could get in from any other areas...

Water pretty much can only enter in through the head gasket or exhaust cover. I would imagine since you have good compression the head gasket is not the culprit. The exhaust covers often leak because 1) the baffle plate which separates the cooling water from the exhaust gasses has a gasket on both sides of it doubling the places to leak and 2) exhaust covers are easily warped.

I think I would first torque down all of the exhaust cover fasteners to factory specs, working from the inside of the fastener pattern out. I'm guessing those are 1/4" fasteners so if you don't have torque values then something like 70 in-lb would probably be reasonable. Lake test it and if problem persists remove the exhaust covers and check the gaskets. If they look good you need to check to see if the outer or inner covers are warped. If all of that checks out then you may need to pull the head and check the head gasket.

has this motor ever overheated?

To my knowledge it has never been overheated. I just replaced the water pump and all looked good as it was. Also rebuilt the carb.

After posting here I did some more research on the web. Apparently it is extremely common on these Nikasil blocks as as well as Mercosil blocks to burn up the bottom cylinder. This block does not have a bolted cylinder head so there is no way of water getting in that way. The primary cause is the exhaust plate and exhaust cooling plate gasket failing.

The ones I pulled off fell into pieces they were so hardened. I checked the cylinder walls and piston. No damage there :lol:. I must have caught it just in time before the cylinder became washed out and lack of oil destroyed it.

This being a Nikasil block, it cannot be bored or honed. Only replace the block and Pistons. I thank the lord I didn't ignor it....

I am supposed to be going fishing in the AM, stopped at HF and bought a hollow punch set. Also stopped at the local auto parts store and bought some sheet gasket. Going to temporarily make gaskets, I have OEM gaskets on order but don't expect them till next week.

ok thats good!

the reason I asked is many years ago I had an old Chrysler 100hp with a similar problem, turned out the cylinder wall had a crack in it, it did not affect compression and it ran ok but somewhere along the line it developed a crack, took forever to figure it out. I think it was cracked before I bought the boat

Interesting about that block. I am not all familiar with those.

Is the exhaust cover like any other typical 2-stroke outboard? Though I have never done it, the advice on checking for flatness on a head is to place some emery cloth on plate glass (which should be perfectly flat) and put the mating surface down on it and do light and consistent figure 8's with the cover. The low areas will wear first to give the indication. I'm not sure if this is the method for exhaust covers though. Seems like replacing them may be the best route if they are in fact warped.

Finished up my gaskets and was able to make it out fishing. Good news is my water intrusion problem on the bottom cylinder has stopped. I really took my time making the gaskets so I may just keep running them till next season.

The bad, motor starts great first thing in the morning. After warm up it starts to misfire and will not run at idle. I have to pull the cowl and increase throttle to get it to run. Even then it stumbles and has a ton of fuel and blue smoke coming from the water.

Carb has been thoroughly cleaned and rebuilt entirely. Sprayed the coils at night last night to see if I could see a leak on a wire. No voltage leaks. Cleaned grounds. Found last night that with either plug wire disconnected the motor would sputter and die.

The motor would ONLY run with the idle speed screw 3/4-1 turn from lightly seated. I do not have a manual so now I am questioning the timing setting at idle.

I then adjusted the slotted screw on the timing linkage. Left side that has arrow pointing to it. It was almost turned all the way IN. I turned it OUT about a 1/4 inch. This threw off the throttle adjustment. I read the roller on the throttle needs to just touch the bottom line on the throttle cam as you advance throttle. So I adjusted it so. After making these mechanical adjustments I was able to get the motor to run great with the fuel idle screw turned out two turns.

After making these adjustments appeared to run far better on the muffs. Then again, I could get it to run on the muffs before and not in the water. Taking it to the river today to test.

Had that happen to an older Merc one time, it was the seal at the bottom of the power head. As the output shaft spins it causes water from the lower unit to fun up the shaft past the bad seal. Power head has to get pulled and $5 seal at bottom of power head replaced.


Fishdaddy

That's what I am thinking, it has to be either the lower crank seal or a reed valve seal. Going to pull the car off tonight as well as the read valve plate and check that seal.

I think I MAY be on to something here guys! Please help me out of you can. Would this little hole in the reed valve cause this idle/starting problem?? Just pulled the reed plate and found it, I have no idea how it happened,but, it's there none the less. Ordering reed valves, plate seals, lower and upper crank seals (just in case). Lord help me!

Just ordered all the parts. Reed valves are no longer available so I ended up buying Boyasen power reeds. New reed gaskets, new upper and lower crank seals.

I am at the point if this doesn't fix it I will be selling this POS.

Anyone care to throw anything in? I feel like I speaking to myself? Lol

Bowhunter1661 said:

Just ordered all the parts. Reed valves are no longer available so I ended up buying Boyasen power reeds. New reed gaskets, new upper and lower crank seals.

I am at the point if this doesn't fix it I will be selling this POS.

Anyone care to throw anything in? I feel like I speaking to myself? Lol

Click to expand...

As far as the lower crank seal, seems like a problem with the older 80's mercs.
I was servicing one for a buddy, it was a motor I used to own, and when I dropped the LU, saw spent gas/oil on splines, looked up the medsection with a flashlight, there was spent oil around that seal.
Mech said not to worry unless I was getting water in the lower cyl, or losing compression, or rpms under load which it wasn't. I would've checked that specific problem before ordering those seals, because pulling a powerhead even that small ain't no fun.
And... you might've already posted this, but did you happen to check the idle circuit inside that carb for debris? I spent half of a 4 day out of town fishing trip in the rain trying to clean and rebuild a merc 25 carb, and that was the culprit. Also, when was the last time you rebuilt the fuel pump on the side of the carb?

I think I may have found my problem... And I feel like an idiot. I bought this boat and motor from my ill grandfather. Because of his current state he couldn't tell me how to run the motor. Well, I FINALLY found some info online for an operators manual.

When starting the motor I would prime the engine, but, I would not adjust the idle speed knob (ignorance). In fact it was always set to full CCW, which is the slowest setting. So I believe that my issue is my idle was just too low and it would die.

Not it totally sure until I get the motor back together Friday when my parts come in. But it sure seems likely. Here's part of the instructions. Let me know what you think.

sure it could be the problem

kind of strange you have to adjust the idle speed each time you cold start it if I am reading this correctly

what happened after you took it out to the river to test it?

you never did say or I missed it

Bowhunter1661 said:

I think I may have found my problem... And I feel like an idiot. I bought this boat and motor from my ill grandfather. Because of his current state he couldn't tell me how to run the motor. Well, I FINALLY found some info online for an operators manual.

When starting the motor I would prime the engine, but, I would not adjust the idle speed knob (ignorance). In fact it was always set to full CCW, which is the slowest setting. So I believe that my issue is my idle was just too low and it would die.

Not it totally sure until I get the motor back together Friday when my parts come in. But it sure seems likely. Here's part of the instructions. Let me know what you think.

Click to expand...

Pretty standard for a merc on the idle setting, and thats just factory recommendations. I always just set mine and left it there after motor was warmed up. also when you advance the throttle in gear and go back to neutral it will change the idle to a touch higher then where you left it, so i always check it by putting in gear, advancing throttle and taking it back out of gear to see how low or high the idle is at the stop. if too low, go up visa versa for too high, then I leave it set. I pull choke when cold like normal and it usually fires right up and stays high until i take it off choke, if its set right, once off choke it should stay around 700-900 every time.

=D>

perchjerker said:

sure it could be the problem

kind of strange you have to adjust the idle speed each time you cold start it if I am reading this correctly

what happened after you took it out to the river to test it?

you never did say or I missed it

Click to expand...

When I took it to the river it did the same exact thing. Fired up great when it was cold, idled smooth. Took off on a full throttle run. When I spun around and got back to the launch it would die out immediately when I went to idle. Then wouldn't start unless I pulled the vowel and advanced the throttle manually. Now that i know how the "idle adjust" knob is supposed to work. This may help with idling. But then again, I can't imagine merc would build a motor that needs constant idle adjustments just to stay running. Who knows, last order of parts come in tomorrow. I will have it back together tomorrow night and will be running it then. It has always ran ok on the muffs, but, as soon as it is in the water (with exhaust back pressure) it will die on me.

On another note, if I kept the idle at around 1,200 rpm in neutral it would not die and fire right up, in gear it would run at 800-900 rpm and not die. I was able to obtain this by adjusting the linkages. However, after doing so it would cut out at full throttle. So I readjusted the linkages and then it wouldn't idle. So I feel the problem may just be the proper use of the idle speed knob.

I had an 88 Johnson 25 before this. Thing was bullet proof. Performed a link and sinc, which is extremely easy in those motors. Basic maint performed and it ran like a champ. This merc is making me grow grey hair, and is actually putting a strain on my relationship with my fiancé lol. When I can't figure out an issue it is on my mind 24/7.

Any adjustments to the motor should be done in a tub or tank to create back pressure. Running on muffs will create false running conditions. Think of it like running on a tread mill vs sand.

Sent from the dust in front of you!

It may help you to buy a knock off tiny tach from Ebay or Amazon. You can get them for about $15. It will help you get it to the factory recommended forward idle speed (in tank or water) plus give you your WOT rpm's just in case you need to re-prop it.

wmk0002 said:

It may help you to buy a knock off tiny tach from Ebay or Amazon. You can get them for about $15. It will help you get it to the factory recommended forward idle speed (in tank or water) plus give you your WOT rpm's just in case you need to re-prop it.

Click to expand...

Yes sir, already have one. My idle speed prior to making linkage adjustments was 650-700. I thought it was idling low due to being rich and subsiquently misfiring. But now I think it was misfiring due to the low idle. There was fuel bubbling up from under the water when it would die