20 HP Merc issues

I’ve been having issues with my Mercury Thunderbolt 200 20HP 2-stroke not wanting to throttle up. I’ve changed the spark plugs, have all fresh gas, used seafoam, drained the carb, tried a different gas can/hose with the same oil/gas mix and spent quite some time running it in all gears, it just won’t “kick in.”
It doesn’t die on me, it just won’t throttle over 4 mph. It will give off a hum as if it is about to rev up and go, but just putters at the brink of going. In idle it gives a slight kick about every 5-6 seconds.
Is this a fuel pump thing? There is no visible mechanical malfunction that I can see, but that doesn’t necessarily mean there isn’t one either.

Could be the fuel pump. Have you tried squeezing the bulb while throttling up (taking the place of the fuel pump).

Otherwise, it could be a bad/intermittent coil?

Make sure there is no cracks in the hoses around the carb that could be sucking air into the carb. One other thing and it could get costly is the power pack is bad. I had one go bad and if I remember right it did about the same thing as yours. I had to idle back around 9 miles up steam so I was lucky because I lived down steam.

Have you taken the cover off and used your fingers to advance the throttle on the carb? That would tell you if its a carb or linkage issue.

That motor does have a linkege that limits the throttle when not in forward or reverse.

If bypassing the throttle does not fix the issue I would look into a full carb cleaning.

Another thing to try is figuring out if you are running on one cylinder. Pull one spark plug boot then start the motor. If it runs the same or not at all you can figure out which cyl is down. Do this on both cyl, if it does not run then you most likely unplugged the dead cyl.

Do not pull plug boots while motor is running unless you want the surprise of your life…

Had this issue this spring. There was a small crack on the inlet to the tank which was losing pressure, if I kept pumping the bulb it would be fine, but you don’t want to be on the water and have to fix it. Would take a close look at the gas connections closely, really was a tiny crack that wasn’t noticable from a few feet away.

I have tried pumping while throttling and it does absolutely nothing

My next step here would be to check and see if a cylinder is bad, and clean the carb.

How would one diagnose an intermittent coil?

You can look at the resistance specs and use a multimeter, but the easiest way is as described above, by pulling plugs one at a time while running. If you do this make sure you use INSULATED pliers.

“Otherwise, it could be a bad coil?”

My money would be on a bad coil. A lot of small hp engines will start and ideal just fine with only one cylinder firing. Try to rev them up and the dead cylinder puts too much drag on the good cylinder that it never quite is able to rev up.

Yep, and if you find a dead cylinder, you can easily swap the coils and check again to see if the problem follows the coil. If so, you know the culprit.

My 9.9 merc did this same exact thing. Found a bad coil pack. Replaced it and runs perfect now.

Thanks guys, I’ll give’r a whirl

Same problem with my 20hp Merc. Great information.

If pumping the bulb did nothing, and you’re SURE about this, then it most likely isn’t an air leak or a fuel pump. But I’d say try it one more time just to be sure, because these are cheap and relatively easy fixes, so you want to be sure you eliminate these before moving on to the complicated / expensive stuff.

Next, I’d do a total teardown and deep clean of the carb. Paying special attention to the fuel inlet at the float bowl and the main jet, but clean EVERYTHING. If you haven’t done it recently, I’d buy a carb kit and replace everything as this is <$30 and it allows you to be sure everything in the carb is good with no leaking seals or plugged passages.

Finally, have the coils and points ever been replaced?

If you look on Youtube, there are videos for how to diagnose coil issues. If you can narrow it down to one, that’s fine, but then you have to ask if it’s worth the peace of mind to just replace both.

Grouse

You might want to check the magnets on the flywheel. I have had them come loose resulting in the performance you describe. If they have come loose you can jb weld them back in place.