20HP Mercury full throttle issues

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Kier

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