why no Carb Icing on boats (or cars) only aircraft?

Discussion in 'Gas Engines' started by Squidly-Diddly, Nov 4, 2018.

  1. Squidly-Diddly
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    Squidly-Diddly Senior Member

    about 3:38:00 mark.

    Says mostly happens at 80+% humidity and under 70'F.
    Says remedy is to throw "Carb Heat" switch for warm air to carb.

    I'm thinking 80+/70- is a lot of boating, and unlike cars many boats run close to max power most of the time. Many cars/trucks have intake hoses that tap fresh cold air at grill instead of semi-heated air from under hood. Never heard of a car getting Carb Icing even at 100%/40'F.

    Cars (used to) get "vapor lock" if fuel line gets too hot and fuel vaporizes and blocks flow.

    This about light planes that do most of their flying below the elevation of many lakes.

    What gives? Are light-plane carbs fundamentally different from their surface-borne counterparts?
  2. Mr Efficiency
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    Mr Efficiency Senior Member

    And temperature does decrease steadily with increase in altitude.
  3. Blueknarr
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    Blueknarr Senior Member

    The consequence of an automotive or marine carburetor freezing up are far less dire than an airplane's.

    Airplanes have extra failsafes because they can never be parked on the shoulder or let drift for even a moment.
  4. Barry
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    Barry Senior Member

    attached an interesting video of an explanation
  5. gonzo
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    gonzo Senior Member

    Car carburetors ice up too. That is why there is a stove connected to the air filter intake.
  6. Barry
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    Barry Senior Member

    Not sure what you meant by "stove"

    The hose coming from the top of the valve cover to the air intake is the PCV hose (positive crankcase ventilation) that takes any fumes created in the engine body itself and runs it back through the intake so the fumes do not reach the atmosphere or the engine compartment. These fumes would be created by "blowback" gases leaking past rings or even just hot oil fumes.

    There may be, (so many hoses on the new emissions engines to figure out) another source of hotter air controlled by the EGR valve, Exhaust Gas Recirculation that takes a portion of the
    exhaust gases and returns some of it back to the intake to help in reducing nitrous oxygen from the emissions.

    While the warmer recirculated gases may reduce the chance of automobile engine carb icing, I wonder how important that this function is. With the new EFI engines, there would not be the cooling effect from the evaporating gasoline as it changes from liquid to vapor which I thought was one of the reasons for carb icing at low intake air temperature conditions
  7. ondarvr
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    ondarvr Senior Member

    There is a ducting system that pulls warm air off the outside of the exhaust manifold on old carbed cars.

    I’ve had carbs on just about every type of application freeze up at one time or another in cold weather.
    Last edited: Nov 6, 2018
  8. gonzo
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    gonzo Senior Member

    The sheet metal cover over the manifold is commonly called "stove".
    fallguy likes this.
  9. Magnus W
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    Magnus W Senior Member

    The procedure for normal carb heat operation basically says to use it at low power settings (as in descending) or in visible moisture.

    Most boat engines does not operate at low power settings and high revs (ie low manifold pressure) and its the low pressure that's the main cause of carb ice. In addition to this, some intakes are heated or at least they are sitting in a hot engine room. The common flat airplane engine is air cooled and the non heated intake sits below the engine in the constant stream of air passing by. And since power to weight is critical the standard way of doing it is to supply the intake with as cool air as possible, which as gonzo stated is not the case with cars (some car tuners have cold air mods at the top of the list of must do's for more hp). Anyway, there's a noticeable difference in power with the carb heat activated so it makes sense to run without it when possible.

    Some motorcycles have carb heat, most often the coolant is circulated through the carb body. The carbs are mounted on rubbers so little to no heat is transferred from the engine to the carbs, I guess that's why they need it.
  10. JamesG123
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    JamesG123 Senior Member

    I once had a Suzuki SV-650 that had electric carb heaters. After finishing a re-jetting (remember those?) , when hooking it back up I mistakenly misconnected the carb heat plug with the radiatior fan thermosat. When the bike got hot enough to kick on the fan, "pop" the bike would blow a fuze and die. Took a while to figure out.
  11. sdowney717
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    sdowney717 Senior Member

    It stays pretty warm in a boat engine room. Mine can be toasty in summer with 2 engines running. In my case, the carbs would get too hot, I use a wood spacer to insulate the carb, 3/4 inch thick piece of epoxied plywood I made. The carb body stays cooler. I use Rochester quadrajets now with a single central fuel chamber, when I got boat, it used 4GC rochesters with 2 chambers and they are not a good design. With no spacer and the 4GC, at shutdown, fuel would boil out of carb flooding the motor.
  12. KeithO
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    KeithO Senior Member

    Since the advent of fuel injection and no vaporizing of fuel in the intake, that got rid of throttle icing altogether. I know a lot of kids who have no idea what a carb is or how to start an engine that has one... Its only in aviation where the pace of progress is virtually zero where people continue to die every year from this problem. But people have less faith in a system that requires pressurized fuel and electronics to control it, so one has to pick your poison. Outboards too have generally been way old technology. The last Mercury I had with 3 carbs and a cam rotated distributor... What a complicated POS. Today a megasquirt controller and a fuel pump and a crankshaft sensor and your in business in the 21st century with fully mapped fuel and spark. No proprietary CDI's that cost a fortune to replace like the OEMs....

  13. brendan gardam
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    brendan gardam Senior Member

    I had a merc 60 that iced up the carbs in minus 7 celsius. Kept stalling at low speed, was fine at cruise speed. I have also had cars with iced up carbs too.
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