I have recently aqcuired a 70 HP Johnson and the previous owner says that boat will do 30-35 mph with same prop and gear. The boat sat for a year without running but when I got the boat it started up first time. I was really happy with the easy starting and ran it on the trailer. It runs up top 3-4000 rpm with no load. Sounds good like its hittin on all 3. When you put the boat in the water it only gets up to about 1500-2000 rpm. Sounds fine but no revs. Boat only does about 10 mph. I checked throttle linkage and it opens butterflies wide open. Choke is closed and not stuck. Checked for blockages in fuel line, sqeezed fuel bulb, no difference.What to do? where should I look next. I though about timing or water in gas. How to check this?The prop is the same prop that was doing 30 mph so I'm sure its not over-propped.Water pressure OK, temp seems ok. When you rev the engine at idle it screams, but in gear its a slug..
Krave, Go back to 5/10 and check my note. I didn't have a repair manual but have one en-route. I want to sell the boat as I have a new Triton SF-19 with a 200 OPTIMAX due in at the dealers by the end of this week or early next week. By the way, the dealer told me that Merc went to the 250 cu inch block for the 200 Optimax. I fired a note to Merc and got back the complete list of engines with the 185 cu.inch highlighted. Guess the dealer was in error. Anyone know different? Uncle Fred
Hi, Krave. You are probably running on 2 cylinders. How it sounds and revs without a load means nothing. You may be missing spark on one, but it is more likely to be a gummed up carb.Check each cylinder for power by shorting/disconnecting each plug in turn. If there is not a drop in rpm that cylinder is not making power. If you have spark on that one overhaul that carb.Good luck.
Thats good info JB. I think you are on to it. I'm going to go home tonight and try shorting out the wires on each. I had also planned on taking the bowl off the carbs and wash them out, look for water, floats etc.Anything special to watch out for?Also is there a jet adjustment on each carb? I dont see one like on a convention engine.I was also looking at the filter where it connects to the engine. Strange looking device. Has like reed valves in it or something. Probably eliminates pulsations in the line?The filter itself consisted on a wire mesh, that was it. Should there be more in there? Also can anything go wrong with the reed assembly.This is a great web site. I've got more help in a matter of hours..We must love our boats, we work all day at our regular job then we go home and dink around till dark so we can be back out fishing ( or beer drinking ) by the weekend.:-)DL
Sounds like the exact problem with my 78 70hp Johno but I found whilst running mine at home the top cycl wasnt firing so I tapped the coil plate and it seemed to break into 3 cycl, I am not sure but I think it may have been causes by a bad earth with this plate, so I took the rubber boots underneath the plate off and bolted driectly to the motor.
So far so good it runs on all 3 and it VERY responsive. Maybe this plate is a common fault in all the 70hp ?? as it made a HUGE difference to my motor.