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Low Compression - Cause of high fuel consumption?

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  • Low Compression - Cause of high fuel consumption?

    Hey all. Have this 95 force 70 that I bought this spring as a project/fun motor. Cold starts, idles and runs pretty darn well, except perhaps hot restarts, where its a little finicky. Had it out several times this year, and found that it devours gas. If I ran at 3/4 throttle, it used about 6 gallons of fuel in under 1 hour! Ive read on here about the 7gph rule of thumb, but I think this motor would chew 7 gallons very quickly at WOT. I did recheck compression just now and is 90-100-100. Do you think that this motors near worn out state is the reason that it burns extra fuel?

    Im running it on a 16 ft four winns (1100 lbs), and sadly I have no tach yet. It runs about 31mph at wot. Running 115/8 x 11 prop.

    I am sort of looking for an excuse to tear into this powerhead, but I suppose I probably wont if you dont think the low compression is the most likely contributor.

    Thanks alot.

  • #2
    Heres what these cylinders looked like back in the spring....

    Comment


    • #3
      assuming you are commanding 50hp worth of power

      your motor should have a BSFC of about 0.55#/HP/hr. at higher RPM's

      so 0.55*50*1 = 25# of fuel, so you should have burned about 4 gallons

      if your burning more, my guess is you are simply washing fuel down the cylinders and that may be exasperating the low compression. start with the fuel system

      your compression numbers are also low, not sure how much that will push the BSFC up.
      +------------------------------------------+----------+----------------+-----------+ | Engine | @ 1K RPM | @ Peak Torque | @ Peak HP | +------------------------------------------+----------+----------------+-----------+ | 4-stroke (low compression, carburetor) | 0.62 | 0.47 | 0.52 | | 4-stroke (high compression, carburetor) | 0.60 | 0.42 | 0.47 | | 4-stroke (high compression, closed loop) | 0.52 | 0.35 | 0.42 | | 4-stroke (supercharged, carburetor) | 0.75 | 0.50 | 0.55 | | 4-stroke (turbocharged, closed loop) | 0.57 | 0.45 | 0.50 | | 2-stroke (low compression, carburetor) | 0.85 | 0.55 | 0.60 | | 2-stroke (high compression, carburetor) | 0.80 | 0.50 | 0.55 | | Diesel 4-stroke | 0.35 | 0.25 | 0.30 | | Diesel 2-stroke | 0.40 | 0.29 | 0.34 | +------------------------------------------+----------+----------------+-----------+
      Cheesehead boating the Gulf Coast of FLA 27.51° N, 82.53° W

      1988 Cruisers Rogue 2420 -VP 290 DP now powered by custom 468 - https://forums.iboats.com/forum/owner...988-rogue-2420

      Past Boats
      1970 Wooster Hellion - Merc 9.8
      2002 SeaRay 190BR - 5.0 - A1G2 - "Cheeseheads in Paradise"
      1984 Avanti 170DLI -3.0 stringer- "Ship Happens"

      What’s behind you doesn’t matter.Enzo Ferrari

      Comment


      • #4
        The comp at 90-100 is low but could be the way it's taken or the equipment used??
        Harbor Freight tools= crappy results.
        With a good test/tester it should be 150# or close.

        The "good" the comp is close all within 10#
        The low on #1 is inherent to that motor/carb setup.
        That cyl. is usually the first to go bad if the motor dies.

        Any idea how many miles to the gallon?
        My twin 88/85hp got 3.2(?) MPG

        What do the plugs look like when you pull them.
        Pay close attention to the #2 plug as it operates the fuel pump.
        Possible the excessive fuel gets pulled through the pump and right into the cyl.

        BSFC ??????
        I didn't understand anything Scott said.


        Buy American made Cars and Trucks!!!Towing insurance for a boat,a GREAT thing.
        ​Tools to make my life easier: Battery tester, die grinder, temp.gun, compression tester, DVA meter.

        In Scenic , Beautiful San Sebastian by the sea Florida

        88 Bayliner 21CC 2001 Mercury 225 HP EFI

        PLEASE,NO PERSONAL MESSAGES IF THE QUESTION CAN BE ASKED ON THE OPEN FORUM I WON'T ANSWER AS MY SYSTEM CAN'T REPLY TO YOUR EMAILS

        Comment


        • #5
          The carburetors respond to air flowing through them.----What happens to that air afterwards , compression in the crankcase and then compression in the cylinder does not affect fuel amount used.

          Comment


          • #6
            Used my now old craftsman thread on comp gauge. Has been accurate on all past use (building harley motors with new parts and got expected compr readings). Is it worth doing a decarbon to see if this brings up comp?

            wow mpg cant be good. Im guessing that its less than 3 mpg at 25 mph cruise (3/4 throttle ish).

            Comment


            • #7
              Looks like the route i took that day was about 12 miles round trip in 5+ gallons of fuel. 3 mpg at very best.

              Comment


              • #8
                Can anyone comment on this prop for this setup? I have a mini tach ordered.

                Comment


                • #9
                  Originally posted by atengnr View Post
                  Can anyone comment on this prop for this setup? I have a mini tach ordered.
                  https://www.generalpropeller.com/pdf/QS_Prop_Guide.pdf

                  You should of really ask this question in the "prop section"
                  Last edited by may the force be with you; December 3rd, 2019, 05:25 PM.
                  "Success is walking from failure to failure with no loss of enthusiasm." - Winston Churchill

                  Comment


                  • #10
                    Thanks. It appears that i am way under propped if im reading this chart correctly.

                    Comment


                    • #11
                      Sign up today
                      3 mpg is good.
                      Some boats get 1 if lucky.

                      My 88 85's came with 17p props.about 32mph.
                      I wanted more speed so I changed to a 19p.
                      I got 2-3 mph more and the fuel usage went up 15-20% with the change.


                      Buy American made Cars and Trucks!!!Towing insurance for a boat,a GREAT thing.
                      ​Tools to make my life easier: Battery tester, die grinder, temp.gun, compression tester, DVA meter.

                      In Scenic , Beautiful San Sebastian by the sea Florida

                      88 Bayliner 21CC 2001 Mercury 225 HP EFI

                      PLEASE,NO PERSONAL MESSAGES IF THE QUESTION CAN BE ASKED ON THE OPEN FORUM I WON'T ANSWER AS MY SYSTEM CAN'T REPLY TO YOUR EMAILS

                      Comment

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