Re: Johnson 70hp 1978 model have bogging down problem
This is a common problem on an otherwise outstanding engine. The idle circuit on these loop-charged engines is very lean, and any slight disruption to fuel flow can cause this "lean bog" you are experiencing on acceleration. Mine is a 1976 Evinrude 70, and I about went nuts trying to correct the same problem. Here are a couple of things related to the carbs, even after a re-build:
1. If I remember correctly there are 3 very tiny fuel passages for the idle circuit of each carb. These are the tiny holes you see in the throat of the carb just behind the throttle plate. These ports need to be perfectly clean all the way through. Even after a "cleaning" during a rebuild, there can be deposits left in these ports. It's not so much a blockage, as a slight restriction by having slight deposits remain. It's worth it to have the carb bodies professionally boiled out to be sure all deposits are removed.
2. It's common after many years of use for the throttle plate shafts to wear a slight oblong shape into the carb body where the shaft passes through. This allows a slight amount of air to pass between the shaft and the carb body, and will slightly lean the mixture particularly at low speads. These particular motors are very sensitive to this, and it doesn't take much wear in this area to begin to cause a problem. Short of trying to find replacement carbs that aren't worn, a "fix" for this is to pack a small amount of heavy grease around the outside areas where the throttle shaft enters the carb body. OMC (now BRP) Triple Guard Grease is perfect for this as it's thick and tacky. Don't make a mess of things, you need just a thick film around the crevace area where the shaft enters the body of the carb. Do this only on the outside of the carb and do both sides, as the shaft exits on the opposite side of the carb body. The spring on the shaft isn't a problem, just pack a little grease at the edge where the shaft, spring, and carb body all meet. It's amazing how much this can help, and it lasts. I clean and re-apply the grease only about every other year on mine. It's quick, easy, and it DOES work if the engine is drawing air through here, as the grease seals this small air leak.
3. After doing some research I've concluded that OMC didn't have the idle jetting quite right on the earlier models of these engines. I don't have the jet numbers in front of me, but I replaced the low speed jets in mine with new ones that are 2 sizes larger. This completely solved my lean bog problem, and the engine still idles fine. Not quite as "clean" an idle as before, but still fine. Now, mine is a 1976 model. In 1977 OMC made a change and they started coming from the factory with an idle jet one step larger (richer) than my 1976 came with. This fact is what told me they recognized they had a problem. Your 1978 came with the same jets as the 1977 model, which means you already have idle jets that are 1 step richer than my originals were. By increasing 2 steps, my idle jets are now currently one step richer than yours. The jets are easy to experiment with, as they can be removed from the front of the carb without disassembly. Just be extremely careful not to allow the tiny low speed jet to drop in the carb throat when you change them. These jets are still readily available from a good Johnson/Evinrude dealer.
Installing the richer idle jets and keeping the throttle shaft entrance packed with a small amount of grease has completely solved my lean bog problem and it has remained solved for several years.
Good luck!
