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#569091 - 09/11/17 12:25 PM Gas sheen after startup  
Joined: Apr 2007
Posts: 20
swopo82 Offline
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swopo82  Offline
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Joined: Apr 2007
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Hi, I come to the well of experience looking for advice again...

I have a twin Mercruiser 5.7 inboards that is leaving a slight gas sheen on the surface just after a cold startup. I've tracked it down to the carb running rich while it waits for the choke to open. If I manually force the choke farther during the warm up I can make the sheen go away. The choke pull off is adjusted to the specs listed in two maintenance manuals I've found. 1995 Merc 5.7 w/ Weber Carb. I can see that the carb seems kinda flooded during this time of warm up in that I can see fuel. Once it's all warmed up and the choke is fully open these is no sheen and no fuel visible in the carb.

My question is....should I just tweak the choke pull off so it is greater than spec or should I really be looking for something else? The boat is new to me so I don't know any other history as far as tune ups, etc.

P.s. I had a mechanic from my marina look at it and he said....oh yeah....weber carbs always do that. Just warm it up faster. Anyone else have similar experience with 95' era Mercs with Weber carbs?

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#569099 - 09/11/17 04:01 PM Re: Gas sheen after startup [Re: swopo82]  
Joined: Jan 2003
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Jack T Offline
Admiral
Jack T  Offline
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Joined: Jan 2003
Posts: 3,669
Southern California
This is one of the reasons government agencies have required clean burning marine engines.

Are both carburetors and their chokes operating the same way?

Do you give the throttle (out of gear) a few pumps before starting? If so, in addition to choking, you are injecting gasoline through the accelerator pump into the intake manifold. Add the two together and you are running very rich for a short time.

In any case, chokes were put on carburetors for starting in all environments. Most of the time, boats are used only in the warm weather, and the chokes do not have to be active for so long because the engines are fairly warm to begin with. I would see if I could adjust the chokes so they open sooner and see what happens. Before making any adjustments, be sure to know where your starting place is.

On my previous boat, which we ran at 5000 foot elevation, I just tied the choke linkage to hold the choke open all the time. Pumping the throttle before turning the key worked just fine.


Have a great day of boating
2012 Monterey 224 FS,
300 HP Volvo Penta with catalytic converters

#569102 - 09/11/17 07:43 PM Re: Gas sheen after startup [Re: swopo82]  
Joined: Jan 2003
Posts: 1,476
Lou C Offline
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Lou C  Offline
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Posts: 1,476
Long Island NY
It depends on how the choke is supposed to get heat in order to open. Most marine carbs now a days are using electric chokes (Edelbrock, Holley) and they use a heating element in order to open the choke. This basically does not follow engine temp so much rather is based on a timer. If you have an electric choke and the choke is staying closed too long, then the electric heater element may be getting old, or the wiring may have corrosion, bad ground etc. If it uses a thermostatic spring heated by the exhaust crossover in the intake manifold (Pre-Vortec Chevrolet small blocks have a cross over that heats the intake manifold, the Vortec engines do not) then the causes of the choke not opening are different. This system uses a thermostatic spring that sits on top of the intake manifold and depends on the engine reaching normal operating temp in a reasonable amount of time to open the choke. If the thermostat is not closing and the engine runs cold (like less than 145) with this system the choke will not open all the way. My old '88 OMC 4.3 has this set up, and it works pretty well as long as the thermostat is not sticking open a bit.
So look at what you have and report back.

Last edited by Lou C; 09/11/17 07:44 PM.

88 Four Winns 200 Horizon 4.3 OMC
98 Jeep Grand Cherokee 4.0 Six Selectrac
07 Jeep Grand Cherokee 5.7 HEMI Quadradrive II
#569110 - 09/11/17 10:08 PM Re: Gas sheen after startup [Re: swopo82]  
Joined: Feb 2003
Posts: 11,635
Frantically Relaxing Offline
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Frantically Relaxing  Offline
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Specs smecks, chokes on boat engines should never be fully closed above 40° ambient. I've always set my chokes to leave an 1/8" opening when fully closed at around 50°-55° outdoor ambient. That's plenty closed enough to cold start an engine and keep it running. Both SkipperLiner's engines will start quicker than most FI engines, cold or warmed up. Their chokes are electric heated and only take about a minute to full open. Whether they have them or not, boats with carbs SHOULD have electric choke unloaders because of the low temps they run...

FWIW and this is JMO, an always wide-open choke is preferable to one that won't fully open. Besides wasting gas and fouling spark plugs, it's a fire hazard...

#569112 - 09/11/17 11:10 PM Re: Gas sheen after startup [Re: swopo82]  
Joined: Jan 2003
Posts: 1,476
Lou C Offline
Admiral
Lou C  Offline
Admiral

Joined: Jan 2003
Posts: 1,476
Long Island NY
Forgot to mention....in this world of fuel injection I forgot to say we used to have as part of our yearly tune up in the day of carbs, cleaning the choke shaft with carb cleaner. It does not take much on those shafts or on the linkage (esp on a Q-JET) to cause a slow choke opening problem. And I agree, due to the low temp of raw water cooled engines electric chokes should work better than a divorced (well type heated by exhaust cross over) style choke. Going back really far, my 1970 Ford with the 302 (5 liter) had an Autolite 2100 2bbl. The auto choke was closed by the thermos spring and opened by suction of hot air from a tube that went into a hollow recess in the exhaust manifold. On our '75 Olds Delta 88 with the 350 and QJet, it also used an exhaust heated tube, this one went into the exhaust crossover and vaccum sucked air through the tube which was heated as the engine warmed up. Our '72 Chev Impala 350 with the 2bbl Rochester carb used the same well type divorced choke as my OMC 4.3. They all work but the electric works better in marine applications....


88 Four Winns 200 Horizon 4.3 OMC
98 Jeep Grand Cherokee 4.0 Six Selectrac
07 Jeep Grand Cherokee 5.7 HEMI Quadradrive II
#569116 - 09/12/17 06:51 AM Re: Gas sheen after startup [Re: swopo82]  
Joined: Aug 2005
Posts: 2,245
tpenfield Offline
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tpenfield  Offline
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Cape Cod, MA
I would suspect that you may not have all cylinders firing during the warm-up and are getting some raw gas out the exhaust. Have you checked the spark plugs to see what they look like?


Regards, Ted

Formula 330SS

My Boat Web Sites
#569121 - 09/12/17 10:10 AM Re: Gas sheen after startup [Re: swopo82]  
Joined: Jan 2003
Posts: 3,669
Jack T Offline
Admiral
Jack T  Offline
Admiral

Joined: Jan 2003
Posts: 3,669
Southern California
Good point, Ted


Have a great day of boating
2012 Monterey 224 FS,
300 HP Volvo Penta with catalytic converters


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