1979 mercury 70 thunderbolt

[bigjayhoffman]

hello all
last year I bought a '73 Rinker with a 1979 70 horse merc on the back and it runs well. except when I'm running it hard for a while, towing a tuber for example. then it doesn't seem to want to idle. it dies off... it starts not proplem but I have to use the fast idle to keep it idling. any ideas.. I replaced the spark plugs last year and it didn't make a difference.
thanks a bunch
BJH

 

[Windykid]

Re: 1979 mercury 70 thunderbolt

Have you de-carbed it yet?

 

[Laddies]

Re: 1979 mercury 70 thunderbolt

If when it cools off it idles OK again, you have a heat problem that could be water pump/thermostat/popit valve or the timing it to far advanced

 

[bigjayhoffman]

Re: 1979 mercury 70 thunderbolt

If when it cools off it idles OK again, you have a heat problem that could be water pump/thermostat/popit valve or the timing it to far advanced

this is exactly the issue I'm having. the pump seems to work fine (well it pumps water, but I'm gonna take to the local Merc dealer and see what they say about it.
oh and I havent de carbed it yet..
thanks a lot for info folks

BigJayhoffman

 

[Faztbullet]

Re: 1979 mercury 70 thunderbolt

There is a old service note on this. Engine uses a water cooled fuel pump and had vapor locking problems. Try a new impeller 1st and if this does not fix it drill relief hole in fuel pump check valve.