I just recently had a coil go out on my 50HP Mercury. I got a brand new coil and put it in and now have lost around 10MPH at full throttle. I have also rebuilt both carbs and changed the plugs but no improvement. My thought is the new coil is bad. Is this possible? I hate to spend another $50 on a coil just to see if the top end speed improves. Any thoughts?

Are the butterflies opening all the way? How about timing advance? Is it sticking? Bulb losing prime? Float levels are correct ?

Check compression too.
That's all I have for ya. I dont know anything about Mercs, sorry.

One more thing....any extra weight in the boat? ( water in hull)

All are good, when I put the old coil back in the motor runs great for about 30 seconds then goes to [censored] again. So it has to be the new coil.

I would try to return the newly purchased coil and either exchange for another one or get your money back and go someplace else. There should be some kind of warranty...

I am past the return date on the coil being it took me all summer to trouble shoot the problem. I first thought it was a carb, then the coil, back to the other carb and then finally the coil. The problem was so infrequent that it took all summer to pinpoint it. I was hoping someone could say yes try another coil so I had a second opinion if you know what I mean. I was hoping someone could confirm the new coil can be bad from the factory

Check all of the wiring real good. These motors are bad about the wiring insulation deterioraing. Lots of problems with wires out of trigger and stator too. Pretty easy to check the coil for firing.

throw that junk in the trash before you spend a bunch of loot and turn gray haired, or it all falls out. merc parts are expensive, can be hard to locate, and they changed them abt every 3 years. if u want to play with old motors, go with evinrudes and johnsons. like was aready mentioned, if you havent completely rewired the motor, all of the insulation will fall off and you will have breaks, shorts, ect. I hate everything abt a merc,10 pounds of [censored] in a 5 pound bag. the carbs get clogged very easily also, and are usually a pain to get off and rebuild, the way the cylinders are made you cant pull the head off to replace a headgasket, you have to diassembly the whole top half, they have notoriously weak lower units, the model / serial number system they use is very confusing ,, geez, how anyone could ever want one of these motors just amazes me. they made improvements on the big mercs , but the older small motors or trash. you likely have a wire shorting to ground, causing intermittent problems.

That is a good idea to check the wiring so I did it this morning and everything looks great. So I am still thinking the coil is the issue. As for leanin post comment, I found it to be unhelpful, uninformed, and generally a waste of time. I have had two older mercs and have enjoyed but of them. I find the motor to be very reliable, easy to work on and generally problem free. Any parts that I have needed are easy to find and easy to install. As far as parts being expensive for mercs, well I find all boat motor parts to be expensive but that is what is expected for a hobby like fishing. Since this motor is in good working condition and gets me to the fish I do not plan on throwing it in the trash anytime soon.

ok well I am hoping maybe someone can help me narrow this down. This is what I have done. I have checked all the wiring and it is looking tip top, I tightened a few loose wires so good to go. I checked spark on each cylinder and have good blue spark coming from the plugs. I decided to check the compression and have 120 on each cylinder. I checked the wires on the stator and all look good. Does anyone else have an idea on what to check? I am still not at full power and the only thing I can think is trying another coil.

Fuel lines are new, both carbs are brand new. I have new clean fuel, good spark, good wiring, and good compression. I am at a loss, any help or thoughts is appreciated.

Do the rpms get up there.....but it just doesnt go? If so, might be a spun prop hub.

Otherwise, it sounds to me like it might be starving for fuel. Double check your float levels, hoses, fuel line, bulb, etc....cuz something isnt right. (duh).

New spark plugs, I assume? ( dont try to use previously fouled plugs on a 2 stroke....just chunk them in the trash)

Who installed the carbs? Are the butterflies opening all the way at full throttle? What about the timing advance? Is it moving properly at full throttle?

I'm just guessing here...and going by what little I know about older Johnson-Evinrudes...so bear with me. Make sure that timing advance isnt sticking.

I had a similar problem with a Merc 50 HP. Turned out that the old coil was running super hot for a short time, then cutting off. That was giving us extra power for a short time. We retarded the timing in order to compensate for the screwed up coil, but it finally went out. We replaced the coil, then had less power, advanced the timing and got the top end back, most of it, but the boat runs steady and smooth now. During this whole saga we also replaced the stator and the regulator, and I am not sure how that played in to the equation, or if we even needed to.

So, MAYBE it is a timing issue.