20HP Mercury full throttle issues

[Kier]

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

 

[Kier]

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

 

[Tallpine]

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.

 

[Tallpine]

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.

 

[JMichael]

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.

 

[Kier]

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?

 

[onthewater102]

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?

 

[Pappy]

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.

 

[onthewater102]

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.

 

[Kier]

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

 

[Kier]

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.

 

[Pappy]

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.

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!

 

[Triple obsession]

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.

 

[Kier]

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.

 

[Pappy]

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.

 

[Kier]

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.

 

[onthewater102]

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...

 

[Kier]

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.

 

[Kier]

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.