You need to buy a manual for your motor a good FACTORY one. Then you need to look at your fuel system. The only way that 3 and 4 can foul out at an idle is if the accelerator pumps are leaking. Because on you 2+2 motor the bottom carbs have no idle circuits just a small hole to provide enough lube for the cylinder.. It is not uncommon for the accelerator pumps to fail on those. BS on the low speed operation I trolled my 125 Merc all day one day and skiid the next and never had a problem. I changed my plugs pnce in 10 yrs. I also never had to tilt my MERC 125 to get it to run right either. Do a compression test before anything and buy a manual. You should not have to do anything special to get your motor to come out of the hole correctly.
Great. I guess I'm chasing butterflies but it works for me and that's what's important. My compressions are 3 at 129 and 1 at 130 done at the dealer where I bought the used boat. He marked the numbers on the water jacket.
I have the 2015 revision of the OEM manual published back in 1998. Some things have changed from the previous OEM manual. It was not a revision. Accelerator pump, and fuel pump have new kits. Fuel filter and secondary filters are new. Carbs are clean and lines aren't degrading.
Acceleration kicks in around 1800-2000 like the manual says it is supposed to. Plugs are gapped to 0.040 as stated in the manual. Fuel is 91 gasoline and oil it Pennzoil Premium TC-W3 semi-senthetic, engine oiler works. Engine doesn't smoke. Acceleration is more than needed to plane out a 17 ½' Crestliner Fish Hawk in a couple of seconds. Running a square port 20P Laser II. Boat ran 47 early in the year when it was cool at rev limit of 5250 and has dropped a couple of k with the water temp at 81 and air at 95. The engine is 18 years old.
If the engine is idling in gear and is moving back and forth, with the trim sucked in and all you do is to raise the rear and it smoothes out like my towers used to idle, There IS something to tilting and as I said before, Mercury designed that into some of their fishing engines....Mark 10A, 15A and 28A, 1959 to name 3.