merc 500 fuel pump problem

[allymoss]

Hello merc geniuses

I have a merc 500, 50hp 4cyl 2 stroke. serial no. is 2948379

Right, i got new carbs for it and put fuel in float bowls manually via syringe as fuel pump not working. It would not start. Not even fire once. Plugs have nice big blue spark on each one. Loads of compression on every cylinder. I don't know if fuel is getting to the cylinders although when plug was removed it was smelling of petrol. I sprayed easy start in the carbs the other day and it buzzed up for a few seconds so im thinking that fuel is not getting from the float bowl to the cylinder. The carbs are practically new, have been stripped down and cleaned with carb cleaner. As there is not a constant supply of fuel to the float bowl, will this stop it starting.

Question 2.

The fuel pump is strange. When I removed it, it was full of crap, i cleaned out, looks like new now. Goes like this. Outside bit is metal, then there are 3 gaskets of sorts. When I try to put back on the screws do not go in all the way as it looks like I am missing a spacer or other part of the fuel pump. I don't know what im missing. There is defiantely something missing as I can see the imprint on the old gaskets and the shape on the engine is different. If someone has a picture of a fuel pump complete, that would be great.

Thanks

 

[allymoss]

Re: merc 500 fuel pump problem

also, the carbs are apparently originally off a 65hp, will this affect the engine. I have been told they will work but dont know!

 

[tmcalavy]

Re: merc 500 fuel pump problem

Put some pre-mixed fuel in a spray bottle, little nasal inhaler bottles work good , and spray some fuel in the carb throats when trying to start it. If it fires off, you have a restriction somewhere in the carbs. If the plugs were wet when you pulled them, you may just need to adjust the carb high and slow-speed settings and do a link-n-synch so the carb butterflies open at the right time. I had a Merc 500 73 model a few years back and paid about $50 for a link-n-synch. I had pulled and cleaned the carbs just fine, but didn't know that the timing/firing has to be in synch with the carbs opening.

 

[allymoss]

Re: merc 500 fuel pump problem

I synced them myself by just looking at them. They open exactly at the same time. I don't know where to adjust the idle speed although there is a screw at the top of the whoel carb. Silver screw, looks like a mixture screw or something.

 

[TriadSteeler]

Re: merc 500 fuel pump problem

I synced them myself by just looking at them. They open exactly at the same time. I don't know where to adjust the idle speed although there is a screw at the top of the whoel carb. Silver screw, looks like a mixture screw or something.

The link-n-synch that tmcalavy is referring to is much more than the butterflies opening together. You will notice that when you move the throttle linkage open that not only do the butterflies open, but the the ignition driver (the upside down distributor looking thing) rotates as well. As you open up the throttle, the rotation of the driver advances the timing simultaneously with the opening of the butterflies.

Anytime you disassemble the carbs or the ignition system, a link-n-synch is practically compulsory. Here is a post to guide you through it step by step.

http://forums.iboats.com/showthread.php?t=168855



This should help in identifying the idle screw. However, If you follow the link-n-synch procedure, your idle will be set perfectly and at the correct RPM.

 

[Chinewalker]

Re: merc 500 fuel pump problem

I can't recall if the fuel pumps on those motors has the base cast in or separate. Where the pump bolts on, is there a recessed area in the shape of the opening on the two gaskets, or is the boss flat? If the boss is flat, that's why the pump isn't working - you're missing the pump base. The pump diaphram needs space on either side to flutter so it will pump.
- Scott

 

[allymoss]

Re: merc 500 fuel pump problem

Thanks for all that.
I will do the correct syncing thing tomorrow. Maybe get my mechanic friend to help me

Chinewalker
The side of the block has the fuel pump area with 2 holes, one bigger and one smaller. I presume these go through to the crankcase or something. There is definately something missing as the diaphram can't "flutter" at all! There is a shape indented around the diaphram which shows I am missing something.

Does anyone have an exploded diagram like above but of the fuel pump that would be great. Thanks

 

[allymoss]

Re: merc 500 fuel pump problem

Hi,

Do I NEED a fuel pump fitted for the engine to work? Can I plug those holes just so I can test the engine works.

 

[Chinewalker]

Re: merc 500 fuel pump problem

The holes must be filled/covered for the motor to actually run, as crankcase compression would be lost on those cylinders if they're open.

It is possible to make a base piece out of a couple pieces of aluminum flat stock, maybe 1/8" thick or so. Cut one in the shape of the pump base and drill the holes accordingly for the mounting screws and breather holes. Cut the other in the shape of the base, plus cut a hole in the middle in the shape of the pump diaphram gaskets (not the diaphram, the gaskets on either side of it). Make a new gasket to go between the plates (holes cut accordingly) and assemble to the pump to the motor with this new base in place. Shouldn't need more than a drill, and a couple of round and flat files to do it.
Good luck!
- Scott

 

[allymoss]

Re: merc 500 fuel pump problem

To be honest I would rather go and find a used spare off ebay or marine broker because it wouldn't work because I made it so I would end up getting one anyway. I know its simple but if I can find the spacer for ?10, I would rather buy it. Thanks for the tip anyway. I might try it if I have some time spare!

Thanks

 

[allymoss]

Re: merc 500 fuel pump problem

and just another thought. There are two sets on the side of the cylinders for fuel pumps. There was only one fitted. Will I have to plug one set and put pump on top set or get another fuel pump!! Or will the other set be blocked.

Thanks

 

[Chinewalker]

Re: merc 500 fuel pump problem

One set should be blocked. The other pump base is kinda like your appendix - useful at one point, but no longer necessary. Vestigial, if you will. When they first designed those motors, they thought it would need the extra fuel flow. Turns out they didn't, but the dies had already been made with the extra boss.

If you weren't overseas, I'd send you one. I'm sure I've got a few in a cabinet somewhere. Postage would a bear, though...

- Scott