1973 Merc500 thunderbolt 4cyl 50hp

[cbcrawford]

I am slowly getting my old mercury to run better, but I need a suggestion on what's next.<br /><br />The predominant symptom is an unreliable RPM -<br />Weight seems to affect it, but sometimes not. I normaly fish by myself, and 3 out of 4 times it will get up on plane at about 23mph (15' fiberglass boat) and up to about 4500 RPM's. It should rev higher.<br /><br />The other times it won't pass about 3000 RPMS and plows the water. Unfortunately, there is no pattern to the problem, engine warm or cold, weight distribution, etc . . . it just seems that sometimes when I hit it, it goes, and others not. It is usualy consistent 'per start', but not necessarily on the same day!<br /><br />I had the carbs cleaned, & plugs changed last season. I have changed out the feul pump diagphragm, changed the impeller, water pump, and lower unit oil.<br /><br />I was planning on taking it back to the shop now for a timing & carb adjust, but wanted to check in here first to see if anyone had another suggestion of "do-it-yourself" fixer uppers before I took it in!<br /><br />CC

 

[cbcrawford]

Re: 1973 Merc500 thunderbolt 4cyl 50hp

oops in title - meant 50HP, obviously.<br /><br />CC

 

[cbcrawford]

Re: 1973 Merc500 thunderbolt 4cyl 50hp

OK, now it looks stupid - just to explain, had 5hp in title, and have edited it to read 50hp, but after I put in the correction post about the title- Oh nevermind!!!! (LOL)<br /><br />CC

 

[cbcrawford]

Re: 1973 Merc500 thunderbolt 4cyl 50hp

anyone? trying to be patient, but fell off the first page . . . I'll take it in this coming week for the timing & carbs unless I hear anything . . .<br /><br />CC

 

[erniex]

Re: 1973 Merc500 thunderbolt 4cyl 50hp

Hi,<br /><br />May just need timing and sync to get it right:<br /><br />Try this link to good instructions.<br /><br /> http://www.iboats.com/cgi-bin/ubb/ultimatebb.cgi?ubb=get_topic;f=29;t=014508#000000

 

[MGuckin]

Re: 1973 Merc500 thunderbolt 4cyl 50hp

I would think that it may be more of a fuel problem as opposed to a ignition problem. If the link and sync problem was the cause it would be consistent unless something was loose in your linkage. I assume you already checked for anything loose. If you have dirt somewhere in your fuel system it may move around, sometimes work and sometimes not.<br />Did it function correctly upon last years service?

 

[cbcrawford]

Re: 1973 Merc500 thunderbolt 4cyl 50hp

Not sure what to check for (I'm basicly a newbie) as far as what's loose? - The control cables have a LOT of slack before they engage - but in my searches on this site I've seen lots of folks say that is normal for this engine (sure doesn't feel normal). I looked through my manual to see if there was a procedure to adjust the control cables, and couldn't find anything . . .<br /><br />It runs *much better* than it did before last years service (used to stall under load, etc) , but it has always been inconsistent (I've had it for just over a year now). Doesn't seem to be getting worse.<br /><br />Does have new feul pump diaphragm and tank, guess I'll change out the primer and line just to be sure there's free flow . . . .<br /><br />Thanks to all on this site - It (and you) are truly invaluable resources!<br /><br />CC

 

[hooksetter62]

Re: 1973 Merc500 thunderbolt 4cyl 50hp

had a similar problem one time.it turned out that my float in the fuel tank came off and was floating around and when under power it would suck up to the fuel inlet tube and stop or slow down the engine . had to remove the cork float as it would not float anymore anyway.now i just watch things very close,it is a 12 gal tank so that helps.