1975 Merc 500 50hp

[gustafsonerik]

I hope someone can help me i've given up- so has my boat mechanic.. I have a 1975 Merc 500 50hp,The engine starts right up the only thing is it only goes about 15 mph.. I've cleaned the carbs and put new seals in it(i didn't replace the floats but they look pretty good)..I put new fuel pump in it I think i installed it right( when i squeeze the primer when its running wot it will want to stall out) also when i run it,it seems that the primer will go soft after a few minutes of wot..also new plugs and changed distribitor..my questions r as followed

1. The compression in all cylinders is about 110,Can there still be something wrong internally????
2.Could this still be a fuel problem if so how???
3. Could the switch box be related to this?? any suggestions would be appreciated thanks Erik

 

[Laddies]

Re: 1975 Merc 500 50hp

You need to ck each cyl for spark it sounds like it is running on 2 cyls

 

[gustafsonerik]

Re: 1975 Merc 500 50hp

if i find out that it is what would be the cause distrib,switch box or something else like coil or that thing under the flywheel thanks Erik

 

[Tom P]

Re: 1975 Merc 500 50hp

Just a thought. I also have a 50hp Mercury Merc 500 motor Serial number (A131168 (1985)). The top speed was 6-7 knots. Everything checked out ok on the motor, and it baffled more than one marina. Ran fine with the muffs on, reved up fine. In playing with it on the water I took the cover off a couple of times. The last time I got the electrical shock of my life. The call me Curly now. After some testing at night, with the muffs on and sparks to a screwdriver from the wires. I replaced the wires and she runs great now. It’s simple and cheep. Apparently wires on these can sometimes need to be handmade. A smart marina found wires for a 6 cylinder that fit mine. Merc part number 84-821945A56.

Good luck,

Tom

 

[tmcalavy]

Re: 1975 Merc 500 50hp

Did you have your boat mech do a linc-n-synch after the carbs were re-installed? The carbs have to be in synch with the timing advance or it will do as you describe...won't get out of its own way. My 73 Merc 500 did that after I rebuilt the carbs...had a linc-n-synch done for 50 bucks. If the bulb won't stay firm you've got a problem. The vent is open on the fuel tank, right? Your fuel pump is pumping fuel right? The connectors to the tank and to the motor are good, right? The hoses are good? You have good spark on all cylinders/plugs?

 

[cdella]

Re: 1975 Merc 500 50hp

He claims to have done all adjustments according to merc specs. Didn't ask about the linc-n-synch but will question him tomorrow. The bulb does occasionally get soft, in fact the last trip out that happened and had trouble with restart. The smoke did seem to get worse after his rebuild so I will persue this issue.Will also check the vent to the tank. All else seem to be in order.

 

[tashasdaddy]

Re: 1975 Merc 500 50hp

linc & sinc

http://www.iboats.com/bbboard/bbBoard.cgi?a=viewthread;fid=29;gtid=891893

 

[gustafsonerik]

Re: 1975 Merc 500 50hp

i'll check it again thanks guys li'' let u know
on the link and sync

 

[gustafsonerik]

Re: 1975 Merc 500 50hp

could i take it out on my own and turn the distribtor by itself to give it a rough estimate i tried this before i took the whole timing off and kept the throttle at wot during the whole trip and turned the distribtor till it got about 10mph but thats all it will go??? should it have gotten better??? thanks Erik

 

[ricksrster]

Re: 1975 Merc 500 50hp

Has it ever gone over 15 MPH?

 

[rogerwa]

Re: 1975 Merc 500 50hp

If it wants to die when you pump the bulb you've got either a bulb adjustment issue or a needle that isn't seating. I would check these out before going further.You could be overloading on fuel..

 

[gustafsonerik]

Re: 1975 Merc 500 50hp

okay to may another long story short..maybe the timing is off i totk the flywheel off without putting the 1# cylinder to TDC so i had to do it and guess with a screw driver i got it close so please look at the picture and tell me if its right...the third picture, where my finger is pointing does that need to be grounded with a wire?? thanks Erik

 

[steelespike]

Re: 1975 Merc 500 50hp

It is not unusual for the bulb to be soft when in operation.
It shouldn't collapse.
Not normal for squeezing the bulb to cause a change in operation.Indicates a float valve that is not closing properly.Could be dirt,float adjustment,bad float.
All that being said if it starts and idles the float is probably not causing your wot problem.
Also if the timing is close it would run close to normal.
Lets confirm that all plugs are firing.Check spark at each cylinder.Should be blue and you should hear it snap.Check each plug.Should be brown to black,not shiney, and not furry.
Ground one wire at a time and note operation at WOT.
let us know what you find.

 

[gustafsonerik]

Re: 1975 Merc 500 50hp

okay i tried everything heres a question..the #1 cylinder would i be able to put that to TDC and manually set the timing from the belt??? than take it out on the lake and twist the distribtor to get a rough estimate on the timing???? THe timing is off so what i'm going to do is put the 1# cylinder to TDC and change the belt and the spark to match it..i know this is a crude method but would it work????

 

[Laddies]

Re: 1975 Merc 500 50hp

Yes, you need to see if if belt is on correctly, remove the plate on top of the mag/dist. and you will find a small arrow cast in the wheel, with the engine at TDC the arrow should be pointing directly at the center line of the crank. Then with the engine running at WOT the max spark advance should be set at 35 degrees with a timing lite

 

[gustafsonerik]

Re: 1975 Merc 500 50hp

not getting fuel...the numbers 1 and 4 cylinder aren't getting any fuel at all??? why the carbs must be pumping fuel to 2 and 3 and theres different carbs??? what could this be please advise..rem the engine hasn't run in 10 years but i rebuild and cleaned the carbs whats up???

 

[gustafsonerik]

Re: 1975 Merc 500 50hp

could this be my reed valves sticking?????

 

[steelespike]

Re: 1975 Merc 500 50hp

I'm not an expert on the intake design but the manual does say there are 8 leaves per reed valve assembly.It is possible there are 4 leaves per cylinder but nothing is said about this. It says the unit supplies fuel to the 2 adjoining cylinders.reed valve replacement requires removal of the crank.
Have you tried priming each cylinder at the plug and starting it?Try it with one and 4 only.