Engine bogging at start of run

[9758JB]

I have a 1981 Johnson 70hp outboard. I bought it from a guy who had it sit for 2 years. And I wasnt expecting much. First time I had it out, it ran great. Next time it would bog after running for about a min, and wouldnt get to full speed. I ran a tank of gas and carb cleaner through engine, and seemed to bee doing good. Replaced feul lines and carb gaskets.

Now here's the important part

Took engine out, first min trying to get outta the hole it bogged like a dog. 7 mph. Then all a sudden, it took off. 32 mph, rpms prob around 5800, ran great for the day, could turn off and let it sit, fired up and ran without a problem. Figured my prob was fixed. Figured there was something in a jet that worked loose.

Day 2. The same thing. Engine bogged but after a min or two of running it just took off.

Any idea why this is. Or are these things just that cold blooded. Maybe air in my line at the beginning of each run. I prime it till its hard.. and ideas?

 

[oldboat1]

water in the fuel?

 

[9758JB]

I dont believe so. The fuel was one week old and I store it in my garage. And it only happened at start. I'd let engine sit for a hour or so and itll be just fine

 

[kbait]

‘Just took off’ sounds like ignition.. Clean/tighten all ignition (coils/CDI box) ground wires, new spark plugs w/proper gap, and give it a run. I would like to test all spark leads with an open-air spark tester when it’s acting up, to see if it’s a spark delivery issue.

 

[9758JB]

kbait, I'll do that and take it to the lake to see if it does anything. It's just odd that it only does it on the first start up of the day. The rest of the day it's fine with no sign of anything like that whatsoever

 

[9758JB]

Kbait, I dont think I can test whenit's happening. It's only for a minute .and only when under load

 

[oldboat1]

Might repurpose a timing gun. Put it somewhere visible (tape or use a plastic tie to hold down the trigger, if an antique -- like mine). No flash means no spark.

 

[Bosunsmate]

Good tips from Old Boat and Kbait. Those grounds for the powerpack/coils into the head/crankcase do get rusty and will cause this issue. Electrics are a mystery as to how they work intermittently but ive a 1981 60hp and cleaning these bolts/internal threads has worked for it before

 

[jimmbo]

Corrosion on connections can cause intermittent opens. A bit of voltage and/or vibration can reestablish the circuit for a while, then it's back to open. Not just visible connections, those Plug-in connectors can be very good at hiding poor contacts and corrosion