How long can i run wide open on a '74 merc?

[merc_500]

Just wondering, i have a 15' fibreglass boat, not very heavy, with a 1974 mercury 500 50hp 4cylinder engine with a brand new 12 pitch mercury prop. <br /><br />the motor runs great and has good compression, the lower unit waterpump is rebuilt as well as drive shaft and prop shaft seals replaced. <br /><br />was wondering if its ok to run it wide open? currently i'm running it at about 3/4 when cruising afraid to blow something up if i have it wide open for a longer period of time...<br /><br />from 3/4 to wide open it dosnt just rev it does pick up a good amount of speed but just seems like the motor is really revving, maybe its supposed to, i'm not sure, i dont have a tach, but as i said i dont think its overrevving as the boat picks up a considerable amount of speed between 3/4 to wide open throttle. <br /><br />is it ok to run this old engine wide open for longer periods of time (say towing a water skieer) or should i stick to 3/4??? <br /><br />thanks- Neil

 

[rodbolt]

Re: How long can i run wide open on a '74 merc?

how many RPMs are ya turning at WOT? if your overspeeding it may die. if its turning in its RPM range it just depends on the maint done. at or near WOT there is no margin of error for half clogged carbs or worn out water pump parts. but if the fuel system and cooling system are in top shape it would depend on how long can ya keep gas in the tank.

 

[Scali]

Re: How long can i run wide open on a '74 merc?

also remember thats a cross flow engine with heavy pistons , I have a 1994 40 same engine basiclly, I do spurts of WOT almost every time out but only for maybe 30 seconds max then back off to maybe 4 grand sometimes a lot less.

 

[Laotzu]

Re: How long can i run wide open on a '74 merc?

I kind of do what Scali does. Generally don't run more than 4k. Do go up to 5k just to see how fast I can go but go right back down after a minute or so. Mine's a 91' 50hp. Probably be a good idea for you to install a tach just to make sure your rpm limiter is set right. I don't think it's a good idea to run all out for any length of time. Maybe that's why I lived to be this old.<br /><br />Jerry

 

[Clams Canino]

Re: How long can i run wide open on a '74 merc?

4k is a good cruise speed. Wide open sucks DOUBLE the gas and isn;t doing anyone any favors.<br /><br />-W

 

[SeaKaye12]

Re: How long can i run wide open on a '74 merc?

Rodbolt; could you explain further your comment regarding half-clogged carbs in relation to engine durability at WOT?<br /><br />Aside from limiting overall performance (and reducing WOT RPM's...) can partially clogged carbs cause lubrication deficiencies ? Is that what you mean?<br /><br />I never realized that.<br /><br />Thanks, Chuck

 

[heybaylor]

Re: How long can i run wide open on a '74 merc?

Im not rodbolt , but partial clogged carb/carbs ,can cause a lean condition ,,results can be diasterous..<br />they always clog in the jets/fuel bowl ..<br />darn things never clog on airflow ..<br />the fuel /oil mix stays the same ...but just not enough to sustain cooling and proper fuel/air mix..<br />thats at any rpm ,not just wot.<br />clean properly jetted carbs are the life of a two-stroke..along with waterpump<br /><br />lean runs hot to a point of melting pistons

 

[Nos4r2]

Re: How long can i run wide open on a '74 merc?

^^^^^ yep-that and as you suspected seakaye, not enough fuel means not enough oil.<br /><br />A few years ago a vespa/lambretta club went on a ride through Yorkshire,UK. During that ride they coasted down a 1 in 5 hill for nearly half a mile using the engine as a brake and the throttle shut. A full half of them seized before they got to the bottom.A Mate happened to be the recovery guy who collected them.