mercury 500 wont get rpm anymore

[dutchman89]

Hello guys,

Yesterday i was cruising with my boat. I have an mercury 500 thunderbolt. This was the first run for this motor in about 2 years. Motor is from mid 80s. The motor ran perfect for around 1.5 hour. Then suddenly during full throttle the motor dropped rpm with an bwaaaaap sound. Like its low on fuel, but it isn't. She starts perfectly with one push on the button and is also cooling perfect. But when I hit the throttle I now hear the bwaaap sound again. What can this be? Spark plugs? Carburetors? Please help me out.

I changed for half a month ago the gearoil and impeller. I use 1:45/50 mixture with quicksilver oil.

Greets from holland

 

[hotrod53]

Re: mercury 500 wont get rpm anymore

After sitting for 2 years I would have to suspect carbs. Are you sure that you're running on both carbs? That motor will run on 1 carb but won't have any power. If it were me, I would disassemble and clean the carbs first. You can also measure each spark plug wire to ground to see if they are all the same, mine read about 1150 ohms to ground. Take a look at the plugs and clean them if necessary. Start with the easy stuff first. That bwaaaap sound is the carb opening and you getting no fuel to raise the RPMs, your problem sure sounds like fuel related.

 

[dutchman89]

Re: mercury 500 wont get rpm anymore

Thanks a lot for your reply!

I`m also thinking it is a fuel problem, but are new to outboards. I`ve cleaned a carb or 2 on my scooter and other 2 stroke machines but never on a merc. Does anybody have any advice were to start by removing the carbs. Are their things that I have to take in notice?

When I remove the carbs do I have to replace all of the gaskets? An how-to would be very helpfull to me because I don`t know which bolt to remove first and don`t know the position of filters etc..

Your help will be much appreciated!!

 

[hotrod53]

Re: mercury 500 wont get rpm anymore

A couple pieces of advice .... remove the starter first, its only 4 bolts and you'll never get the carbs off otherwise. If your carbs are Tillotsens and have the blue round piece where the fuel line goes in, if you remove it, you'll probably have to replace it. These lose their curve and will leak ever after .. they're only $5 each. Carb kits don't include all fuel pump pieces, you need both kits. If you have old original carb needles with rubber tips, replace them, they are not Ethanol compliant and will swell and stick. The choke linkage is a PITA to remove and you have to bend it. It's plastic but its tough, it won't break. The syncronizer link between the carbs is fragile, be careful with it, it WILL break if not careful!

Other than that, just clean away and keep your parts on a clean rag.

 

[dutchman89]

Re: mercury 500 wont get rpm anymore

A couple pieces of advice .... remove the starter first, its only 4 bolts and you'll never get the carbs off otherwise. If your carbs are Tillotsens and have the blue round piece where the fuel line goes in, if you remove it, you'll probably have to replace it. These lose their curve and will leak ever after .. they're only $5 each. Carb kits don't include all fuel pump pieces, you need both kits. If you have old original carb needles with rubber tips, replace them, they are not Ethanol compliant and will swell and stick. The choke linkage is a PITA to remove and you have to bend it. It's plastic but its tough, it won't break. The syncronizer link between the carbs is fragile, be careful with it, it WILL break if not careful!

Other than that, just clean away and keep your parts on a clean rag.

Thanks a lot! I cleaned out the carbs and indeed one of them was full of dirt and sand. The hole where the fuel goes in was also full of dirt and I couldn`t see trough it. I all cleaned it out and im sure the problem has gone now.

I did remove the side of the carb (pomp) but only the 4 bolts, so i could clean the filter. I didn`t remove the center (blue) part. I alligned them up good and reassembled them. Are they going to leak now or doe you mean when you remove the center blue part?