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Thread: 2 barrel 2 banger

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  1. #1
    Team Member JohnsonM50's Avatar
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    Default 2 barrel 2 banger

    An Idea to re-ask, Ive been thinking about putting a 2 barrel carb on a '70s 31.8ci OMC twin. Ive been given some good ideas so am in progress. Ive got a 2 barrel from a 4 cyl OMC. that used 2 carbs. The throats at the rear are 1 3/8", same as a stock single for the 31.8ci motor. The venturi sizes are very close between the carbs, the 2 barrel appears to be equal side to side.
    The single barrel stock carb feeds a common intake that both reed sets draw from so in effect the carb singularly feeds each cyl. Every charge to each cyl is from the full bore of the single barrel drawn back & forth thru the reeds.
    If the above is correct, I theorize that if I separate the intake plenum so the each barrel feeds 1 cyl Ill essentially [short of working out bugs] have the same thing. If so then the performance.. where would be the gain? Im thinking the smaller more direct intake per cyl might cause an increase in velocity [this is a crossflow] and therefore send a better atomized, possibly slightly larger charge in. The carb would be drawing from each feed tube 1/2 as many times per barrel as the single barrel so might be more capable at top end. Im wondering if in alternate operation the feed jets would cause a siphoning effect on each other but doubt it because the air vent should cover that.
    Worse case it doesnt work & I cut up a spare intake & sunk a bunch of time.
    Im ready now...Point-n-Laff, Ask why!?, make remarks,or add wisdom Rest assured Im enjoying try-n it & theres more to the story than just stickin this on any motor. Thanks

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    Team Member JohnsonM50's Avatar
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    Default

    so far...

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    modifiedoutboard OUTBOARDER's Avatar
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    Default Sleeved 2bbl Carb

    1-3/8" O.D X 1-1/4" I.D Alum tube will fit on butterfly side, drill thru hole for throttle shaft and make/get a 1-1/4" dia butterfly....................

    oops also Drill out idle and transition holes thru sleeve
    Last edited by OUTBOARDER; 02-07-2010 at 12:29 PM. Reason: and another thing

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    Team Member JohnsonM50's Avatar
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    At 1st glance I thought to make a plate to attach to the intake but it became apparent that to thru bolt to whole thing was the only way. Ill have to go thru the carb of course, there appears to be orifices in the rear cutouts between barrels. Will separating the intake effect them is 1 concern, how to compensate the other. Im not concerned with idle/troll speed so can sacrifice purr like a kitten in favor the roar.



    Thanks Anthony, I will do that if needed but am going to test 'as is' just in case.

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    If you want power at high revs you will need to use reeds on a vee block not the flat ones

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    Team Member zul8tr's Avatar
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    Talking 2 x carb

    Thoughts

    Have you checked the size of the total venturi area of the 2 x carb compared to the total venturi area of the single carb you are replacing? It appears to me that the 2x has a whole lot more area than the single. While this may be good for max high rpm output (?) it most likely will be negative for lower to mid range performance due to poor throttle response from reduced air velocity at the venturi. The new 2x carb setup would require proper jetting to obtain the correct A/F ratio but such a larger venturi area then stock might make jetting out of reasonable range.

    You note correctly that each cylindere feeds on the single carb and theorize that a 2X carb divided at the plenum will allow each cylindere to use each side of the 2x carb and that should provide a more efficient intake. While it sounds reasonable the intake air still mixes atthe reed cage. I think that unless you keep the intake seperate all the way to the cylindere intake ports the 2x carb will have about 1/2 the air velocity at each venturi as the stock single venturi setup based on the similiar geometry dimensions you note.

    Since you are already on the way go for it and see what happens. Edison tried 1000 filiments before he hit on carbon impregnated cotton, but he missed it on his incorrect belief that DC power was the way of the future. It took Nikola Tesla (his former employee) to set the light and gave us AC, Edison was pissed - thank you Mr Tesla.

  7. #7
    Team Member JohnsonM50's Avatar
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    Default 2 banger x 2 barrel update...

    From questionable technologies* a subsidiary of Rude RaT motors.................
    >>>>>>>>>>>>>>>>>> 2 banger x 2 barrel >>>>>>>>>>>>>>>>>>>>>>>>>>

    Heres some pics of the maniflood project, as you can see if there is a function form follows it. Pretty aint in the picture, pun perpetuated. Im hopin to rig up a powerhead & start the process of starting the motor to see what I can see then ponder pond testing. Well, enuff of that- work or not other serious speed considerations are under delerious speed consideration. me, myself & I - Prez, V-Prez & cleanup boy of Rude Rat un-inc.



  8. #8
    FFX-61
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    Thumbs up


    Part one of "Secrets of the OMC Mod 50" : http://www.boatracingfacts.com/forums/showthread.php?t=2052

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    Team Member JohnsonM50's Avatar
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    [QUOTE=Fast Fred;86867]
    Thanks Fred, Just hope I get it goin ok. It'll be a novelty if it goes as good as stock, get-n more from it to any degree will be neat. Ive got a couple OMC 31.8s that are faster than stock but no great improvement yet. With the hefty crossflow pistons & the flat reed plate, limits are in place.
    The carb Im working with is from a 90hp, year unknown. I found a "72 OMC 85hp carb & am glad to see the venturi is significantly smaller than the 90 while the throats & bolts are the same. With that Ive got 2 to test & theres others float-n around.
    The maniflod is done, mocked up with all fasteners & the carb bolted up. If I learnt anything about fab-n, the parts need to be made to fit as well as bolt patterns worked out so they dont interfere. That determines whether or not its worth doin as much as the parts workin.

  10. #10
    Team Member JohnsonM50's Avatar
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    Thumbs up Start test

    After some leak chasing...

    I mounted a spare powerhead on an old tower, roped to a stand [no clamps]

    Made up a couple quicky gaskets & assembled it.

    It started right up after a few 'choked with a rag pulls', idled sorta high, [the timings full]. It rev's nice..Wicked nice! So on to testing on water. Hose tests always sound good.. I know, but it shows promise as is.

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