My son has a circa 1985 Johnson 70 HP two-stroke outboard motor. It ran well
until recently when it began "bogging down" when the throttle/shifter is
pushed full forward. Now, I'm no boating expert, so anything I say comes
from the viewpoint of a mechanic in another field and is subject to scrutiny
from those knowledgeable on power boating.

This is a 3 cylinder in-line outboard that my son got second-hand, so
there's no manuals to which we can refer. We did notice that when either of
the "lower" two plug wires were pulled the motor almost stalls but pulling
the "top" plug wire did nothing. So, we tried switching the coil paks for
the top two cylinders to see if the problem would "move" but that did
nothing to solve the problem.

Next we pulled the top carb and took it apart. I noticed the float bowl was
dry and the float needle was stuck closed. I was elated thinking this was
the solution and disassembled the float to clean the fuel varnish from the
needle and seat. I should mention that he confessed he hadn't sprayed some
sort of protective solution into the carbs when he stored it for the winter
and that that was probably the reason. Since the other two cylinders weren't
giving any apparent problems we didn't screw with them, we just re-assembled
the motor and took it out for a test. It ran ok for a few "starts" but then
returned to the same "bogging" problem when goosed.

He's played with the remote throttle adjustors with a little success, but
the problem still remains. He's also emptied the fuel tanks and added fresh
fuel.

There doesn't seem to be a way to adjust the fuel/air mixture on these
carbs, so we haven't been able to screw *that* up in any way, as far as I
know. The last thing we tried was removing the lower two carbs to
disassemble and clean them, but the problem persists. The linkage that
synchronizes the throttle to the carbs is pretty solid so we don't think we
screwed that up.

Also, the plate that I assume holds the pick-up coils for the magneto (or
which trigger an electronic ignition) seems to advance all the way as soon
as the throttle is moved up a little. That may be normal, I'm not sure, but
we don't know how to adjust it properly. I've looked for shorts in the
ignition system as far as I'm able without major disassembly and everything
looks ok. My next move was to pull off the flywheel which I assume must
contain a magnet(s) to trigger some sort of induction coils on that plate I
mentioned to see if there was something amiss under there, but we didn't
have anything big enough in my little service tool kit for that.

It seems to be one of those problems where almost anything you do "cures" it
for a short while and the motor runs beautifully. Then at some point, when
you go to take off the darn thing bogs down and almost stalls. He can get
going by exercising the throttle/shifter lever rapidly a few times and then
"babying" it into high speed. Once it gets going it runs fine at high speed.

If anyone has any suggestions I'd be very open to them, or perhaps a link to
somewhere with instructions for setting the throttle cables, fuel mixture
and/or timing. I've looked without any success. Sorry about the length of
this post but I wanted to include as much pertinent info as possible. If
I've left needed info out let me know and I'll try to get it.