Johnson 70 hp spark test question

I am troubleshooting my 1978 Johnson 70 hp. It has been bogging so I am going through everything and decided to do a spark test. Went to autozone grabbed a spark gap tester and tried it out...nothing on all three cylinders, yet the motor starts and runs albeit with difficulty. What gives? I grounded the tester and set gap distance to 1/2" fired over the motor and got no spark whatsoever on all three plug wires. Bad tester? Bad power pack? Would engine run with a bad power pack?

I can start the motor fine, idle it fine but when I give it throttle it likes to bog down, however if I rev it up a little in neutral then shift into drive it will run wide open just fine, if I idle a little more then give gas it bogs. This motor is driving me crazy. :shock:

Don't worry about the spark as it is obvious that you have spark. I think it is time to look at the carb(s). Either the jets are clogged( or at least partially) or the adjustments are off.
The carb(s) could also be dirty.

I cleaned carbs...did NOT rebuild. I was originally thinking that the float needle is sticking in one of them. I think I need a carb rebuild. I checked to make sure that all butterfly valves open and close in sync with each other, there aren't any carb adjustments I can make with these older carbs, just jets and float.

Its kinda weird that I can rev engine in neutral just fine (great throttle response), then put in forward quickly and it wont bog down actually ran 36 mph for about 5 minutes or so, then after putting around a little bit in idle it will bog when I give it throttle trying to go fast.

I had a similar problem - AFTER I rebuilt the carb and did other stuff I finally discovered I had a fuel problem - gas line from tank to engine was either cracked or a poor fit (never did find out - purchased new tank and lines and all was fine!)

Same thing you have - revs up and occasionally works great but under load would usually start to rev up and then just and die to idle speed

i had this exact same problem with a 77 70 hp sea horse. cleaned the carbs... no fix, rebuilt the carbs still no fix..., had the carbs professionally synced and still no fix. ended up running fine after i rebuilt the fuel pump and replaced the line and ball from the tank. not sure which did the trick but my guess is fuell pump since i used the old line for my smaller hp motors and am still using it to this day. good luck. i hate those multi carb engines. that's why i'm sticking with my little tinny and 10 and under motors from now on... lol

oh and one more thing... drop the gap on your spark tester down to 1/4 of an inch and try that. 1/2 is a bit much... i've never had spark related issues with an ignition system that would jump a quarter inch gap. check each carefully and make sure you dont have one cylinder not firing. also look closely at your spark plugs after running/attempting to run and see if one is dry, this could mean a clogged jet or stuck float and no fuel to that cylinder. running on two cylinders will also give those symptoms... i found that out the hard way as well. you would be surprised how smooth it will sound running on two. again, good luck

I had a real similar problem and was getting ready to rebuild the fuel pump but figured I'd try with a new fuel line first and bingo that solved it for me I'd try that first now whether mine was the bulb or the line doesn't matter but is fixed