Mikuni carb size and Jet size

Discussion in 'Half VW' started by Tuneturkey, Mar 26, 2019.

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  1. Mar 26, 2019 #1

    Tuneturkey

    Tuneturkey

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    Need some help with sizing the jets for my 32mm, carbs. Engine is a 1/2 VW, std. crank, 92mm cylinders, set up for 2 carbs. Intent is to derive about 32hp @ 3500RPM.
    My two carbs come from different sources, and the jets under the plug below the bowl have different numbers on them, 250 and 280. The mikuni website lists jets up to 500, with lots of discussion, more tuned to motorcycles then 1/2 VW's.
    Need some help from anybody who has gone through the process of elimination in selecting a jet which will give the proper mixture for both takeoff power, and reasonable cruise speed, with EGT 1200-1300.
    it has come to my attention that many are useing 28mm carbs in lieu of the 32mm, so hopefully I make the 32's serve the purpose by adjusting the jet sizes.
    Also, any comments on the subject matter will also be welcome.
    Thanks,
    Johnc
     
  2. Mar 26, 2019 #2

    Steve hurst

    Steve hurst

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    According to Leonard millholland, the 32 hp version needs to be dual 28 mm’s. The 32 mm in his plans (legal eagle) are incorrect. You might call him and ask for the jet size, I’m not sure. When you find out please post. His phone number is on his website I think.
     
  3. Mar 27, 2019 #3

    Tuneturkey

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    Steve - I spoke with Leonard about it, and he gave me the same info. His plans for conversion, which I purchased, provide info for 32mm mikunis and in addition, call for drilling out the jets with a #60 drill for full power. Based on that, I purchased the 32's and surely would like to use them if at all possible. Surely the main jets on the 28's are same as on the 32's, at least seems that way, since the only difference between the 28 and 32 is the slide cyl. diameter. Per leonard, the 32's pull too much air if the throttle is advanced to rapidly messing up the mixture. As for the #60 drill, that's a 0.04" dia hole, however, I have not found source that correlates the 0.04" j to a specific Jet size.
    I also have a set of Conversion plans from Simplex Aero who also recommends VM32-282, however, I am not sure if the 282 has anything to do with jet size. So far as I know, jet sizes are in increments of 5 or 10, which would invalidate the "282" as jet size. usually the second number relates to the length of the spigot, but a 282mm long spigot would be wierd!
    Can you tell me what size jets you are using on your 28's. At least I could use that as a starting point. As i mentioned in my post the sizes on the carbs are 250 and 280 stamped on the rim of the jet.
    Thanks for the help!
    johnc
     
  4. Mar 28, 2019 #4

    fly2kads

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    Generally speaking, it seems better to err on the small side when selecting a carburetor. The reason being that fuel metering is driven by the vacuum signal. If too large a carburetor is used, the velocity through the carb may be too low to produce the required pressure drop through the venturi. If the vacuum is too low, you're not going to pull enough fuel out into the airstream.

    This would be one possible explanation for Leonard's comment about messing up the mixture. Too low a vacuum would make it hard for the carb to respond to the changing fuel requirements. If the mixture was okay at those midrange RPMs in a steady state, but lagged badly on acceleration, that would seem to confirm that hypothesis.

    My understanding is that the numbers after the dash in the Mikuni carb number (your -282) is a code for the carb's configuration. This would be analogous to what Lycoming does with their engines (e.g. O-320-H2AD). This indicates things like flange vs. clamp mounting, left or right side idle screws, etc. I haven't decoded the list of options, though.

    I have been reading up on the Mikunis for a different application, but haven't bought one yet. The good news is that those VMs, in particular, can be finely tuned. The bad news is that the different jet and needle options makes for a big matrix of possibilities, so there is a steep learning curve.

    I don't know if you will get a satisfactory "cookbook" kind of answer here, but I hope so. You may need to go through the same tuning procedures that the motorsports people do when setting up a race motor. There are some good articles online on how to do it. If worse comes to worse, you can hire that job out to a motorcycle tuner in your area.
     
  5. Mar 28, 2019 #5

    pictsidhe

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    Mismatched 'pair' of carbs is likely to be a huge headache. You need to match every jet and mixing tube, or your engine will run unevenly. Unless they have identical codes, I'd suggest selling them and buying an actual pair. I'll be using CVs from something, they have inherent altitude compensation.
     
  6. Apr 27, 2019 #6

    Tuneturkey

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    Great - What constitutes a "code", the number stamped on the part, if any is present? You mention "CV's", what are you referring too?
    Tuneturkey
     
  7. Apr 28, 2019 #7

    Turd Ferguson

    Turd Ferguson

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    CV = Constant Velocity
     
  8. Apr 28, 2019 #8

    Turd Ferguson

    Turd Ferguson

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    I used to have a tackle box with Mukuni tuning parts in my motorcycle days. The "motorcycle type" Mukuni carburetors had a number stamped on the main jet. The number corresponded to how much fuel (in cc's) could flow through the the orifice in one minute. There is definitely no standard for numbering metering jets. Some manufacturers, the jet size corresponds to the dia of the orifice.
     
  9. Apr 28, 2019 #9

    pictsidhe

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    The carb code I am referring to is on the body somewhere. I haven't had my paws on a mikuni for years, but something like vm32-123a The vm32 is the basic type and size, the 123a ia a cryptic code that refers to that carbs particular set of main, needle, air and idle jets etc.
    CV carbs are 'constant velocity'. Unlike slide carbs, they have some inherent altitude compensation. That seems a jolly good idea for an aeroplane.
     
  10. Apr 29, 2019 #10

    rickofudall

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    One thing you might want to consider if it does not have it is a balance tube between the two intakes. Both the Legal Eagles built at 18KS did not have them as delivered. The HKS and the Rotax 9 series both have them. It helps balance out the two cylinders so there is less of the tendency to be two one cylinder engines with a common crankshaft. Another benefit is having a port for synchronizing the two carbs.
    As for finding the right jets there surely must be similar engines from which you can make a start. With all the needle jet controlled midrange don't forget that the clip on the needle is your best tuning device once you get close. I've taken too many carbs apart (Bing 54 on Rotax engines) and found some home brewed combination of jets that caused the owner fits trying to tune them. I put them back to stock and raise the clip one notch for local field altitude and voila, the engine runs fine.
    Same with the Bing 64 on the 912. A fellow at Stearman field was dissatisfied with the fuel consumption on his Sport Cruisers ( he was a dealer for the plane). He was getting advice to change the main jet. I advised him not to and told him I could probably get results with a minor jetting change. I raised the clip one notch (lowering the needle into the needle jet) and we took the airplane up to check. Fuel flow showed a change of .8 lower GPH. Needless to say he had me do the second plane too.
    I had the same experience with the HKS engine on my trike. When HKS came out with a service bulletin requiring the needle clip to be set at its lowest groove for better exhaust guide wear I went the other way and raised the clip one notch. I run 1 oz of MMO per gallon to lube the valve guides rather than burning a gallon an hour of fuel to do the same thing. 300 hours on the engine and no sign of valve guide wear.
    While I'm on it, don't be too concerned about making sure the slide pulls all the way up in the bore when setting up your throttle linkage. I got to wondering how Rotax makes the Bing 54 run properly on all its two strokes so I did an experiment. In flight I opened the throttle on my 582 powered Kolb Mk III for maximum take off throttle then shut off the engine and glided in. When I pulled the air filters off I was surprised to see that the slide was only 3/4 open. I built a new throttle lever taking this into account and the result was that I no longer had to hunt for a throttle setting. Throttle lever all the way to the stop gets me 6100 rpm for take off and a slight adjustment from that gets the engine to 5800 rpm for cruise. Such a slight change resulted in not having to hunt for throttle settings anymore and made the plane much more fun to fly.
    If you haven't found the jetting in the next two weeks I'll track down the two 1/2 Vdub flyers at our annual HOA meeting and see if they still have their jetting written down somewhere (they've both sold their Eagles). I remember that they both had the same engine and were always amazed that the two engines ran so differently. They are both A & P rated and one is an IA and I think the difference in the two engines played a big role in their deciding to sell the aircraft.
    Anyway, good luck in your adventure.

    Rick
     
  11. Apr 29, 2019 #11

    Tuneturkey

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    Rick - Thanks for the input. No, This is the first "meaty" response i have had to the post, and i am no further along on the jetting issue.
    You mention a "port" for syncing the two carbs. Would you elaborate on that, please!
    Actually I have two basic issues. 1. The Mikuni VM32 is a three, RPM control band device (IMHO) The idle range and mid range adjustments have nothing to do with the main needle and jet, however, having them identical seems prudent, since the two carbs running in parallel should share equally in the load. Don't know how important that is at idle and up to 50% speed or so. Comments would help. 2. at cruise and max power, With the higher engine stresses, sharing the load is paramount to full output (IMHO) so tuning of the jet, needle, whatever, is my goal. With the wide range of available main jet sizes, and needle sizes, Someone out there that is running a 1/2VW at 32hp with two VM32's should have gone down this path before. Finding out what the 250 or 260 number stamped on the jet specifically refers to, be it dia. of the orifice, or what is difficult, and a recommendation for a specific size to at least get in the ball game is not happening. Leonard MilHolland in his conversion plan using a VM32 recommends that the jet be drilled out with a #60 drill bit which is a known dia., however, he no longer recommends the VM 32, in favor of a VM28.
    Leonard's VM28 recommendation is based on engine response to rapid throttle advance whereby, the engine lacks sufficient vacuum to provide proper fuel to match the increase in air, and the engine hesitates, or stalls. If thats the case, I have VM28s in my future.
    In full recognition of the term "experimental" , I am proceeding with my VM32's and plan to use identical jets, run the engine on a test stand, and see how it responds. If you have any other recommendations for "syncing" in the cruise and full power RPM range, I am all ears.
    Tuneturkey
     

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