1978 Mercury 500 50HP problems...

[weasel897]

I've been working on getting my boat up and running here recently. After going through the carbs and throttle linkage, it was running pretty good. Now, I'm trying to figure out why my engine is cutting out at WOT. It will die unless I let off the throttle. I've checked the line for air leaks. I cleaned the carbs(again) and ran some seafoam through it.

It's really weird because after cleaning them again today, I took it out and hit the throttle and it was fine. I made a full pass on a small lake without any issues. I killed the engine and then started it again and hit the throttle. All was well until it started to plane. It started cutting out unless I reduced the throttle. I babied the throttle until it got back up to full throttle. I made another pass around the lake and killed it again. Once I hit full throttle again it started cutting out again. I seems like I can baby it sometimes to get back to fully throttle and then other times, it's just going to not run right!

I'm not sure what's going on.

Any ideas?

Thanks,
Weasel

 

[weasel897]

Re: 1978 Mercury 500 50HP problems...

bump...

 

[oldman570]

Re: 1978 Mercury 500 50HP problems...

If your cutting back from WOT to idle real fast , it could be that the floats are not up to the setting they need to be or the fuel pump is not able to supply enought fuel quick enought to the carbs. Could also be the carbs are set to lean on the idle. Try backing the Idle jets out a 1/4 turn and see if it helps. JMO
Oldman570

 

[weasel897]

Re: 1978 Mercury 500 50HP problems...

Oldman570,

I did notice yesterday after going into the carbs again that the lower carb float felt heaver. It appear to be an OEM part. Maybe it's soaked up some gas over the years and isn't functioning correctly. I'm going to get two new floats and seal kits(just in case). I've found plastic replacements and the original OEM floats. Which would be better???

Also, I've thought of the carbs being too lean. Although after first getting the boat and getting it running right, I had no issues with performance(other than the boat being too heavy, that's another story). If and when carbs get a good supply of gas, the engine runs without any problems. I can make several laps around the small lake unless I kill gas supply or pull the throttle back. I'm going to see if the float fixes my problem before I try to tune anything!

Thanks for you help!
Weasel

 

[weasel897]

Re: 1978 Mercury 500 50HP problems...

Update...

I purchased a new carb gasket kit, fuel pump kit, and new floats. I wasn't sure of the previous fuel pump gaskets as I purchased them separately from a local marine shop. I figured the best way to eliminate that doubt was to get the kit. Which leads me to my first question.

Question 1: The fuel pump kit comes with four gaskets/seals and one diaphragm. The online diagram only shows three gaskets/seals and a diaphragm. There are two of the same gasket, but made of different material. What gives???

I replaced the floats with the new plastic ones and fixed the issue with the lower carb overloading with fuel. Now the top carb overloads. I checked the inlet needle valve on the top carb and with the float all the way up, the new float doesn't even touch the spring connected to the valve. Maybe PO changed the valve maybe the spring is just laxed??? I am going to replace both with OEM parts.

At this point I don't think it's the new float, because a new float fixed the lower carb. As it sits now, I have the original float back in the top carb because I had no issues with it before changing. My second question...

Question 2: Anyone ever have this issue with replacement plastic floats?

I took the boat out yesterday and was pleased with the results.

When I got on the water my first action was to ease into the throttle. At about 3/4 throttle it started cutting out as it did previously. I tried several times to advance to no avail until I pumped the bulb "ONCE". It was already hard for the most part. Once the engine got that shot of gas, it sprang to life. I was able to move to full throttle and the boat got to plane without an issue. I made a multiple passes around my test lake without any more issues. I killed the engine several times and had no more issues after my initial blip.

Question 3: Does anyone think this is just a fluke or another calm before another storm?

I would love to hear any thoughts or comments!!!

Thanks,
Weasel

 

[oldman570]

Re: 1978 Mercury 500 50HP problems...

Might have been that the fuel line was laying down just right to cause a fuel lock that the fuel pump could not get cleared and would not let enought gas flow as the motor needed. About the same as if the gas tank were setting on the fuel line. I route my lines so they will stay above the leavel of the tank to help keep from having it happen. As long as the pump is in good shape you should be fine.
JMO
Oldman570