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Hard to start 7.4 liter big block when warm... Ideas?-gctid396547

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    Hard to start 7.4 liter big block when warm... Ideas?-gctid396547

    My 7.4 liter Mercruiser starts fine when cold. One short starting try to fill up the carb, then it fires right up on the second try. After it has been running, if I shut it down, it is difficult to restart.

    The engine turns over fine, but the ignition won't catch. I usually have to hold the throttle wide open and crank for at least ten seconds before it will catch. Two mechanics have been unable to help. New plugs, wires, coil and distributor in the last year. I only use ethanol-free gas. The engine is a 1996 vintage with a carb, not electronic ignition.

    I welcome your suggestions. If I keep this up, I think I'll be adding a new starter soon!

    Rob

    #2
    What carb ? No electronic ignition ? I know you don't have points exactly what do you have ?
    Be good, be happy, for tomorrow is promised to no man !

    1994 2452, 5.0l, Alpha gen. 2 drive. Sold ! Sold ! Sold !

    '86 / 19' Citation cuddy, Merc. 3.0L / 140 hp 86' , stringer drive. Sold ! Sold ! Sold !

    Manalapan N.J

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      #3
      robster_in_edmonds wrote:
      My 7.4 liter Mercruiser starts fine when cold. One short starting try to fill up the carb, then it fires right up on the second try. After it has been running, if I shut it down, it is difficult to restart.

      The engine turns over fine, but the ignition won't catch. I usually have to hold the throttle wide open and crank for at least ten seconds before it will catch. Two mechanics have been unable to help. New plugs, wires, coil and distributor in the last year. I only use ethanol-free gas. The engine is a 1996 vintage with a carb, not electronic ignition.

      I welcome your suggestions. If I keep this up, I think I'll be adding a new starter soon!

      Rob
      The fuel pump has a check valve in it to keep the line to the carb full of fuel. If this leaks back it will take awhile to get fuel to the carb.

      If it is a 1996 engine it should have a Thunderbolt ignition.(see link below)

      http://www.mercruiserparts.com/Show_Pictures3.asp?

      dnbr=821723965&ivar=images/COMMON/37878.png&inbr=2345&bnbr=5&bdesc=DISTRIBUTOR+AND+I GNITION+COMPONENTS

      The fuel in the carb will boil out if the engine is warm and will take time to refill.

      Also if you let the engine run a idle for at least two minutes before shutting it down that may help your problem.

      Comment


        #4
        These issues are from automobile troubleshooting, but may give you a "start":

        From:http://www.thecarburetorshop.com/Troubleshooting.htmHARD STARTING, HOT

        Difficult starting of a hot vehicle from 5 minutes to an hour after the engine has been operated, can be caused by the volatility of modern fuel. If you have this problem; try using the following method to start the engine: DON'T touch the footfeed (VERY important). Crank the engine over from three to 5 seconds (different vehicles will respond to different times); and then GENTLY (so as not to activate the accelerator pump) press the footfeed approximately 1/3 of its travel. The engine should start, and may run rough. Run the engine at a high idle for about 10 seconds. This issue is caused by volatility of modern fuel. Once the engine has been shut off, the gasoline is heated by the latent heat of the engine, and percolates the fuel from the bowl into the throttle area, forming a mixture that is too rich to fire. If you push the footfeed to the floor (as has been the traditional method of "unloading" a flooded engine) the gasoline continues to flow into the engine (again due to the volatility). By not touching the footfeed, you do not open the throttle plates, and the engine will pump the overrich mixture out of the tailpipe. Once the overrich mixture has been alleviated, gently opening the throttle will allow the engine to start.

        From: http://autos.yahoo.com/maintain/repa...ques093_4.htmlHot starting problems are usually fuel related. When a hot engine is shut off, the temperature of the engine and everything on it continues to rise for awhile as the engine undergoes a period of "heat soak." This can cause fuel to boil inside the carburetor bowl, fuel lines and fuel filter. When you attempt to restart the engine, "vapor lock" obstructs the flow of fuel and the engine doesn't want to start.

        This is much less of a problem on fuel injected engines because the fuel is usually under much higher pressure inside the injectors and fuel line. Even so, a fuel line routed near an exhaust manifold or a fuel rail that's exposed to a lot of heat may still suffer the same kind of problems.

        Heat soak problems such as these can sometimes be cured by wrapping insulation around affected fuel lines, and/or installing an insulating spacer or heat shield under the carburetor.

        Other Causes

        Hot starting problems can also be caused by cooling problems that allow your engine to run too hot (the pistons swell up and may scuff the cylinder walls), or excessive resistance in the starter motor that causes the engine to crank slowly. A starter "amp draw" test can be used to check the condition of your starter. Also, many starters have small "heat shields" to protect them from heat radiating from nearby exhaust pipes or manifolds. If the shield is missing, the starter may get too hot and bind up

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          #5
          I have had this problem and it turned out to be the carb base was dripping gas into the manifold. The causes two problems. 1) the float bole empties 2) The engine floods after shut down. Both these conditions make for hard starting and poor mileage. This condition is common on Rochester Carbs.

          Having gas drip into the manifold will usually allow gas to get into the cylinders, past the pistons down into the oil. See if your oil goes bad quickly or smells of gasoline.

          Try shutting down the engine and listening in the carb throat. Listen for a drip, drip, drip sound.

          This condition will carbon up an engine. Once the carb is rebuilt, change the oil and run some SeaFoam gas treatment through the engine...

          If the carb checks out okay, then you might have an electronic ignition/coil which fails only after running (the part may be failing on the heat of the engine)

          Comment


            #6
            Great feedback - love this site and all the participants. I'll try the starting procedure on my trip this week, and I'm going to get the carb rebuilt later this month.

            Thanks again!

            Rob

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