Atomic 4 Fuel Consumption

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  • Don Moyer
    • Oct 2004
    • 2823

    Atomic 4 Fuel Consumption

    With gas prices reaching unprecedented levels, our webmaster (Bill) has been grousing at me for several weeks to consider a tech note with suggestions aimed at cutting down on fuel consumption. I've been somewhat slow to pick up the torch, believing that the price of gasoline has little impact on a group of sailors who historically worry mostly about using their fuel so slowly that they fear it might go stale.

    However, Bill finally came up with three sets of numbers that are difficult to ignore:

    * An Atomic 4 in reasonably good condition can conservatively cruise at 5.5 knots, burning 1.0 gallons per hour.

    * If you can cut your fuel consumption to 0.75 gallons per hour (quite easy to do with an adjustable main jet), you can motor for 1.33 hours for each gallon of fuel, which represents an increase in cruising range of 33%.

    * If/when the price of gas in your neighborhood reaches $3.75 per gallon; decreasing fuel consumption from 1.0 gallons per hour to 0.75 gallons per hour could save you approximately $50 during an average season (50 hours of engine operation).

    Even if you remain indifferent to the rising fuel costs, a 33% increase in cruising range translates into quite a few less stops at fuel docks, so here are some things that you can do to improve your fuel economy that are quite easy and which create little or no expense:

    1) Recheck your ignition timing by performing a simple timing advance check under power. This check is performed by loosening the distributor hold-down bracket and rotating the distributor a slight amount in each direction until you find the spot of highest RPM. Retighten the distributor in this new location of highest RPM. Operating on either side of this optimum advance setting will have a detrimental effect on fuel economy.

    2) Fuel economy can also be greatly enhanced by staying a bit further away from your theoretical hull speed when setting your cruising speed (getting there is supposed to be half the fun anyway). The power required to move a sailboat to a hull speed of 6.5 knots goes up disproportionately to that required to move the same boat at 5.5 knots. In other words, you don't make up the difference in fuel consumption by getting to your destination that much faster.

    3) Fine-tune your fuel mixture with an adjustable main jet. Our intracoastal cruising customers report that they're usually able to easily achieve the reductions in Bill's above example with the installation of an adjustable main jet alone. Click here to view this product:


    Here are the installation steps to help you decide if it's something that you would like to consider:

    INSTALLING AN ADJUSTABLE MAIN JET:

    a. Remove the fixed main jet from your carburetor, and install the fixed part of the adjustable assembly. The fixed jet of the adjustable assembly looks much like the original jet, except that the orifice in the adjustable one is larger.

    CAUTION: Both the fixed jet and the 1/2" hex-headed plug of the needle assembly have hard washers to seal them after installation. Use care in removing the original fixed jet so as to not lose the original washer. If the original washer remains in the carburetor instead of coming out with the jet, it's usually preferable to leave it in place and save the new washer for a spare. Do not end up using both small washers behind the fixed jet. Also, do not overtighten the 1/2" hex head of the needle assembly, or the sealing washer is likely to extrude out from under the hex head and cause a leak.

    b. After the fixed jet is in place, thread the adjustable needle assembly into the main passage, in place of the main passage plug. Make sure that the adjustable needle of the valve is backed out far enough so as not to seat against the fixed jet.

    c. After the needle assembly is installed and tightened, turn the "T" handle on the end of the needle in until you feel it make contact with the fixed jet. Then back the needle out approximately 1 and 1/2 turns. This should be a good initial setting. After warming up the engine, and at your favorite cruising power setting, turn the "T" handle in and out until you find the best RPM.

    NOTE: You will find quite a large zone in the adjustment process where the engine performance will not noticeably change as the "T" handle is turned one way or the other. By turning the "T" handle in (clockwise) until a definite decrease in engine performance is noted, and then backing it out just until power is restored, the best fuel economy will be achieved.

    d. After the installation is complete, it may be necessary to tighten the 5/16" packing gland nut to seal around the shaft of the needle. Tightening this packing gland after the final adjustment is made will also insure that the adjustment will not slip over time.

    Don
    Attached Files
    Last edited by Don Moyer; 04-21-2006, 10:53 AM.
  • rhaggard
    Senior Member
    • Jul 2007
    • 18

    #2
    Adjustable Main Jet

    For what it's worth: I was running way too rich, could smell raw gas whenever I ran the engine, went through fuel like crazy. Just purchased and installed the adjustable main jet. WOW! The old A-4 has never run so smooth. I started with the recommended 1 & 1/2 turns and only had to back it out about 1/4 turn to get it right. Not only does it run much better, but now we don't smell any raw gas when running. Got to be the best investment we have made in the engine so far. Thanks a million!

    Bob

    Comment

    • CaptainHatty
      Senior Member
      • Aug 2011
      • 33

      #3
      Just to clarify.

      Originally posted by Don Moyer View Post
      1) Recheck your ignition timing by performing a simple timing advance check under power. This check is performed by loosening the distributor hold-down bracket and rotating the distributor a slight amount in each direction until you find the spot of highest RPM. Retighten the distributor in this new location of highest RPM. Operating on either side of this optimum advance setting will have a detrimental effect on fuel economy.
      This seems like common sense, but the instructions I've read for EI haven't been exactly clear what steps applied to a points system and what steps did not apply to EI (and vice versa). I'm assuming this advance timing check applies to BOTH types of ignition systems. Correct?

      Comment

      • ILikeRust
        Afourian MVP
        • Sep 2010
        • 2212

        #4
        Yes. Regardless of whether you're using points and condensor or electronic ignition, you still need to time it to make sure the distributor is generating and delivering the spark at the proper time in the cycle.
        - Bill T.
        - Richmond, VA

        Relentless pursuer of lost causes

        Comment

        • Dave Neptune
          Afourian MVP, Professor Emeritus
          • Jan 2007
          • 5063

          #5
          Points vs electronic.

          Per what Bill said with either type of ignition the proper timing is a must. This is a good spot to mention the big advantage to the "electronic" type is that once you set it it is set for the life of the ignition, however with points once set the TIMING DRIFTS as the points wear and the signal gets weaker.

          Re fuel, I have a 35' boat with a 25' waterline, 14000 lbs and I tow a 10' fiberglass lap strake rowing dinghy all the time ging to the island. With the Indigo prop I run at 2000 RPM's @ around 5.4~5.6 kts running into the seas and morning lite breeze. I have kept track of the hours and the fuel for the season and I am using just under .8 gallons an hour, this is over about 80 hours of motoring. My cruise setting is 2,000 RPM's (Indigo prop) at 8.5 inches of vacuum. If I run harder say 2200 I use a bit over a gallon an hour and far less below 1900. My engine is tired and has two cylinders with low compression, I use a quart of oil in about 50 hours. I am using a fixed jet however I have done a lot of tinkering with the carb!

          Dave Neptune

          Comment

          • JOHN COOKSON
            Afourian MVP, Professor Emeritus
            • Nov 2008
            • 3501

            #6
            Correctly set timing will benefit any (running) engine.

            TRUE GRIT

            Comment

            • smaarch
              Senior Member
              • Jul 2008
              • 47

              #7
              from a sailing point of view

              a non mechanical point of view
              just got back from a 200 mile sail over a period of a week.
              here timing and tides are everything
              I think the biggest issue is how hard you want to press forward. dropping the throttle back off max rpm seems to be the key to economy.
              there is definitely a law of diminishing returns at play here.
              at one point in the Battery, waiting for the tide to change for the north bound trip up the East River, just off idle would allow us to hold our position and gain slight headway, while max rpm literally was shoveling against the tide...why bother? everything is working so hard and the gain in distance seems hardly worth it...barely making a knot against current.
              this was the first day before being hauled and a seriously fouled bottom was powerwashed.

              In the NYC area $3.75 a gallon would be a steal!
              will not mention the name of the Marina, but we paid nearly $6.00 a gallon at one stop....that's a lot of money!

              The Keep it Simple Approach
              swinging on a mooring with no facilities around I usually just fill a jerry can at the local station to keep the tank topped off...just under $4.00/gal at the pumps here.

              In any event, the adjustable jet sounds like a great idea and will consider it over the winter. Thanks Don.
              Last edited by smaarch; 09-27-2011, 01:25 PM.

              Comment

              • Dave Neptune
                Afourian MVP, Professor Emeritus
                • Jan 2007
                • 5063

                #8
                Cost

                smaarch, bare in mind Don's post is from 2006 and fuel was a bit cheaper and very little with ethenol.
                Yopu make a good point re how hard are you pushing with the throttle, relating to economy!!

                Dave Neptune

                Comment

                • smaarch
                  Senior Member
                  • Jul 2008
                  • 47

                  #9
                  hahaha...thanks Dave
                  Well.....then never mind...hahaha

                  Comment

                  • wrapper
                    Senior Member
                    • Apr 2006
                    • 15

                    #10
                    Choke

                    Okay,

                    Installation and adjustment of adjustable main jet was relatively easy. But now I have to use the choke for about a minute or engine tends to stall.

                    Have I leaned it too much? Or is this normal ?

                    Comment

                    • Dave Neptune
                      Afourian MVP, Professor Emeritus
                      • Jan 2007
                      • 5063

                      #11
                      And~~~?

                      wrapper, when is it you have to use the chokje? If it is at startup it's normal if your mixes are about right. With an updraft carb it should be a bit harder to start than a car and the choke should only be necessary for the first couple of minutes while getting the chill out of the cylinders. Once warm no choke should be necessary unless there is a problem. I have adjusted to many A-4's over the years that have the idle set way to rich, it does help the engine "start" a bit faster but then it just fouls or soots up the plugs if you need to idle or run at slow speeds.
                      Good luck with your adj-jet from the midrange up.

                      Dave Neptune

                      Comment

                      • wrapper
                        Senior Member
                        • Apr 2006
                        • 15

                        #12
                        choke

                        It is on first start-up of course. Now, it needs the choke on a little longer, but that is understandable. Just about a minute.

                        But if I have been running the engine for a while and then let the engine cool for say an hour, it now needs choke to stay running or to have any power. Otherwise it stalls when I throttle.

                        What actually happened was we needed to start the engine after some sailing time to get out of a situation. The engine stalls when we give it power even though she is somewhat warm.

                        Before, I never needed choke when it has runned for any time that day. Now I need choke to have power when the engine has been turned off for an hour or so.

                        I can live with that, but just wondering if I leaned her too much.

                        Comment

                        • JOHN COOKSON
                          Afourian MVP, Professor Emeritus
                          • Nov 2008
                          • 3501

                          #13
                          wrapper

                          The new main jet may have uncovered some other adjustment(s) that were made to compensate for a less than perfect old main jet setting.

                          There is an interplay between timing, dwell, idle mixture setting, and idle speed setting. I recall a recent thread in which a chap had to adjust the timing to get rid of blow by. After he got the timing right he had to adjust the idle on the carb.

                          So here's what I'd do: Make sure the dwell is to spec (if you have points) then adjust the timing. After that, if necessary, adjust the idle speed\mixture.

                          Here's how my A4 starts - I consider it to be normal:
                          Cold start: Full choke. Starts on the second or third seven second cranking burst. Full choke for ~15 seconds then partial choke for ~45 seconds. Fast idle for ~45-60 seconds then I can shift into gear and throttle up.
                          Warm start: Momentary full choke untill the engine starts then choke full open. Fast idle for ~30 seconds then shift into gear and throttle up.
                          I have my idle set a little on the lean side.

                          If you need to use the choke once the engine is warmed up to keep it running or to get power there is a problem - most likely an air leak somewhere.

                          TRUE GRIT

                          Comment

                          • Dave Neptune
                            Afourian MVP, Professor Emeritus
                            • Jan 2007
                            • 5063

                            #14
                            Quick first try this.

                            FWIW, mine too starts a runs just like John described. Yes and I too tend to get the idle as lean as I can.

                            wrapper, before you start futzing around give your adj-jet a tleas a 1/4 turn to rich and see if the "bogging" (stalling) lessens a bit. I have had mine to lean and it would run but I had to accelerate slowly, if I hit the throttle she'd stumble and sometimes die but I still could work her if I did so slowly.

                            Dave Neptune

                            Comment

                            • hanleyclifford
                              Afourian MVP
                              • Mar 2010
                              • 7004

                              #15
                              The adjustable jet will be giving you a new education on A4 fuel matters. If you adjust for max rpm at idle or off idle, you will be too lean at cruise rpm. If you adjust at cruise rpm you will be too rich at idle and off idle. This reveals that the carb even with the adjustable jet is not able to deliver constant fuel ratio throughout the rpm range. This is true for all carbs; the adjustable jet just makes us aware of it.
                              Last edited by hanleyclifford; 05-24-2012, 07:34 PM.

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