custom tuning on 1978 85 hp

[monte100727]

I have a 1978 johnson 85 hp, did not run when I got it with a boat for cheap,compression was 112-118, so I insalled new water pump,and rebuilt carbs and with new plugs she was running good,installed new fuel pump,all four coil, one power pack,and the rectifier,motor ran great top speed on the 1985 cajun was 41 gps light load 38 heavy but with four batteries in the back it was a little slow out the hole,not having much in this motor I found a complete hump back exhuast with tuner and intake fillers that I installed with new gaskets total $100, now the issue is the carbs, as this motor is only 3.375 bore it will not move as much air as the 3.500 motors so I need help,I got it up and running pretty good with a 61 jet and enlarging the vinturies from 1" to approx 1 1/4" by hand grindding,first time out today, she had improved hole shot and it felt fast on top end but I forgot gps in garage so I really dont know,but she would start coming out of plane below 3300 rpm so I guessing to much carb for half throttle,I am going to get a set of possibly 1 3/16" carb,any thoughts,thanks.

 

[tashasdaddy]

Re: custom tuning on 1978 85 hp

four batteries in the back , simple you are tail heavy, distribute your load. or put smart tabs on it.

 

[monte100727]

Re: custom tuning on 1978 85 hp

well I had her out today with gps, and raised motor 1 1/2", she ran 41 mph gps with heavy load,and raising motor seems to helped hole shot even more,she will rev to 6000 rpm so may up prop a size,it would hold plane till 3300 rpm and 18mph anything below 3300rpm and the power starts to drop off and come out of plane.motor hight seemed perfict as I had no ventilation and plate was just on top of water, this motor sound so much happier at idle and cruising but she smokes a little more at idle than I ever noticed before, Im going to check plugs today to see if she running rich.still wondering on carbs for this set-up if any one has any thoughts.thanks

 

[emdsapmgr]

Re: custom tuning on 1978 85 hp

I had a 72 100 hp which was also a small bore crossflow. I put a set of 125 carbs on it-which should be the 1 5/16" ones. Spend a day or two rejetting, but the engine ran great, I picked up about 200 rpm on top and seemed better at midrange. Normally when you go to large carbs, on a small bore, you will see a decrease in holehot, but I never felt it was poor out of the hole at all. The 125 carbs happened to have the adjustable idle screws, so the idle setup was really quick and easy to tune.