1973 Mercury 500 dies when throttle advanced above idle

[aban_trackerman]

I have a 1973 Mercury 500 50 HP. Seems to run great when cold. Also sounds like it is runnig good out of the water. After engine is warm, with the prop in the water, it usually starts and idles ok. When throttle is advanced from idle to low/mid range, it starts running rough and dies. Choking and pumping the prime bulb helps it recover. After it dies it is hard to start again. I have found that pumping the bulb before every start helps for a little while. It doesn't matter if the engine has been off for 2 minuites or 20, I have to prime.When squeezing the bulb it does get hard, but doesn't stay hard for very long. I have replaced the gas tank, gas tank hose and bulb. I have also replaced the diaphragm on the fuel pump, checked the filters on the carbs and replaced the carb bowl gaskets(one was leaking). The float and needle/seat looked good. I also decarbed and changed the spark plugs. I checked for leaks in the fuel system by closing the vent on the tank and priming until the bulb was hard. The bulb stayed hard for several hours(haven't looked at it since). I did notice when sqeeezing the bulb hard, gas would come out of one of the carburetors inlet and the others vent hole. I also might have a very small fuel leak on the throttle shaft. Any suggestions would be greatly appreciated. Thanks!

 

[moderator1]

Re: 1973 Mercury 500 dies when throttle advanced above idle

Moving to Mercury/Mariner Troubles and Repair.

 

[aban_trackerman]

Re: 1973 Mercury 500 dies when throttle advanced above idle

Should I try adjusting the idle mixture first? Do both carburetors need to be adjusted for forward idle? I'm wondering if I getting a compression leak after it warms up- I don't think the powerhead has ever been rebuilt.

 

[Chris1956]

Re: 1973 Mercury 500 dies when throttle advanced above idle

Trackerman, It would seem that your problem is either misadjusted idle mixture or dirty carbs. You can try to adjust the idle mixture screws, and if you do not have success, best to remove and clean your carbs. The motor probably runs better when it is cold, cause you use the choke to richen the mixture. To adjust idle, Start with the needles 1-1/2 turn out from lightly seated. Warm the motor and set it to idle in forward gear. If it idles OK and will accelerate smoothly, your done. If not and the idle is poor, adjust the mixture screws until it idles better. Now try to accelerate. If acceleration is smooth, your done. If not, turn the top carb's idle screw out (CCW) 1/8, and try to accelerate. If it is smooth, your done. If not open the bottom carb 1/8 turn and try again. Repeat the top carb, bottom carb sequence if necessary. The object is to have a rich a setting on the idle jets as possible with an acceptable idle performance.<br /><br />PS The head and cylinders are one piece on that motor, so you cannot get a compression leak in this area. I suppose the rings could leak, but I do not know if it would only happen with a warm engine.

 

[aban_trackerman]

Re: 1973 Mercury 500 dies when throttle advanced above idle

Thanks Chris, Hopefully will get a chance to take it out this evening and try adjusting the carbs. It would be nice if that is all it is. Thanks again!

 

[aban_trackerman]

Re: 1973 Mercury 500 dies when throttle advanced above idle

Before I took the boat out last night, I popped the cowling off to adjust the carbs. When I pumped the prime bulb, I noticed a small amount of wetness on one of the carb fuel lines. When I grabbed the line and moved it, a pinhole sized stream of fuel shot out. We found the problem!!! Should have the fuel line in a couple of days.