[C320-list] Brown Smoke from Exhaust

[Annie Bennett]

Can anyone give me a pointer on the meaning of brown smoke from exhaust.

Today was my second day out on Topcat after a month overseas.  Topcat's engine was serviced just before we went away.  This morning before going out I checked the oil and all was fine.  I thought the coolant might be a bit low in the visible container and called the mechanic to see if he could change it as I noticed in the engine log it hasn't been changed for a while.  The wind was light so I motored for a couple of hours - all fine - then sailed with the engine off for another hour and only turned the engine back on for the last 15 minutes for getting the sails down and coming into the mooring.  In the last 5 minutes I noticed some white smoke.  It must be smoke not condensation as it was a very warm day and the engine was warm.  There was no smoke earlier in the day.  

After my guests had gone ashore I decided to turn the engine back on to test it, initially nothing, just the normal water, but then white smoke came out - no smell.  Still In neutral I turned the revs up to 28 and then brown smoke started coming out and there was a faint oily residue on the water that I think but am not certain came from Topcat.  I turned the engine off.  I wanted to double check before calling the mechanic that I really had a problem and turned the engine back on, still in neutral revved again.  First nothing, then  white smoke then quite a lot of brown smoke again but this time a slight burning rubber smell.  I've turned it off and the mechanic is hopefully coming tomorrow afternoon.  Plenty of water is coming out as normal.  The impeller was changed in the service in August and today is only the second time I've been out since the service.  Someone at the marina wondered if the smoke could be caused by my fouling the propellor on something in the water but he changed his mind when I said the brown smoke came out when revving in neutral.  Does anyone have any ideas that might help the mechanic.  I've done about 1000 hours.  My googling suggests it might have something to do with fuel injectors.  If so is this a big / difficult / time consuming problem for a mechanic to fix?  I'm hoping not as I have a charter booked for a birthday on Wednesday and I'm hoping I won't have to cancel and let them down.  Ive never had a problem with smoke of any colour before.

Thanks in advance to anyone for their help / advice.

Kind regards
Annie Bennett
Topcat #1073