3GM30F runs rough when starting

Jan 5, 2017
120
Hunter 356 SF Bay / Delta
My Yanmar is running very rough for about 5 seconds after starting then smooths out:

It started doing this immediately after I replaced throttle and shift cables, replaced coolant hoses, replaced fresh water pump, and removed injection pump access plate to clean/lubricate the throttle lever. I did not adjust the throttle stop screw.

I took it out for a motor cruise last weekend for about 4 hours and it ran normally during that time at about 2800rpm.
 
Oct 22, 2014
16,409
CAL 35 Cruiser moored EVERETT WA
Perhaps in replacing the Throttle cable the idle point is just a little less then it was before. The motor is getting enough fuel too start but is a little lean tail the pump gets to a balance point. Then all is good. Keep an eye on it. Watch for any further changes.
 
Jul 23, 2009
504
Beneteau 31 Oceanis Grand Lake, Oklahoma
Maybe the old throttle cable was holding the idle up a little. I think I would try turning the idle up slightly.
 
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Jan 5, 2017
120
Hunter 356 SF Bay / Delta
Have you had an opportunity to start her from cold again?
I have. The video was probably the third cold start.

I'll be back down to the boat on Friday -- I'll try starting it with the throttle up a bit, see if that helps.
 
Mar 20, 2004
1,665
Hunter 356 and 216 Portland, ME
Perhaps in replacing the Throttle cable the idle point is just a little less then it was before. The motor is getting enough fuel too start but is a little lean tail the pump gets to a balance point. Then all is good. Keep an eye on it. Watch for any further changes.
ditto! our 3gm does that too; the idle is probably a little low for an initial cold start and it takes a sec for the injector lines to pressurize. I just advance the throttle a bit before starting and she starts smoothly and then you can throttle back if you want
 
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Nov 21, 2012
295
Yamaha 33 Port Ludlow, WA
The Yanmar Operating Manual GM Series says open the throttle half way for starting. I do that and then immediately reduce RPM to 1000-1100 for a few minutes then down to idle at 800. I'm quick - no roaring engine, just pull the throttle back after start. I have another version of the operating manual specific to the 3GM30F and it recommends full throttle at start. I opted for the more conservative version.

1613012967536.png
 
Last edited:
May 24, 2004
6,838
CC 30 South Florida
Yes it sounds like it may be starting on just two cylinders and the third cylinder may be lagging in building compression. Possible solutions, adjust the valves, clean the injectors and advance the throttle opening some when starting the engine.