Newbie needing help with Mikuni Super BN carbs on a V8.n
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  1. #1
    PWCToday Newbie
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    Newbie needing help with Mikuni Super BN carbs on a V8.n

    Hi All

    Joined site for your expert's help!

    I am from UK and have built a race car fitted with an Evinrude V8 - 3.6 litre engine as per photo attached.

    I have it running BUT cannot get the eight Mikuni Super BN 44-40 carbs tuned.

    These are all new carbs fitted with "std". jets

    At startup the 120 Pilot and 260 Main jets were obviously way too large.

    Calculations indicated that for 37hp per cylinder, I should be using 80 Pilot and 100 Main jets which I have fitted. Engine ticks over at 1000 RPM OK, but refuses to load up at 2000+RPM - just spits & coughs?

    Can anyone help with "Theoretical" jet sizes needed for a 450cc capacity cylinder producing circa 40 hp at 7000 RPM - i.e. 320hp from 8 cylinders.

    ANY help/advice appreciated to get the engine running ready for fine tuning.

    Peter2012-07-11 03.27.25.jpg

  2. #2
    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

    I would think that should be real close to Kawi 550 jetting. Try 100 pilots and 125 mains. Popoff pressure should probably be pretty low, like 15-20 psi. If your popoff(spring) pressure is too high, try lowering that first.
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  3. #3
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    Re: Newbie needing help with Mikuni Super BN carbs on a V8.n

    Quote Originally Posted by Myself View Post
    I would think that should be real close to Kawi 550 jetting. Try 100 pilots and 125 mains. Popoff pressure should probably be pretty low, like 15-20 psi. If your popoff(spring) pressure is too high, try lowering that first.
    Hi Myself - thanks for posting. ??? Is the Kawi 550 a single or twin cylinder?. If two does it have one or two carbs with 100 and 125 jets?. I am running pre-mix std pump fuel at 40:1. Oil is:Motul 800 2T Factory Line Ester Synthetic Racing Motorcycle Engine Oil. I have 17 psi pop off. With 80 pilot and 120 mains the engine starts, tick over circa 1000 rpm and OK up to 1/8th throttle. Above that throttle opening it will not run cleanly. I have oiled a number of plugs. The oil is low smoke but have excess on the 120 jets. Everything indicates I need smaller main jets. A set of 8 main jets in UK is US$50 here in UK. Just trying to get closer to required without wasting money! Any further info/thoughts appreciated.

  4. #4
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    Re: Newbie needing help with Mikuni Super BN carbs on a V8.n

    Each Cylinder has about 450cc displacement. This would be similar to a 2-Cylinder Yamaha GP800R which has Dual 44mm Carbs (400cc Cylinder with one 44mm Carb).

    The 800R stock jetting is #90 Pilot Jet and #150 Main Jet with a restrictive Air Box. These jets should be the absolute minimum sizes.

    I would start with a #100 Pilot and a #155 Main. I think the pop-off is low enough, could be too low if the engine shakes and vibrates too much, the Needle Valves will leak. Maybe somewhere 'tween 20-25 psi Pop-Off might be better.
    Last edited by JC-SuperJet; 09-24-2020 at 07:30 AM.

  5. #5
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    Re: Newbie needing help with Mikuni Super BN carbs on a V8.n

    No need to waste a bunch of money on jets when trying to dial in an unknown combo. All you need is a set of numbered drill bits and some plumbing solder. Drill bigger, solder and redrill to go smaller. I have a chart for reference that puts the numbered jet close enough to drill bit size that the adjusters can take care of the rest.
    http://www.facebook.com/twinlakesjetskirepair

    '89 Kawi 650SX 42.4 GPS
    +3" stock pipe, ported ex. manifold, drilled waterbox, blueprinted pump, Ocean Pro ride plate

    '99 Yama GP1200 65U 61.8 GPS
    ported cylinders, matched cases, milled head, blueprinted pump, long ride plate

    '96 Seadoo HX 717 53.13 GPS
    light porting, massaged cases, lightened flywheel, port matched manifolds, Rossier pipe, stubby pump cone

  6. #6
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    Re: Newbie needing help with Mikuni Super BN carbs on a V8.n

    Kinda disagree with most of what has been said so far.

    The 550 is a twin cylinder with a single carb, which is a very different situation than one carb per cylinder.

    The GP800R uses I-Body carbs that are completely different.

    The thing with the drill bits is probably perfectly valid advice if you have a single carb, but since your drilling is almost certain to be imperfect it means you'll probably have inconsistencies between individual carbs. Maybe still okay when you're just doing very rough "first try" type adjustments. Also the sizing of the Mikuni jets is the size of the bore in millimeter. For example a 150 main jet is 1.5mm ID, a 110 main jet is 1.1mm ID.

    Lots of things are going to affect the jetting but the closest comparable motor would be a 760 yamaha (or 1200 yamaha) which has a 44mm carb per cylinder at around 380cc's per cylinder, but still overall a hell of a lot different than what you have there, and the oem Yamaha 44's on that motor are at least SBN's but they are slightly different than aftermarket sbn's that you are using.

    Based on my expirience with a 780cc motor with bn44 carbs I think your pilots are too lean, I'd be looking at more like 120 on the pilots and maybe 120 on the mains but you'll likely end up going a little leaner than that.

    But it's all a guess. Soooooo I'm no help either!

    That thing is cool as hell though!

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

    Good test for pilot size is this.....adjust idle rpm, turn low speed adjusters out 1/4 turn at a time. If you make it to 2.5 turns out and idle speed is still increasing= need bigger pilots. If idle rpm drops but raises as you turn adjuster in towards closed then pilots are too large. If the pilots are very close to optimal then around 1 turn out will produce a good idle....unscrewing will start to drop rpm and blubber rich.....screwing in might pick u some rpm for 1/4 turn then start losing rpm as you go towards closed as you're shutting idle fuel down. When it's all dialed in very close I use a digital tach and fine tune each cylinder for highest rpm plus 1/8 turn richer for best throttle response. If there are no air leaks all adjusters will be within 1/4 turn of each other when finished.
    http://www.facebook.com/twinlakesjetskirepair

    '89 Kawi 650SX 42.4 GPS
    +3" stock pipe, ported ex. manifold, drilled waterbox, blueprinted pump, Ocean Pro ride plate

    '99 Yama GP1200 65U 61.8 GPS
    ported cylinders, matched cases, milled head, blueprinted pump, long ride plate

    '96 Seadoo HX 717 53.13 GPS
    light porting, massaged cases, lightened flywheel, port matched manifolds, Rossier pipe, stubby pump cone

  8. #8
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    Re: Newbie needing help with Mikuni Super BN carbs on a V8.n

    Quote Originally Posted by Myself View Post
    Good test for pilot size is this.....adjust idle rpm, turn low speed adjusters out 1/4 turn at a time. If you make it to 2.5 turns out and idle speed is still increasing= need bigger pilots. If idle rpm drops but raises as you turn adjuster in towards closed then pilots are too large. If the pilots are very close to optimal then around 1 turn out will produce a good idle....unscrewing will start to drop rpm and blubber rich.....screwing in might pick u some rpm for 1/4 turn then start losing rpm as you go towards closed as you're shutting idle fuel down. When it's all dialed in very close I use a digital tach and fine tune each cylinder for highest rpm plus 1/8 turn richer for best throttle response. If there are no air leaks all adjusters will be within 1/4 turn of each other when finished.
    Hi Myself - Now thats a very good guide, something that is indicative of correct pilot jet size. Any similar guide for the mains. The race season is now finished here so getting ready for re commissioning after the winter.

  9. #9
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    Re: Newbie needing help with Mikuni Super BN carbs on a V8.n

    Quote Originally Posted by Storbeck View Post
    Kinda disagree with most of what has been said so far.

    The thing with the drill bits is probably perfectly valid advice if you have a single carb, but since your drilling is almost certain to be imperfect it means you'll probably have inconsistencies between individual carbs. Maybe still okay when you're just doing very rough "first try" type adjustments. Also the sizing of the Mikuni jets is the size of the bore in millimeter. For example a 150 main jet is 1.5mm ID, a 110 main jet is 1.1mm ID.
    Hi Storbeck - Thanks for posting. I agree about soldered jets. I soldered a set of 260's that originally came with the stock carbs and drilled 1mm =100 jet. These would just not run equally across the 8 cylinders. 3 plugs oiled, the other 5 came out in various states of being "Wet". Unfortunately the race season has finished but getting ready for re commissioning after the winter. Have purchased a set of 100's and will start from there.

  10. #10
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    Re: Newbie needing help with Mikuni Super BN carbs on a V8.n

    Forgot the other part of your question. The Kawi 550 is a twin cylinder making between 36 and 41hp depending upon the year model. That's with a single carb. We're all just guessing but since hp is about the same, guessing fuel requirement would be too. You can't know for sure on the mains until the pilots are correct. From mid throttle on the pilots are added to the total jetting. So, pilots first, then mains, then fine tuning, then laugh at your competition. I spent many many evenings a couple years ago dialing in an FCR carb for a Raptor 250 race quad. Between pilot, main, needle, and acc pump.....using plug reads, seat of my pants feel, and O2 readings......I had it close enough that when he took it to the cycle shop to run on the dyno, the guy couldn't believe how close the a/f ratio was throughout the throttle range. The kid was a great rider and would routinely wipe up 450r machines. I know how big of a headache you're in for but trial and error will always get you there in the end.
    http://www.facebook.com/twinlakesjetskirepair

    '89 Kawi 650SX 42.4 GPS
    +3" stock pipe, ported ex. manifold, drilled waterbox, blueprinted pump, Ocean Pro ride plate

    '99 Yama GP1200 65U 61.8 GPS
    ported cylinders, matched cases, milled head, blueprinted pump, long ride plate

    '96 Seadoo HX 717 53.13 GPS
    light porting, massaged cases, lightened flywheel, port matched manifolds, Rossier pipe, stubby pump cone

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