'99' 40hp 4-stroke stalls on throttle up

[olphart44]

my '99 mercury 40hp, 4 stroke, on an 18' pontoon will not go past about 3000rpm and motor dies when i try to push throttle past about 1/2 throttle. Starts fairly easy, idles rough for about 2 minutes then smooths out and idles very smooth and quiet and IF I increase throttle slowly it will gradually get up to about 3000 rpm. tried using a new temp fuel tank, checked fuel line filter, bulb pumps ok, and tried turning back the carb adjustments 1/8 turn at a time and nothing seemed to change except idle got poor so put the screws back to old position. also put in new plugs. this is my 1st boat and bought it over the winter without having it in water first...

 

[olphart44]

Re: '99' 40hp 4-stroke stalls on throttle up

Any help for this novice appreciated ^

 

[Angler Don]

Re: '99' 40hp 4-stroke stalls on throttle up

Hi olphart44,<br />had a similar sounding problem with my 2000<br /> 40 / 4 stroke / 3 cyl / 3 carb. Died on acceleration, or when driving up onto the <br />trailer etc. Synchronizing the carburetors <br /> worked for me. Good luck !<br /> Don

 

[olphart44]

Re: '99' 40hp 4-stroke stalls on throttle up

to really show my ignorance, how do u synchronize the carbs?,, I wasn't too clear in the original post. NO matter if I go up slow or all at once, when I get to about half throttle, the engine dies like I just turned off switch?

 

[Angler Don]

Re: '99' 40hp 4-stroke stalls on throttle up

This is done with a synchronizer. It is a tool, basically a gauge assembly, hooked to the engine carburetors, which measures cylinder vacuum on multiple carburetors simultaneously. This is necessary for any multiple carburetor engine, whether marine, auto, motorcycle, etc. The goal is for each cylinder to contribute an equal amount of power to the engine. I have one of these tools, so I can do my own. Marine repair facilities in my area charge $75 - $100 to do this. My boat also acted this way, I could barely coax it up to 20 - 25 mph. Just like shutting off the key. Now, tops out at 40mph, smooth as glass.<br />This may or may not be the cause of your boats troubles, but fuel and carb are perhaps the two leading causes of outboard motor performance problems. You bought a used boat, like I did, not knowing how it was maintained, used or prepared for off - season storage. Worse case, drop it off at the shop, explain the nature of the problem, let them perform their diagnostics and repairs, just like your car. It can get expensive, though.<br /> Good luck, post how it works out, OK !<br /> Don

 

[olphart44]

Re: '99' 40hp 4-stroke stalls on throttle up

Thx Don, will try to locate a synchronizer by the weekend.

 

[Angler Don]

Re: '99' 40hp 4-stroke stalls on throttle up

For my own use, I purchased the <br /> Morgan Carbtune II , primarily because it<br /> does NOT use mercury, like other brands.<br /> Mercury is highly lethal, no point getting<br /> killed trying to tune up the boat !<br /> Contact them at carbtune dot com