1976 Merc500 losing power when warm

[mrw28]

I have a 1976 Merc 500 - one previous owner (my father), well winterised between seasons and in fairly good shape.<br /><br />I've used it three times this season (pushing a 15foot runabout) and it runs really well - we're up on the plane in a hurry and it feels nice, then the power drops, and if I come off the plane I can't get back up again. I've been told it was showing the same symptoms in 2003 (it wasn't used in 2004), and if it was turned off for a few minutes and turned back on again full power would be restored for a little while. I haven't had the chance to try something similar.<br /><br />We've got a new impeller in, new plugs, and the pee stream is strong.<br /><br />So does anyone know what I can do to sort this out?<br /><br />Here's a pic of the engine for identification purposes:<br />

<br /><br />Thank you, Gubbins.

 

[Laddies]

Re: 1976 Merc500 losing power when warm

Ck the engine for spark when warm, ie sounds like you are losing a cyl--Bob

 

[mertthedog]

Re: 1976 Merc500 losing power when warm

Check fuel line bulb. Try squeezing it hard again while losing power if soft. Need new bulb? Then check & clean carbs, switchbox test and spark (Laddies post). See the posts on 8-21-05 on "cant get over 2000 rpms".

 

[joblo33]

Re: 1976 Merc500 losing power when warm

If squeezing the bulb helps the problem could be a fuel pump diaphragm.<br />Eric

 

[mrw28]

Re: 1976 Merc500 losing power when warm

Hi Laddies, thanks for the advice. I'll get a spark checker and give it a whirl (I'm not a technical guy so for each check there's going to be shopping for me).<br /><br />Mert, I don't think it's a fuel problem - I took her for a spin yesterday and had 10-minutes at high throttle with no problem. I'm linking to the thread you mention as there's all sorts of useful stuff there:<br />Topic: Merc 150 will not run above 2000 rpm <br /><br />If the spark check doesn't show a problem I'll look into my carbs as you suggest.<br /><br />Might not be able to report anything for a few days now - I've got a 30min drive to the boat and have to launch her each time to check anything so I need an extra pair of hands. I'll post here when I have news.<br /><br />Thanks for the advice so far, Gubbins.