Runs good but stalls at full throttle in water

[ToadFish]

I have a 1985 65HP Mercury that seems to run overall pretty good except when you give it full throttle. The funny thing is when I have it out of the water and run full throttle it doesn't stall at all. Any ideas? I've replaced the plugs and fuel filter and replaced the gas. Any help is appreciated. Thanks.

 

[funpilot]

Re: Runs good but stalls at full throttle in water

Welcome to iboats, ToadFish.<br /><br />I assume that out of the water you are running the motor on a pair of muffs. I wouldn't recommend running at full throttle out of the water, It's hard on the motor.<br /><br />The motor will act differently in the water with a load. If you've done the plugs, fuel filter and new gas, I would check for leaks in the fuel line or primer bulb. It's easy to get air in the lines if they leak. Another idea is to clean and rebuild the carbs. You don't say when the boat last ran well, has anything changed?<br /><br />good luck

 

[ToadFish]

Re: Runs good but stalls at full throttle in water

Thanks, there's a lot of good information on this site -- I just replaced the primer bulb and fuel line yesterday but it's still stalling out at full throttle. The motor appears to run really good at low speed until you push the throttle down all the way. Do you think that I might have a fuel pump problem? <br /><br />Also, do you recommend spraying internal carb cleaner into the intakes. I heard that this could lock up a small 2 cycle engines.

 

[99tango]

Re: Runs good but stalls at full throttle in water

As long as the fuel pump is sending fuel to the carbs it should be ok. The fact that it runs fine at cruising speeds also is a good sign that the fuel pump is ok. Of course I would check the fuel filter and see if you can blow through it backwards nice as easy or if it is stopped up and hard to blow through. You may consider the possibility of an ignition problem. Check to see if you are losing spark when at WOT (full throttle). Use a timing light and check each of the spark plug wires at WOT. Take good notice to the way the timing light flashes just before it acts up and then give it a little more gas until it acts up and look at what the timing light is doing. You might notice erratic light flashes on one cylinder or all cylinders or maybe one bank of cylinders.

 

[Barnacle_Bill]

Re: Runs good but stalls at full throttle in water

A pinhole in the fuel pump diaphram would let it run good at low rpm's but not higher becuase it loses pressure.

 

[KCLOST]

Re: Runs good but stalls at full throttle in water

Time for a carb rebuild, in my opinion...

 

[ToadFish]

Re: Runs good but stalls at full throttle in water

I was kind of leaning toward the rebuilt carbs but wanted to check off some of the easy stuff first. <br /><br />Although, I still think it might have something to do with fuel starvation. I'm wondering if my next step should be to rebuild the fuel pump.

 

[funpilot]

Re: Runs good but stalls at full throttle in water

Actually, rebuilding the carbs is really easy. A carb/gasket kit will come with extra parts for other applications, so don't let that spook you. Take one carb apart at a time, and don't mix up parts between carbs. You are mostly concerned about cleaning passages, orifices, and resetting the float and needle. You can screw the jets out, and make sure you can see light through the orifices. Don't take off the throttle shaft if it looks and functions OK. Use a good quality carb cleaner, and then use compressed air to clear out the cleaner.<br /><br />Go ahead and do the fuel pump at the same time. It's probably due too.<br /><br />The only other thing I can think of is a timing problem, but unless it has been messed with, that hardly ever shifts. I had a '63 45hp merc which lost its timing once, but not on any other motor, unless I screwed it up in the first place!