50 HP Mariner dying when in gear

I have a mid 1980's Mariner 50 HP outboard that my dealer says "runs like a top" (I'm thinking about getting a new dealer).. It starts and runs fine until I put it in gear, then about every 100 yards (If I'm on a no wake lake, which is where I fish the most), it dies. It acts like it is running out of fuel. The bulb on the fuel line remains hard & it re-starts just fine. and then dies again.. I replaced the old metal fuel tank (no problems inside it, looked new on the insides), with a new plastic tank, put new non-ethanol gas in it, as well as enough oil (supplied by my dealer) to get a 50:1 mix. Still have the same problem. Any ideas? I don't want to tear the carb apart if I don't have to..
Thanks

Re: 50 HP Mariner dying when in gear

did you rebuild the fuel pump they need a rebuild every now and again.

Re: 50 HP Mariner dying when in gear

I have not done anything to it. I just got the boat & took it to my dealer to have him check eveything over & make sure everything was working properly. He was also instructed to fix anything on the boat that needed it. When I picked it up, he went on for several minutes about how well the motor runs & that for it's age it is in excellent shape. Silly me, I thought that meant that he did what I asked him to and actually TESTED the motor... The fuel pumps on these (unless they've changed since the last Merc I worked on & since the Mariner I have says it was made in Fon du Lac, I assume it is a Merc in disguise), are pretty much just a diafragm. I'll check it later to make sure that it is good. The Service manual I bought for the motor says that the old black diafragm needs to be replaced with a new grey one (I assume it's an ethanol resistant material). If the black one is there, I'll know the guy didn't do anything other than take some money...