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?

On Mon, 06 Dec 2004 21:14:42 -0500, 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?

Low speed jets - either adjustment or dirty.

Get some Gumout Carb Cleaner, run the engine for a couple of minutes
to warm it up, the spray the hell out of the carbs at a mid-range
throttle setting (that's just to keep the engine running - no other
reason).

Do that a couple of times and see what happens.

Other than that, you need to adjust the low speed jets - pretty easy
to do.

In My Most Humble And Strictly Amateur Opinion that is.

Later,

Tom

On Tue, 07 Dec 2004 02:24:54 GMT, Short Wave Sportfishing
wrote:

Low speed jets - either adjustment or dirty.

====================================

That was my first thought also. How does spraying Gum Out into the
carburetor throat clean the jets?

On Mon, 06 Dec 2004 21:36:32 -0500, Wayne.B
wrote:

On Tue, 07 Dec 2004 02:24:54 GMT, Short Wave Sportfishing
wrote:

Low speed jets - either adjustment or dirty.

====================================

That was my first thought also. How does spraying Gum Out into the
carburetor throat clean the jets?

Probably won't, but just in case there is some varnish buildup in
throat. I had a similar problem with a Johnson once - rude, crude and
purely a guess, but it worked and I honestly didn't need to touch the
low speed jets.

Those might be factory set though now that I think about it.

Oh well, try the carb cleaner anyway.

One NEVER EVER knows. :)

Later,

Tom

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

"Short Wave Sportfishing" wrote in message
...
On Mon, 06 Dec 2004 21:14:42 -0500, 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?

Low speed jets - either adjustment or dirty.

Get some Gumout Carb Cleaner, run the engine for a couple of minutes
to warm it up, the spray the hell out of the carbs at a mid-range
throttle setting (that's just to keep the engine running - no other
reason).

Do that a couple of times and see what happens.

Other than that, you need to adjust the low speed jets - pretty easy
to do.

In My Most Humble And Strictly Amateur Opinion that is.

Later,

Tom

Check for air leak around the carb. Take some spray carb cleaner and spray
around the base of the carb and see if speed changes.

Is it lean popping at all? Is it sneezing at you, like it has a bad
cold?

You could have a host of problems here, especially if the engine is 15
years old and you don't know the history of it. I'd be curious to know
if you solved the problem. Often a 2 stroke engine will run just fine
with a host of problems at mid or full throttle. But when you slow down
to idle, that's when they show up. And that's the most important time,
as that is when you dock.

There is an entire diagnostic procedure for these little Yamie's. And
the best place to get one is, you guessed it, your service manual. By
all means, try the "gumout" treatment. But I doubt it will solve the
problem. But the nice thing is, these little Yamaha's are easy to work
on with the right tools and a service manual.

Decarbonize the engine first, before trying all else.

wrote in message
oups.com...
Is it lean popping at all? Is it sneezing at you, like it has a bad
cold?

You could have a host of problems here, especially if the engine is 15
years old and you don't know the history of it. I'd be curious to know
if you solved the problem. Often a 2 stroke engine will run just fine
with a host of problems at mid or full throttle. But when you slow down
to idle, that's when they show up. And that's the most important time,
as that is when you dock.

There is an entire diagnostic procedure for these little Yamie's. And
the best place to get one is, you guessed it, your service manual. By
all means, try the "gumout" treatment. But I doubt it will solve the
problem. But the nice thing is, these little Yamaha's are easy to work
on with the right tools and a service manual.