Carburetor main jet holders (diaphragm springs)

Discussion in '1st & 2nd Generation 1983-1989' started by DKC'sVFR, Aug 13, 2009.

  1. DKC'sVFR

    DKC'sVFR New Member

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    I have successfully rebuilt a set of Vf500f carburetors only to stumble onto the following perplexity regarding rebuilding a set of Magna V45 carburetors. Making a generalization from the Vf500f carburetors I took all the internal parts of the Magna carbs apart and put them in the carb cleaner basket only to later realize that the V45 "main jet holders" are not alike. This is also true of the diaphragm return springs. The different spring lengths can be understood by virtue of the gravitational pull on the different carb pistons in the carburetor. But which is it? Do you need the longer spring in the side draft carbs (read FRONT) because they are not aided by gravity like the down draft carbs (read REAR). Seems to make sense :confused:. And then there are the main jet holders :crazy: ??

    There have to be some old hand Vf carburetor rebuilders (like MasonV45 and some others, YOU KNOW WHO YOU ARE) who have a logical explanation for the difference between the FRONT and REAR carb holders. How about sharing it with me and the rest of the First & Second Gen crowd. I don't want to guess when I put the V45 carbs back together. I had a 'bike stripper' show me a picture of V65 carburetor main jet holder differences (with an explanation of the proper placement of each type), but I didn't copy it or write it down and my memory is not as good as it used to be. I am not going to even hazard a guess as to why one holder goes here or there instead of in the opposite position. Just be brilliant and tell me which goes where and why, if you know why. PLEASE
     
    Last edited: Aug 13, 2009
  2. squirrelman

    squirrelman Member

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    The set of VF 500 carbs i just yesterday finished cleaning/rebuilding had springs all the same length......

    main jet holders were different front/back as you describe.

    different number and placement of holes may be due to horizontal or vertical placement angle of float bowls.
     
  3. DKC'sVFR

    DKC'sVFR New Member

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    HELP The Vf 500 and the Vf 750 are different.

    Hey Squirrelman, I need someone with a lot of Vf750/1100 (C or S) wrenching experience to take my hand and explain where and why the main jet holders on these early V four bikes are configured as they are. No time to guess at it. I need HELP!!!!

    You need to take a look at a 82-8 Magna V45(750) or Magna V65 (1100), or a Sabre V45 or V65 sometime. I'm not sure about the 83-85 Vf 750f Interceptors, but I'd bet they are similar to the Magna/Sabre model Vf 750's. Don't let the nomenclature get you confused, the V45 stands for 45 cubic inch displacement, which is @ the same as 750cc and likewise the V65 stands for 65 cubic inch displacement which is @ 1100cc. When you do look you will notice that the engine cylinders are turned so that the centerline between the two cylinder banks has been rotated forward, toward the front of these bikes by about 35-40 degrees. This was probably done to get a best fit in their "cruiser" style frames. This results in the front cylinders being more or less parallel to the ground and the rear cylinders being perpendicular to the ground. This results in the front cylinders having "downdraft" carbs and the rear carbs having "sidedraft carbs". But if you take the carbs off the bike they have more or less the same carburetor bodies.

    So how do they get them to work. They put long springs in the front carb diaphragms to compensate for the lack of gravity pulling down on the pistons in the down draft carbs (think about it). And they put the holes drilled in the main jet holders (air mixture perforations or emulsion tubes) at different locations on the back and front jet holders. You can think about that too and you might come up with a logical explanation for why this or that jet holder goes into a front or rear carburetor. Never mind the guessing. You want to know exactly where which kind of jet holder goes in which kind of carb before you put it back together. Logic be damned. It takes two or three knuckel busters to get the jets and holders out if you put them in wrong. No thanks. I prefer to get it right the first time.

    Whole truck loads have been written about rejetting carbs. But narry do I see an article on the fact that the carbs on these early V fours have different jets and holders depending on where they reside on the engine. How many bozo's have rushed into "getting it done" with the holders and springs messed up? Not this bozo if possible.

    If you take the time to look at the (86-7 VFR 700) you will notice that it's cylinder banks are set symetrically about the vertical line between the cylinder banks. So the first VFR's have similar carb internals, much like the Vf500f that you are familiar with. I am not sure if succeeding VFR models (750's and 800's) continued to use a symmetrical cylinder bank configuration, but I would suppose that they did.
     
    Last edited: Aug 13, 2009
  4. squirrelman

    squirrelman Member

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    You're looking for a theoretical explanation that would best come from a gas/fluid dynamics Ph.D.....?

    I'd suggest that you might find your answer after studying the best book on carbs i ever found : John Passini's "Weber Carburetors--Tuning and Maintenance"

    Since Weber makes both downdraft and sidedraft setups you might discover something in the emusion tube discussion to provide your answer.

    It's probaby evident that hole placement is related to fuel/air level within float bowl.
     
    Last edited: Sep 13, 2009
  5. DKC'sVFR

    DKC'sVFR New Member

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    The full story has been handed down

    Masonv45 :thumbsup: or a VFRWorld tech guru @ VFRWorld computer central :hail: has sent me a private message regarding the "FINAL WORD" on these carb issues :biggrin:. They are as follows:

    1. The main jet holder with the emulsion tube perforations furthest from the main jet go in the REAR CARBs.
    2. If different needles are installed front to back, the thinner tipped needles are installed in the REAR CARBs.
    3. If different main jets are installed front to back, the larger main jets go in the REAR CARBs.
    4. If different vacuum piston springs are installed, the longer springs go in the FRONT CARBS. The short springs go in the REAR CARBs.

    It's hard to say why the emulsion tubes et.al. are different :nerd:. But if you think about it, the fuel bowl fluid level/position is a gravity issue, like the piston issue that I referred to earlier. However, I am sure that there are other contributing factors like heat of the head (rear cylinders run hotter) and the whole issue of fuel atomization (gravity mixed with gas diffusion). But that discussion is moot. I know where everything goes even if I don't know why :rockon:. Enough talk, back to action :flame:. DKC
     
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  6. kennybobby

    kennybobby New Member

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    Springs, Jets and Emulsifier Tubulars

    Over the weekend i was cleaning and rebuilding a couple of sets of carbs used on the 84 VF700, one was VD52B, the other VD52D. i had similar questions and found this thread, and would like to add my findings.

    Looking at the Honda VF700 Service Manual the shorter springs go on the rear bank, which agrees with #4 above. Also the smaller main jets go on the rear bank, which conflicts with #3 above. All the needles were the same taper, OWJ, so nothing to say about #2 above.

    The front holder tubes had 5 cross-drilled holes in one axis and 1 in the other axis. The rear holder tube only had 4 holes in one axis, and they are further from the main jet, which agrees with #1 above.

    The interesting part was finding that there seems to be no reason for the cross-drilled holes in the main jet holder (aka emulsion tubes), because there is no air port or air jet feeding air to the main jet circuit--it is blocked off or was never drilled out.

    So why the different hole patterns in the holder tubes?

    btw, i've seen Mikuni sidedraft carbs that are very similar design to the Keihins which had no cross holes in the main jet emulsion tubes--guess emulsion tubes were being phased out without anyone noticing...

    The carb body casting has internal tubes and passages that are drilled and filled with little brass plugs in order to complete the various circuits--it looks like the plumbing is there to have an air feed to the main, but for some reason it is not being used. Anybody want to cut one open? i want to see why it's missing.

    Maybe the Honda manual is wrong? It indicates to put the stiffer (shorter) springs in the rear bank with the thinner needles and the smaller main jets (110), and the softer (longer) springs in the front bank with the fatter needles and the larger main jets (112). Seems counter-intuitive, but they say the front bank runs hotter than the rear, hence the larger jets in front.

    For a given airflow thru the venturi, the longer springs will let the needle rise higher than the short springs, so that is an enriching move to add extra fuel to the front bank. Thinner needles help enrichen the rear bank at WOT. If the emulsion tubes actually had an air feed, then that would be an additional variable available for tuning the mixture at partially open throttle settings.

    Lots of tuning variables--maybe they got lazy on dyno day, found something that worked and stopped there. It's a lot more trouble to pull a set of carbs on a vee motor than an inline, takes longer to make changes, etc... e.g. the cbr900 carbs can be popped over to change the jetting with all the cables attached, about 10 minutes to pull the bowl covers, replace mains/pilots, and back on and running, versus 30-45 minutes to do the same on a VF rack.
     
    Last edited: Jul 24, 2011
  7. ligurt

    ligurt New Member

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    I know it's an old thread but still of some interest to me? I have a vf400f that has different needles front and rear named 99p and 99q respectively. Main jests are 85 all around, but HRC manual indicates 115 and 118 front and rear in accordance to what is logical about hotter rear cylinders. I actually didn't check emulsion tubes and needles shape, but I'll give a look tomorrow and be back to you. On the 400greybike forum I found something about the vacuum piston hole that seems related to the speed the engine rev up. Have you any report about it?
     
  8. squirrelman

    squirrelman Member

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    F/R main jetting difference isn't a fixed rule. for example, the honda hawk nt650 uses a larger jet in the front cylinder and slightly smaller at the rear of the V.

    Picture1-12-2020 199.jpg
     
  9. ligurt

    ligurt New Member

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    Yes indeed. In any case I stripped the carbs today and I can report a 10% thinner needle at the rear side all along the needle lenght. Springs are 2 of the same lenght and two slightly longer but different. In addition they were one on the front side and one the back. So it could be due to the age of the bike. I drilled the vacuum holes from the original 2mm to 2,5 and made a minor mod to the needle retainers to lift needles 1,5 to the back and a 1mm to the front.
     
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