Yanmar 3GM idles too high

[gregjames]

I've just installed a new Yanmar 3GM30 and find that it runs very rough at anything less than 1100rpm. It runs smoothly over 1100rpm. "Rough" means that it seems as though it will vibrate off its mounts. Prior to starting the fuel pump and injectors were professionally serviced so for now I'm assuming there are no fuel issues. The installation book says the idle speed should be 850rpm. Any suggestions as to what I could try ?

[SteveInMD]

The idle setting is a set screw on the throttle arm.

I would try reconsidering a problem with the fuel system. You have classic symptoms of an air leak. Start at the tank connections and check each one. Make sure your fuel filter is sealed well. Check all the bleed point bolts too see they have good crush washers and they are snug.

[GBurton]

Agree with Steve. Make sure you have enough fuel in the tank that the pickup tube is well under the level. Most tubes don't go to the bottom of the tank (they feed in from the top)

[SteveInMD]

If you open the final low pressure side bleed point (the one where the fuel enters the high pressure injection pump), and you activate the lift pump manually, do you get a clear strong flow of fuel, or is does it have any bubbles? Make sure you open the bolt until you can see the hole on the threaded part of the bolt. If you get any bubbles you have an air leak somewhere. Even if you don't I would still check over all connections. A small air leak can be hard to find when operating the lift pump by hand.

[gregjames]

Thanks all.
I revisited each connection, tightened it a little more, gave a couple of pumps on the manual pump lever to bleed at the fuel pump, and hit the start button. After a minute or so I tried to adjust the idle down and managed to get it running quite well at 900rpm. So it appears the problem is resolved and you correctly predicted an air leak.
Thanks again.