Newbie needing help with Mikuni Super BN carbs on a V8.n - Page 2
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  1. #11
    PWCToday.com Is My Home Away From Home fastgtfairlane's Avatar
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    Re: Newbie needing help with Mikuni Super BN carbs on a V8.n

    ive also seen where too large a main jet can affect the low speed circuits too. when you try to rev it out, does it gurggle or want to shut off?
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  2. #12
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    Re: Newbie needing help with Mikuni Super BN carbs on a V8.n

    Ditch the SBNs and get some VM carbs. That's what all the Thai Long-tail river boat dudes do.
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  3. #13
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    Re: Newbie needing help with Mikuni Super BN carbs on a V8.n

    you'd have to mount the engine 90 degrees, so the VM floats work correctly
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  4. #14
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    Re: Newbie needing help with Mikuni Super BN carbs on a V8.n

    Some more thoughts, and please keep in mind I'm just throwing this stuff out there I'm no expert.

    Take a look at this thread:

    http://www.x-h2o.com/threads/importa....176798/page-2

    With the very odd combination of parts you have there, and the large number of carbs that you are simultaneously tuning, it might not be a bad idea to drill out the individual restrictors in the carbs and combine and tune the return restrictor size. How many to group together I'm not sure (all into one restrictor? maybe combine the two cylinder banks? I don't know) but at least being able to tune the fuel pressure might be beneficial especially if you are having any issues with consistency between cylinders.

    What kind of exhaust are you running on that thing? I'm wondering if it is something that would be likely to result in differences between cylinders and thus need different jets for different individual cylinders. Also ignition?

    Do you have individual pulse lines for every cylinder/carb? What kind of fuel pump setup, are the carbs drawing directly from the fuel tank or is there some "boost" pump somewhere?

    Post more pics please just because I'm fascinated by this thing and I want to see more pics.

  5. #15
    PWCToday.com Is My Home Away From Home Myself's Avatar
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    Re: Newbie needing help with Mikuni Super BN carbs on a V8.n

    Storbeck, not a bad idea at all, balancing the fuel pressure. I think I would do left and right bank seperate. Probably need in between a 100 and 120 size jet orifice to get wide open fuel return pressure around 4-6 psi.
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  6. #16
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    Re: Newbie needing help with Mikuni Super BN carbs on a V8.n

    Hi All for your posts. I will try to answer the various points raised plus a little history. I am 76 years old and tuned my 1st two stoke 60 years ago, a 200cc Go-Kart gearboxed engine. In those days it was "Stuffing" the crankcase to increase transfer pressure, installing extra external transfer ports, converting to "Full Circle" crankshaft webs instead of the T shape and then we had the expansion chamber arrive which really sorted the engine. Since retirement I have built 4 race cars. The ist one has a 500cc Rotax twin and I used a gearbox, the 2nd was a F500 single seat car I bought in USA and fitted a 600cc Rotax twin (tuned by Olaf Aaen in the USA to 135 hp) + a CVT transmission from him. Then, to learn about fuel injection, I converted a 3 litre V6 4 stroke engine to full EFI which I put into a Marcos GT sports car. Then a 7 year project to build the current V8 Sports car - 4 years thinking - 3 years making and now 1 year trying to get it working correctly.
    Onto forum answers:
    1. Why SBN carbs. Race cars turn over with driver strapped inside and engines are hot. With 8 float chambers of petrol escaping, fire is a big risk, hence diaphragm type to reduce risk.
    2. I have two external fuel pumps each feeding a bank of 4 carbs. I flow and return the fuel to/from the SBN "Chamber" at 5psi. The Mikuni pumps then transfer fuel required to the jet chamber. Once the engine is running, I can
    turn off the external pumps and there is no noticable difference to the apparent richness I am experiencing. The external pumps ensure all fuel lines are full and then the Mikuni pumps work correctly. With the external pumps
    and engine running, the fuel supply pressure is 4 psi.
    3. The low speed circuit runs OK, ticks over at 1000 RPM OK, problems start , not running clean, when I open throttle above 1/4 i.e. main jet circuit.
    4. Running a CVT transmission, I have it set to engage (grip) at 3000 RPM, so I am only really interested in 3000 + RPM operation.
    5. Thanks Storbeck for the H-h2o link, great info.
    6. To incorporate the CVT primary pulley, I had to remove the original flywheel, rotor, and CDI ignition system. I then built my own ignition using 8 "Hall" sensors onto a circular disc which rotates to give ignition timing between
    the 4 degree BTDC for starting and 16 degree for running above 2500 RPM. (Degees as per Evinrude manual). Via a M & W ignition module (from Australia) I signal each of the 8 coils to produce a 45000V spark to try and keep
    the plugs clean.
    7. If anyone is interested in the project, I have produced: http://www.uphillracers.com/showthread.php?t=133976. It covers the complete build process.

    So extended answer(s) but they may trigger a memory of you experts to lead to a solution. I think its a matter of test - test - test. Here is the car running slowly at 1st event.
    Shelsley.jpg

  7. #17
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    Re: Newbie needing help with Mikuni Super BN carbs on a V8.n

    Having re - read the: http://www.x-h2o.com/threads/importa...176798/page-2: post its identical to the problems I have, so looks like a solution.

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