Runs Well, Can’t Reach Top Speed??

[Shaner41]

Hello all! Thanks in advance for any comments.
I have a 1981 Yamaha 9.9 outboard. I recently put a new carburetor on it. It runs very well and idles perfectly. It also runs fine when revved up in neutral, and it runs fine in drive when I’m out on the lake. However, I can only reach a top speed of 6mph (just over 5 knots). It sounds fine when I have the throttle opened up, but it seems like it just isn’t reaching top RPM when it’s in drive (forward). It just seems like I need to give it more gas, so to speak, but everything is wide open.

I did adjust the carburetor linkages and related items so that the tiller handle “slow” and “fast” are where they should be.

Thanks again!

 

[brodmann]

Why was the carburetor replaced? Did it have the problem before the carb replacement? Are you 100% sure it's the correct carburetor? At full throttle, are the butterflies completely horizontal?

 

[Shaner41]

Why was the carburetor replaced? Did it have the problem before the carb replacement? Are you 100% sure it's the correct carburetor? At full throttle, are the butterflies completely horizontal?

Thanks for the response. A certified mechanic told me it was the carb after he kindly looked at it and did the tests (compression, spark, etc.). It was running terribly before the new carb. Now it’s running perfectly. And yes, at full throttle the butterflies are horizontal - wide open. The new carb matched the old one exactly. A friend suggested the float of the new carb might be off. Thanks again.

 

[Scott Danforth]

Still don't understand why a mechanic would suggest a new carb over a $20 carb kit

Take it back the the mechanic and have him make it right since he didn't finish the job

 

[99yam40]

has the motor ever pushed the boat like you want it too?
maybe it is just too small of a motor, or it is propped wrong for the load it is trying to push.

 

[flashback]

Still don't understand why a mechanic would suggest a new carb over a $20 carb kit

Take it back the the mechanic and have him make it right since he didn't finish the job

Key word here is "mechanic" I think he's a salesman!

 

[Shaner41]

Key word here is "mechanic" I think he's a salesman!

Ha! The old carb had been JB welded in places by a previous owner. The mechanic was going to purchase parts for the carb, but I said I’d try a cheap new carb first because that was quite a bit cheaper.

 

[Shaner41]

has the motor ever pushed the boat like you want it too?
maybe it is just too small of a motor, or it is propped wrong for the load it is trying to push.

Good question, thanks. Last year before the carb crapped out it did go faster, around 15-17 miles per hour.

 

[Scott Danforth]

Was the new carb an Amazon china special?

 

[Shaner41]

Yes. Gave a cheap carb a shot.

 

[Scott Danforth]

Get an OEM carb. The internal passages in the chineseum carb are most likely not drilled correctly

 

[jimmbo]

I did adjust the carburetor linkages and related items so that the tiller handle “slow” and “fast” are where they should be.

What exactly did you "adjust", and Why?
Did you do a Link and Sync, to assure that the Carb is properly Synced to the Timing?

 

[Shaner41]

Get an OEM carb. The internal passages in the chineseum carb are most likely not drilled correctly

That could be, but it was acting the same way when I was using the old OEM carb.

 

[Shaner41]

What exactly did you "adjust", and Why?
Did you do a Link and Sync, to assure that the Carb is properly Synced to the Timing?

Yes, that's what I did - link and sync. When the carb is wide open, the timing is also advanced. I messed with that a bit with muffs on and when it was in neutral - got it just right.

 

[99yam40]

did you check with a timing light to make sure idle and WOT timing are both in spec?

 

[jimmbo]

did you check with a timing light to make sure idle and WOT timing are both in spec?

There is no Idle Timing Spec

 

[99yam40]

never heard of a motor without an idle timing spec.
motors need to have timing set for start and idling and timing advanced to a set timing for them to run properly at higher RPM

 

[jimmbo]

never heard of a motor without an idle timing spec.
motors need to have timing set for start and idling and timing advanced to a set timing for them to run properly at higher RPM

Then you don't know much about 2 stroke Outboards

 

[99yam40]

then what would you call this?
seems to me with the throttle at idle the timing is full retarded
and the timing advances to full advanced as you open up the throttle.

But this is on my C40 not the OPs motor

 

[jimmbo]

On pretty much every 2 Stroke I have ever seen, there is no Idle Timing spec, as the timing is retarded to determine the Idle Speed, and that will vary depending on the Load on the engine(prop Pitch). The Carb(s) remain fully closed, and are not touched to set Idle Speed.