1989 Yamaha 9.9 4 stroke loses power after 30 mins then 5 mins later runs sweetly for hours

I am new to boating and this outboard is driving me mad. It will start up easily on auto choke and then idle smoothly after 2/3 minutes. After approx 30 minutes running it loses power but will idle and not cut out. There is a strong smell of petrol as if the plugs are flooded. When left to idle it will recover after 5/10 minutes and then run smoothly for hours. I had it serviced in April but the fault remains. The outboard engineer says he has no answer! It has done this repeatedly all summer after the engine has been started from cold. Is this a faulty choke being fooled into action, a faulty sensor or a faulty carb? Has anyone out there had the same problem and rectified it or can offer some advice?

bad fuel pump it sounds like

If the fuel pump is faulty why does it run well before and after the blip? Wouldn't it affect the performance on a regular basis?

Use a timing light to see what the ignition does when it acts up.--------Might be running on one when it acts up.

Thanks, will try that when next at the boat. Any suggestion why this might be the case?

Find out if it is an ignition issue or fuel issue first -------------Once you know that you can do further trouble shooting !!

The fuel should be the first thing you should check for. Clean out the carbs and get another gas tank with fresh gas to try it out. Your symptoms sound like a typical of water is in the fuel. At idle it sucks very little gas on the top of bowl. At higher rpms it sucks more gas in and possibly the water at the bottom of the bowl. GL.

Thanks. I have already replaced the fuel tank and fuel line with new ones and also introduced new fuel. I thought this would cure the problem but alas no.

Check your fuel line connections for air leaks. Does the ball pump up easily and hard? I have fought fuel line connections on multiple motors. It might be time to tear that carb apart and clean it.