Merc 500 lack of power in 3 and 4

[joblo33]

I was trying to figure out why my merc 500 is weak this year so I removed the sparks plug wires 1 at a time and found theres hardly any rpm loss when #3 and #4 have the wires off. Those two cylinders do have spark as I painfully discovered (my hand is still numb). The carbs are both spotless and air passes thru every passage of them. I just cleaned them last week for this very problem. Do these cylinders share a reed valve? If so what's the easiest way to gain access to the reeds?<br />Thanks,<br />Eric

 

[jheron]

Re: Merc 500 lack of power in 3 and 4

You need to get a compression tester and test your compression, before you do any thing more you need to know if it needs an overhaul... <br />Jon

 

[joblo33]

Re: Merc 500 lack of power in 3 and 4

that was the first thing I did 120, 120, 115, 120

 

[joblo33]

Re: Merc 500 lack of power in 3 and 4

Never mind those numbers. I just changed the cracked O-ring on my compression tester and they were 135-135-135-140.<br />Eric

 

[joblo33]

Re: Merc 500 lack of power in 3 and 4

When I took out the upper spark plugs and cranked it I smelled gas, but when I put em back in and took the lower ones out and cranked it I didnt't smell any. I'm pretty certain it's fuel and I'm just curious what the easiest way to see the reeds is.<br />Eric

 

[jheron]

Re: Merc 500 lack of power in 3 and 4

I have a 1976 l6 tower (not sure if yours is the same design) and the reeds are around the crank the only way to access them is to remove the power head and split the crank case. You can see part of the reeds if you remove the carbs and look into the block...

 

[joblo33]

Re: Merc 500 lack of power in 3 and 4

Thanks,<br />I was afraid of that. I'll see what I can see from the carb holes.

 

[fishingdan]

Re: Merc 500 lack of power in 3 and 4

Are you certain that you are getting fuel in both carbs?

 

[joblo33]

Re: Merc 500 lack of power in 3 and 4

I think I might know the cause!!! Last time I rebuilt the carbs both float bowl needles were sticking up. Thus the reason for the rebuild, but I lost a needle from the carb kit so I sanded one of the needles, lubed it and called it good enough. Who wants to bet it wasn't good enough and that that needle ended up in the lower carb, where it's sticking up and keeping the bowl dry. I'll find out tomorrow, I have the day off and I guess I'll be taking apart carbs..again.<br />Eric

 

[fishingdan]

Re: Merc 500 lack of power in 3 and 4

Hopefully, that is the primary cause of the problem. If it is not, I would still replace the needle valve and seat with a new one.