Maybe overheating. The exhaust manifolds are not a sure test. Poor
running from overheating is because the head and block around the
combustion chamber get too hot and cause detonation. Does it run on,
diesel, when you shut it down while running bad? What does the temp
gauge say? When did you last replace the fresh water pump impeller?

Make sure the timing is correct. If it is, one thing to try is setting
the initial timing to less advance. It could be developing detonation
after it gets hot. Are you using premium gas? These older boat
engines sometimes need premium.

It could also be vapor lock. Ethanol contributes to vapor lock. Make
sure the gas lines are routed away from direct contact with the engine.
A 1/2" fiber carb spacer often helps with vapor lock.

paul78 wrote:
Hello there,

I have a question about a problem with my 10 year old Mercruiser 5.7
litre (Carb) motor.

When using the boat for the first 30 minutes or so it runs
fantastically smooth - perfect. After that it starts to backfire and
soon will only drive (roughly) at idle speed. If you turn the engine
off and give it a rest for awhile, it will run fine for a bit when its
turned back on (it seems to run better for longer if the engine has
been off longer).

Took the boat out yesterday and when the problem started up I tried
opening up the fuel cap to see if a vacuum was building up in the tank
due to a blocked vent but it wasn't the problem. I also took the fuel
filter off and poured the petrol inside into pot to see what was in
there - there was a small amount of crud but definitely no water.

I don't think the engine is overheating as I can always put my hands on
the exhaust elbows without finding it too hot. Also, all the filters,
ignition leads, distributor and rotor have been replaced recently.

It appears to me to be some kind of fuel starvation but you folks might
know better. Can any of you help?

Thanks in advance,
Paul.