backfires under load mercruiser 3.0

[Mike Spijkerman]

hi new to this forum but i have a problem. my 3.0 merc backfires under load, in neutral it runs fine but higher then 1200 rpm it backfires and runs terrible. I changed the plugs, sets the timing, new fuel filter, new cable between the ign and the coil. but no getting better.
Please help, I don't now what else to do

Mike from the Netherlands Bayliner 175 br 2005 3.0 mercr.

 

[Bt Doctur]

firing order correct 1,3,4,2 ,sparkplugs in good condition, backfiring out the carb can also be a bad intake valve

 

[Mike Spijkerman]

igntion timing ok. new plugs, and in idle it runs great

 

[LostAnchor]

Back fire through the carb can also be a worn cam lobe. Pull the valve cover mark the push rods with a marker. Spin the engine over with coil wire removed if the push rod dont spin you've got a worn lob.