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Adjusting twin carbs

Old 06-26-2010, 08:32 PM
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chestervw
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Default Adjusting twin carbs

Need some help, (info), on adjusting the carbs on the new RCMK TWIN INLINE MOTOR to get them syncronized. Any help or information would be greatly appreciated.
Old 06-26-2010, 09:38 PM
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Justaddwata
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Default RE: Adjusting twin carbs

Hi Chester - Welcome to the forum!!

I have no experience with the RCMK (though not many do at this point i guess) but the way I have been doing it with zenoah inlines is setting the stock carbs to factory basepoint to get it running and tweak by reading plug color (and checking fuel consumption) between the two motors. Running a pair of tanks (one per carb) affords you the opportunity to compare usage (which should be close). You should also ensure that both carbs are throttling at the same rate as this will also alter the fuel consumption.

error on the side of rich (especially with a new motor).

You might also ask the dealer for starting carb settings for the twin.
Old 06-26-2010, 10:22 PM
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Scott Schneider
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Default RE: Adjusting twin carbs

Just finished setting up a new S-520 twin with modified J&G cylinders .... and frankly the coupler between carbs used as the synchronizer is marginal at best IMO.
But aside from my opinion on that subject can share how some of the more basic twins threw the years have been done

Rather than having sync at closed throttle, place an @ 3/16" shaft into carbs throat lightly closing butterfly against it ( be sure butterfly hits the shaft at same location on each carb )... do this at the same time on both carbs. Then lock the two shafts set screws of coupler.
** This is synchronizing .... Mixture being equal on each carb is done the old manual way.
Lightly seat all 4 needles, bump them on and off there seats lightly a few times so all is well seated ... now
Take the low needles ( Closest engine ) out EXACTLY 1 1/2 turns
Take the high needles ( Closest intake ) out exactly 1 1/4 turns.

You very likely won't need to alter the low needles from this setting if near sea level to @ 2000' ... so for now set them and forget about them for now.

Once you have your running engine going roundy round on the lake and find your wanting a richer or leaner setting, just move the HIGH needles EXACTLY the same amount until engine runs like your needing.
That done your ultimate fine tuning of each cylinder will be by plug color using the HIGH needle. Any subtle changes in idle quality or rich load up throttling up from low rpm can be done with the low needles, together and in equal movement.
Old 06-27-2010, 07:01 AM
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chestervw
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Default RE: Adjusting twin carbs

Forgot to mention that the catbs are 180 degrees from each other, (facing opposite directions), the cylinders are rotated so the exaust will come down both sides.

Thanks, Chester
Old 06-27-2010, 09:52 AM
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lapistola28
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Default RE: Adjusting twin carbs

There is a product called ColorTune . I have used it on Car engines with multiple carbs . It along with a small flow meter placed over the mouth of each carb to ensure the same throttle action bettween both carbs . It's really simple with the right tools ...... They aren't very expensive either ...
Old 06-27-2010, 10:29 AM
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Scott Schneider
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Default RE: Adjusting twin carbs


ORIGINAL: chestervw

Forgot to mention that the catbs are 180 degrees from each other, (facing opposite directions), the cylinders are rotated so the exaust will come down both sides.

Thanks, Chester

Better yet once your TIGHT linkages are set up you can then adjust throttle position using the round rod trick.
** Just make sure your throttle linkage design DOES NOT open or close throttle positions even slightly as engine torques within mounts in any dirrection.
If throttle plate positions change independently with movement you will NEVER get a smooth running engine.
Old 06-27-2010, 03:04 PM
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chestervw
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Default RE: Adjusting twin carbs

Thanks for the information guys.
Old 06-27-2010, 08:05 PM
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Default RE: Adjusting twin carbs

ORIGINAL: Scott Schneider
Take the low needles ( Closest engine ) out EXACTLY 1 1/2 turns
Take the high needles ( Closest intake ) out exactly 1 1/4 turns.

Are you talking factory 929's or some other carbs? Sounds like 771 or 813 numbers.
That in my boats with RCMK's with 929 carbs would surely be overly lean. I use 2 LOW and 1 - 1 1/8 High and certainly not too rich. Maybe my 10oz. Of Maxima per gal gas is thicker than the fuel you are running. Just seems awful lean I would surely fry my engines in one bag at those settings if it even ran. They aren't twins but don't see that changing the settings a bunch. Better to ere on the rich side, it won't burn up your engines.
Old 06-27-2010, 08:10 PM
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Default RE: Adjusting twin carbs

If the carbs are 180 degrees I would run a carb with the throttle stop reinstalled as I believe that one has been removed from one of the 520's original carbs and use the throttle stops at wide open to synchronize the carbs. Of coarse it is imperative that all servo horns and bellcranks making there way to the throttle shafts are all operating at the same ratio or you will never get them to sych. Nylon crossshaft horns or bellcranks don't work well on twin carb setups as the flexing alone will have them out of whack.
Old 06-29-2010, 05:34 PM
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rcuheadhunter
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Default RE: Adjusting twin carbs

Chester,

Bring that bad boy with you this weekend. Everyone wants to see it and hear it.
We'll set it up on a twin SU linkage with some bellcranks like an old Spitfire!

FH

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