Wayne.B wrote:
I have a Yamaha 15 hp that is ten to fifteen years old. It was on the
inflatable dinghy of the trawler I recently acquired. The engine will
start in neutral using about half choke and it will rev up quite
nicely. When you throttle back however it will stall immediately
unless you can catch it with the choke and jockey back and forth.

By carefully playing choke vs throttle you can get it slow enough to
put in gear and it seems to develop decent power at higher revs
without any choke. It still won't idle back to lower speeds however.

Any ideas?

Blocked idle circuit????

Probably best to take the carb etc off & clean it all right out
etc............ or :-) to just get at the idle circuit:-)

(i) Take the mixing chamber plate off the top of the carb (2 screws) You
should now be looking down at a machined "S" shaped channel.

(ii) The idle jet is the one closest to the idle mixture screw (which is
at the back closest to the engine), take it out so you can clean the jet
itself.

(iii) Turn the idle mixture screw in till it lightly seats counting the
turns (probably about 1 1/2) so when you put it back in you have a known
starting point to tune from.

(iv) Clean the idle jet & "lightly" blow the channels etc with air.

(v) Don't use aggressive carb cleaning stuff it removes the casting's
coatings.

K