Yamaha 30DMH - Runs Rich

[Yam-4.2Explorer]

New to this forum so apologies if this is posted in the wrong area.

Have a 30DMH (3cylinder) and generally very happy with it however when running at low speed (ie *****ing) it will load up one or two of the cylinders and will not clear again until held wide open for 20 - 30 seconds.
Carby's are balanced and fuel mix is at 100:1 as recommended.

Have added extra idle adjustment screws to carby's 1 & 3 to ensure idle setting is the same for all three.
Air screws adjusted for idle/slow running.
Its fine for a few minutes and starts loading up.
Any suggestions or has there been a jetting change I need to know about?
Many thanks,

Yam-4.2Explorer

[boats.net]

Check fuel pump diaphrams to see if has hole in them, check fire on all cylinders with timing light when loading up.

[Yam-4.2Explorer]

Check fuel pump diaphrams to see if has hole in them, check fire on all cylinders with timing light when loading up.

Thank you for responding promptly.
Have had the motor from new and it has allways behaved the same.
Starts easy, idles fine (after brief warm up) but when driving along in gear ie *****ing it loads up.
Clears again when held wide open for 20-30 seconds.
Have been suspicous re jetting on the slow speed circuit as it occurs under load.
Float levels all checked out OK

[99yam40]

Have added extra idle adjustment screws to carby's 1 & 3 to ensure idle setting is the same for all three.

Not sure what this means, i

If you have modified the carbs by adding something then you probably need new carbs

Not sure about your motor, but on some motors there are different size jets in the different carbs and if you get then mixed up there will be a problem.
I have found the jets in my second hand motor had been drilled out, once replace with new it solved my high speed miss.

Like he said check fuel pump for leak

[almetelo]

Why re-drill carb jets???
I heard some guys do this...
What will improve (or not)?

[99yam40]

Drilling out jets messes up the fuel/air mix that the motors were designed to run.
Some people think they know more than the engineers that designed the motors and do strange things. That is my guess anyway

[Yam-4.2Explorer]

Carbys are as supplied OEM
The standard setup has one idle (not refering to the air/mixture screw) setting screw on the middle carby.
This did not compensate for slack in the linkages and noticed the other (top and bottom) carbys would shut off completely.
Fitted similar idle stop screw to the top and bottom carby as is standard on the middle carb to get a consistant throttle position on idle.
Runs just fine on idle, only an issue on slow *****.
Note no jetting changes.
All runs well when under power/mid throttle etc
It's allways been an issue when on the ***** for a few minutes.
Hence asking if there has/is optional jetting.
Note also, I have owned this from new and its never been apart.
Many thanks,

Yam-4.2Explorer

[99yam40]

Do you have the Yamaha service manual for this motor?
Did you follow the link and sync procedure in that manual to set up and adjust everything?
They do not need to have other screws added to function properly.

They will all close and open at the same time if adjusted properly

If they have never been apart, then maybe they need to come apart and get clean properly and then readjusted like manual states

[Yam-4.2Explorer]

Thanks for your input.
They have been synchronised as per manual and open in unison.
I added the extra idle adjustment screws for finer adjustment of each at idle.
Looking dow the carbs they all lift off together.
Issue is in the 1/8 - 1/4 throttle regios where its run when *****ing.