'78 70hp Johnson back to life

[Topol_86]

Finally got my boat out on the water, while heading out of the bay, gave WOT, took about 5 minutes to put my boat on plane, finally planed out, so I figured I`d go for a test, staying close to home so to speak. Went up river, got choppy, so I headed downwind. On my down-river run, out of nowhere the motor kicked up and ran proper. Now this motor hadn`t been run in 6 or seven years. Rebuilt the carbs, replaced ALL fuel line (before and inside the motor) new spark plugs, new lower unit oil etc. Just wondering if maybe there had been carbon build up, or if the motor just had to "warm up" and come back to life. spent another hour running all possible rpms, idle to mid-range, idle to full-open, full to idle (minding wash-over from the wake into the boat) I'm overjoyed with the performance of the motor, just wondering if anyone can explain exactly what happened, as I'm curious. Thanks in advance.

 

[jonesg]

Re: '78 70hp Johnson back to life

Maybe the ignition advance plate was sticking, could have been anything.
If it acts up again run it without the hood so you can watch whats happening , could be ignition linkage.

 

[jtexas]

Re: '78 70hp Johnson back to life

I can tell you for sure, it'll run on two cylinders but not one. I've never known it to plane the boat while running on two, but I never gave it 5 minutes to try so I guess it's possible. And it won't take that long to warm up. Could it be a fuel quality issue?

Inspect the powerhead wiring, look for burned/chafed/missing insulation, where maybe a wire coming off the timer base can intermittently contact or arc to the block.

Check the choke plates, make sure you don't have a loose one.

Those plastic fittings, that hold the linkage rods to the carbs, and the timer base to the advance lever, see if any are broke or missing -- that'll give you strange symptoms for sure.

 

[Topol_86]

Re: '78 70hp Johnson back to life

Everything under the hood is CLEAN, and in excellent condition. Have had the boat out at least a dozen times since and have not experienced the issue once since the original run. Timer advance base moves freely and all plastic connectors are in great shape, no cracks, breaks etc. Also all wiring is still in excellent condition. Thanks for the info. Seems to have been a one time thing.