# 20HP Mercury full throttle issues



## Kier (Jan 4, 2015)

Hi guys! I need some guidance on where to start with my 1984 20 HP mercury 200. The motor starts fine and idles well. I take off out of the dock area and get up to full throttle and get my 14foot StarCraft on plane. I can go like this for 5 to 10 minutes and then the motor will cut out. I start it back up fine and it will idle and go half way up the throttle. If I go past the half way mark it acts like it is going to die. After 15 minutes or so I can go back to full throttle and it runs fine. Then the process repeats itself. Where do I start troubleshooting?


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## DaleH (Jan 5, 2015)

Do you notice your fuel line primer bulb going flat? It could be that she gets starved of fuel. Try running it with the cap to the fuel tank open. If it runs OK then, then your vent to the tank is clogged.


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## Kier (Jan 5, 2015)

The bulb seemed mostly hard. I have a screw on the top of my 6 gallon tank. I have tried full open to half open on the screw. I tried a couple minutes with the gas cap off but did not make a difference. I bought it a two years ago and the carbs have not cleaned since i have owned it. Could it be something wrong in there?


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## lovedr79 (Jan 5, 2015)

my motor used to do that. I discovered that it was the float bowl sticking. would idle and rev all day, but above 3/4 nothing.


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## lovedr79 (Jan 5, 2015)

I would tap on the side of the carb with a screwdriver handle and it would fine again. i cleaned the carb and it has been fine since.


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## Kier (Jan 5, 2015)

Thanks. I will take the carb off and clean it.


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## lswoody (Jan 5, 2015)

Sounds like some gunk in your carb. When you clean it make sure to get every one of the ports clean. Just a little bit of trash can cause this


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## Kier (Jan 5, 2015)

Do i need to purchase any kit ( gaskets or needles) or do i just take it out and clean it with carber cleaner?


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## lovedr79 (Jan 5, 2015)

i didnt. you might want to get a rebuild kit due to the age of the motor. all i did was completely dissassemble carb, cleaned every orafice and the float valve really well.


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## JMichael (Jan 6, 2015)

If you don't plan to install a rebuild kit, be careful with the carb cleaner and the o-rings/rubber parts of the carb. Rubber and carb cleaner don't always play well together.


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## DaleH (Jan 6, 2015)

JMichael said:


> If you don't plan to install a rebuild kit, be careful with the carb cleaner and the o-rings/rubber parts of the carb. Rubber and carb cleaner don't always play well together.


I would use Seafoam (or their spray, called Deep Creep), as it was designed for OBs. Clean it all out, fill bowl w/ it or spray in, pull through motor to crank with ignition kill removed and let her sit for a day or two. 

I've resurrected many a motor to include lawnmowers, snowblowers, weed-whackers, chainsaws, pressure washer and an old Ford Exploder (Explorer) that people junked and GAVE to me for free ... as they said they no longer worked and weren't worth the $$ to have them fixed. A little time, effort and Sea Foam salvaged all of them. I ran that Ford for 6 more years on the road, then left it in the boat yard as a haul/tow vehicle for another 6 years, before the brake lines totally failed ... thankfully we weren't on the ramp wth a boat :shock: at the time. I even sold that pressure washer and got $300 for it!

*Sea Foam is the BOMB!* FWIW, I do a decarb, via the 'Dunk procedure', on all 2-stroke OBs every 50-hours or so.


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## Kier (Jan 6, 2015)

I did that procedure when i first bought the motor. I agree seafoam is awesome. Is it possible that the seafoam dislodged a big chunk of crap and that is causing it?


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## DaleH (Jan 6, 2015)

Kier said:


> I agree seafoam is awesome. Is it possible that the seafoam dislodged a big chunk of crap and that is causing it?


It sure is possible, which is why I love Sea Foam ... but acknowledge that it is NOT a replacement for a good carb pull-down and service. Always clean carbs and other orifices (orifi?) from smallest orifice to larger.

Another thing, while the spray is good for decarbing the cyclinders, it misses the ports and jets, so I use the 1-can to rest fuel for a 1-gal mix (or less, w/ oil added as needed) for the 'Dunk' shock treatment. 

But back to the original point, I rarely ever have to take a carb down and rely on SF religiously!


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## Kier (Jan 6, 2015)

I took the carb out and it was fairly clean. Nothing was really dirty except the metal valves and the float needle. The float needle had a small amount of black film on it. I stuck fishing line through the needles to make sure they where clean.I wonder if float needle dirt was causing it not to release and then not fill the float chamber? It wont be till February till it warms up enough here in Kansas to get on the water. I hope that was it!


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## Kier (Feb 8, 2015)

First set of nice days in a while, I took the boat out and she started fine and ran for a minute then died. I could not keep the bulb hard. I noticed the fuel line from the connector to the fuel pump had split and was leaking gas everywhere. I guess I should have replaced all the lines while I was in there. I wonder if it was sucking air last year as well, causing the pump to not be able to keep the carb bowl full.


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## lovedr79 (Feb 9, 2015)

did you try squeezing the bulb to keep it running? I did have a bulb go bad once.......


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## JMichael (Feb 10, 2015)

Kier said:


> First set of nice days in a while, I took the boat out and she started fine and ran for a minute then died. I could not keep the bulb hard. I noticed the fuel line from the connector to the fuel pump had split and was leaking gas everywhere. I guess I should have replaced all the lines while I was in there. I wonder if it was sucking air last year as well, causing the pump to not be able to keep the carb bowl full.


I'd say that the odds are in favor of that being exactly what was happening but you may also have other minor things that also contributed to it.


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## Kier (Mar 16, 2015)

Well, had a nice week of warm weather here in Kansas. I took the boat out and opened her wide open. The same thing happened but not as bad this time. The motor recovered and I was able to take it back up to 60-75% throttle. I was able to maintain that for as long as I wanted. I wonder if I still have a small leak somewhere or if I am overheating the electric side.


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## lovedr79 (Mar 17, 2015)

may also want to try tapping on the float bowl, the float may be sticking too.


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## Tallpine (Mar 17, 2015)

My uncle had a 20hp merc doing that and after fighting with it he replaced the coils. One of them was bad and it solved the problem. I also had a 7.5 Evinrude do the same thing.


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## Kier (Mar 17, 2015)

Is there a way to tell which one is bad? Or do I just replace both?


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## Kier (Mar 17, 2015)

Also do you try a used one on ebay or do you buy new?


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## Tallpine (Mar 17, 2015)

I have never tested them myself but understand you need a peak volt meter. I took my Evinrude to a shop and had them test it. You could ask the question over on iboat forum. The first thing they are going to tell you is to buy a service manual for your motor. Which is a good idea. Personally I wouldn't buy a used one. New ones aren't that expensive. I think the one for my Rude was 35.00.


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## Tallpine (Mar 17, 2015)

Another thing, before you replace coils is to check for spark at both plugs and check and clean all your wire connections and plug wires. It could be something as simple as a plug wire.


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## JMichael (Mar 18, 2015)

You can test a coil with an ohm meter and/or a spark tester, but you'll need to know the specs for the coil's primary and secondary windings to test with an ohm meter (service manual should have the specs). If your motors cooling system is working properly, the "electrics" (coil and ign box mainly) shouldn't be any warmer at 60% throttle than they are at 100% throttle. They get hot because of engine heat, not because of current flowing through them. If you still think it's because things are getting hot, run it with the cover off and see if there's any improvement.


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## Kier (Mar 18, 2015)

I did some searching on the net. Someone said to pull your plugs and see which one is more fouled up. My bottom spark plug was way oily compared to my top plug. I pulled the coil for that cylinder.
Per https://www.outboardignition.com/page39.asp , My model 1980-81 20Hp Mercury should have a resistance of 800-1100 ohms. I was reading 980 when I first pulled it. I stuck in front of a hair dryer for 5 mins and i was getting 1080 - 1180, it was bouncing up and down. I was testing from the negative lead on the bottom of the coil to where the spark plug cable goes. From connection to connection on the top i was getting 1. Could this be my random issue?


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## onthewater102 (Mar 18, 2015)

The fact that one cylinder is noticeably more oily does indicate that you're getting incomplete combustion - did you compression test that cylinder to make sure the problem isn't mechanical/internal? Not saying it's a likely cause - but it's an easy one to rule out with a simple test.

1180 is starting to get out of spec - but I don't think you'd feel that significant a power drop at that point. I don't know where 5 minutes in front of a hair dryer left it temperature-wise relative to where it would be if it were running on the motor...if it were significantly warmer on the engine you may be on to something, but it shouldn't be all that warm there either. Easy way to tell - run your same unscientific test with the other coil - if the readings are the same then the reading itself isn't indicative of a problem as the functional cylinder is reading the same.

I take it all the contact points at either end of the plug wires were clean? Do you get much resistance measured across the plug wire itself?

Otherwise if you don't know the age of the wires I'd be inclined to start with replacing them only because they're a wear item anyway (no harm in replacing them a bit early) and if they turn out to be the cause they're much less expensive than a coil.

I had a problem with my mariner (same vintage, but mine was an early production of the XD series) where the fuel float had a pin-prick hole in it, so over time my top end got worse and worse and it got harder and harder to start. Your issue sounds like its outside of the carb as it's unique to once cylinder if there is a substantial difference in the residual oil in one vs. the other with a single common carb feeding them both.

Are you noticing any fuel blowing back through the carb and under the cowl?


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## Pappy (Mar 18, 2015)

You know.....at this point you don't even know if you if you are dropping a cylinder or not and are being talked into changing out ignition coils!!??
For one...your engine would not idle worth a darn on one cylinder, also it would not plane or barely plane the boat on one cylinder. 
Still not convinced? Buy a pair of plastic plug wire pliers and pull a spark plug lead at the RPM where the problem occurs and see if the engine changes. 
If it does then it is firing on that cylinder. If it does not change then it is not running on that cylinder. Be very careful of this because it will shock the Hell out of you for one and will decelerate hard for two. Be prepared for both scenarios. 
Your ohms readings are close enough by the way. Am thinking you have a good coil set.


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## onthewater102 (Mar 18, 2015)

Pappy said:


> You know.....at this point you don't even know if you if you are dropping a cylinder or not and are being talked into changing out ignition coils!!??
> For one...your engine would not idle worth a darn on one cylinder, also it would not plane or barely plane the boat on one cylinder.
> Still not convinced? Buy a pair of plastic plug wire pliers and pull a spark plug lead at the RPM where the problem occurs and see if the engine changes.
> If it does then it is firing on that cylinder. If it does not change then it is not running on that cylinder. Be very careful of this because it will shock the Hell out of you for one and will decelerate hard for two. Be prepared for both scenarios.
> Your ohms readings are close enough by the way. Am thinking you have a good coil set.




WUT??? Are we even reading the same thread??? "talked into changing out ignition coils!!??" Several people have suggested to test them along with a number of other items and only replace them if everything else is ruled out. "your engine would not idle worth a darn on one cylinder" - no-one (save for you, apparently) is reading his description of problem as if he's complaining about running on one cylinder, but he's clearly got a loss of power coming from somewhere or he wouldn't be here asking the question - not a completely dead cylinder - loss of power.


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## Kier (Mar 18, 2015)

The idle is not smooth as butter, it shakes from time to time and coughs some times. I thought it was normal for two strokes so I never questioned it. I did not notice any gas in the engine compartment. When I took apart the carb I did not notice anything wrong with the float. I did check the drop length and compared against the manual. I do not have a compression tester so I don't have the answer to that question. The plugs for this model of mercury are not ones you can set the gap. NGK BUHW-2 5626


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## Kier (Mar 18, 2015)

Looks like NGK BUHW-2 have a heat range of 12. Which is the coldest spark plug NGK provides. I wonder if I need to upgrade to a hotter plug.


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## Pappy (Mar 19, 2015)

onthewater102 said:


> Pappy said:
> 
> 
> > You know.....at this point you don't even know if you if you are dropping a cylinder or not and are being talked into changing out ignition coils!!??
> ...



Ahhh.....so tell me about this "not a completely dead cylinder" Am very interested in how a coil can only partially shut down a cylinder. As far as I know the coil will either fire that cylinder or not. If it is coming and going it would be pretty obvious to the owner as a surge. 
Second...you don't just go and replace coils if you have ruled out everything else as is in your quote. That would be kind of a shade tree approach wouldn't you think? Replace parts until it runs? 
As far as reading the same thread goes, it kind of sounds like you are trying to put me down. If you would like to match knowledge on two-strokes and what makes them run and systems troubleshooting....I read your profile (CPA), be my guest!


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## Triple obsession (Mar 19, 2015)

Kier said:


> I can go like this for 5 to 10 minutes and then the motor will cut out. I start it back up fine and it will idle and go half way up the throttle. If I go past the half way mark it acts like it is going to die. After 15 minutes or so I can go back to full throttle and it runs fine. Then the process repeats itself. Where do I start troubleshooting?



Dirty carbs don't act like this. I have seen electrical components break down because of heat soak, but that's kind of rare & normally won't run again until cooled off.

Sounds like a fuel problem to me. Make sure the inside of the tank is clean, no rust, gunk or water. If your tank has a screen on the fuel pickup in the tank make sure it's clean. 
Make sure the vent on the tank is working, loosen the cap & go for a spin.
If that all checks out it might be a fuel hose de-laminating where the inner ply starts to seperate from the outer & collapse from the suction, or a primer bulb problem.


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## Kier (Mar 19, 2015)

I replaced all lines from tank to carb. I have not checked the tank pickup. I have a 6 gallon attwood plastic tank. I am going out on Saturday. I will take video if it does it again.


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## Pappy (Mar 19, 2015)

If you are going on Saturday bring a squirt bottle of some kind with you filled with a mix of gas/oil. When the problem occurs try my earlier test on the coils but also use the squeeze bottle and shoot a little mix down the carb throat while the issue is occurring. If the engine picks up you have a fuel issue. By doing both of these tests you can see which system is causing your issue.
If the engine exhibits the issue quick enough you may be able to do these tests with the boat tied securely to a dock while the engine is throttled up. You would know the answer to that one. Makes troubleshooting easier if you can.


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## Kier (Mar 23, 2015)

I went out on the river and was going at a good clip and hit a stump. Now my prop is slipping at anything over half speed. So i will have to table this for a bit till i can get a new prop.


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## onthewater102 (Mar 23, 2015)

You probably spun the hub - you shouldn't need a new prop. Depending on the hub system you might be able to just unscrew the prop nut & swap out the hub sleeve - otherwise you have to send it out to a shop with a press to be re-hubbed, but that's not all that expensive, just time off the water...


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## Kier (Mar 27, 2015)

New prop arrived today. 2 blade 9 3/8inch by 11 pitch. Per the repair manual I torqued the prop nut to 11ft lbs. I plan on taking her out next week end to test.


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## Kier (Apr 5, 2015)

Installed new prop. I put the muffs on the motor and went to start it in the driveway. Noticed bulb was getting mostly hard but could keep squeezing it. Fuel was leaking out of the fuel pump. Took off carberatour and tightened down the fuel pump. I took her out to the lake today and opened her wide up and ran like a champ! Thanks for all support and knowledge.


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