Mercruiser 5.7 backfiring

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.

"paul78" <paul.rockley@gmail.com> wrote in message

Eldon

paul78 wrote:

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:

Also, I think I used the word "backfiring" wrongly when I was writing the first message. It seems to be defined as when the fuel/air mixture ignites with either an inlet or exhaust valve open. I'm not sure if that is what my engine is doing; I reckon it is "spluttering" rather than "backfiring". Sorry for any confusion caused.

Would love to hear any more suggestions from any of you people out there...

Thanks and all the best, Paul.

jamesgangnc wrote: