Help! 1984 Honda V30 Magna

Discussion in 'Motorbike Technical Discussion' started by Dave Best, May 20, 2005.

  1. Dave Best

    Dave Best Guest

    I'm trying to help out a friend of mine with his 1984 V30 Magna. The
    problem is that it won't idle unless the choke is on. As soon as you
    give throttle it will starve and stall. This bike has not had the
    preventive maintenance that is should have. I removed the carbs,
    disassembled them cleaned them up, blew out all the passages etc.,
    replaced the rubber mounts, (what a job, I'll never own a V4) put in
    fresh gas and it still runs the same. By playing with the choke and
    throttle I got it off the pilot circuit and it will rev up, so I don't
    think it's bad fuel pressure. Any Ideas?
    Dave Best, May 20, 2005
    1. Advertisements

  2. Ooooh, you're going to hate what I have to say, because you're probably
    going to be doing that job all over again :-(

    This is probably the most frequently encountered problem in the world
    of CV carb-equipped motorbike riding. Your motorbike doesn't even
    *have* a choke, Dave. It has a *starting enrichener* which is a tiny
    carburetor built into the side of each main carb. It has its own little
    throttle valve, and a jet that sucks gas right out of the float bowl.
    When you move the "choke" lever to full ON, you are just opening that
    little valve and bypassing air around the closed throttle buttefly...

    OK, so you decide you're done with the "choke", you figure the engine
    is warm enough to idle on its own, but the engine stalls. The idle
    mixture you need to run the engine has to come through the plugged up
    idle jets and the plugged up idle passages and it has to get past the
    plugged up idle mixture screw that tweaks the fuel air mixture to make
    it rich enough so the engine will run with the startening enrichener
    turned off...

    CV carbs don't have much vacuum downstream of the throttle butterfly,
    Dave. The engine has a hard time sucking any fuel through the idle jets
    and passages if they are clean and correctly adjusted, when they are
    dirty, it's no joy in Magnaville...

    If somebody elese has drilled out the EPA antitamper plugs and tweaked
    the idle mixture screws, you might help get the engine running by
    turning all the idle mixture screws out half a turn and turning the
    idle speed knob down as far as you can go and have the engine still

    (google for [email protected] +EPA for a description of the plug removal
    procedure and fuel/air screw adjustment)...

    But I think you probably didn't do a good enough job cleaning the carbs
    out (google for [email protected] +idle jets to read how to clean out and
    adjust those pesky idle mixture screws and jets)...

    The alternative is that you have vacuum leaks around rubber hoses or
    the rubber boots that hold the carbs on the cylinder heads. If the
    engine runs well enough to allow you to do it, squirt water around the
    boots and the hoses. If the idle speed gets higher while you're
    squirting water around, it's a leak...

    But, if the engine runs well enough so you can go for a ride, and the
    exhaust pipe goes fartPOP! (or piffle-piffle-SPLAT!)when you roll off
    the throttle, it's probably lean idle mixture because your carbs are
    still plugged up...
    krusty kritter, May 20, 2005
    1. Advertisements

  3. Dave Best

    Ari Rankum Guest

    krusty kritter wrote:>
    Man, it is fun reading your stuff in this group.
    Ari Rankum, May 21, 2005
  4. My little Nissan pickup truck has a computer controlled carburetor that
    has a solenoid that shuts off the gas when I'm coasting. The exhaust
    goes fartPOP! and piffle-piffle-SPLAT! like some senior citizen that
    just ate beans...
    krusty kritter, May 21, 2005
  5. Dave Best

    Ari Rankum Guest

    Really, though. You know your shit.

    Thanks for helping me learn some stuff.

    I like your style.

    Props, even.

    RCOS #7
    2005 FJR1300AT [V4? - no]
    1992 GL1500 (sold) [V4? - no]
    1985 CB700SC [V4? - no]
    1984 XL500R (sold) [V4? - no]
    1979 SR500E [V4? - no]
    1971 CL175 [V4? - no]
    1969 Schwinn Sting-Ray (totalled) [V4? - no]
    Ari Rankum, May 21, 2005
  6. Dave Best

    Matt Guest

    Hey, Mr. kritter. Your posts refer repeatedly to CV carbs. What does
    'CV' stand for? "choke virtual", maybe. I guess my CJ360T doesn't have
    CV carbs---the carbs seem to have real choke plates.

    Still no joy with my CJ that was idle for 13 years. Spark, air, and
    timing seem to be okay. About to try the Berryman's B-12 Chemtool as
    you suggest. I removed and inspected and was able to blow air through
    all the jets, but maybe the idle circuit passages are plugged. I hope
    so---otherwise I am stumped. But I'd better not hope for that unless I
    can certainly get them unplugged with the B-12 stuff ...
    Matt, May 23, 2005
  7. I looked at the carburetor parts fiche for your CJ-360T at

    It does have CV carbs. I could see the diaphragm set listed. I could
    see the idle jet, the main jet, and a third jet, a #67 sized jet which
    may or may not be the starter jet. I couldn't see a starter valve or a
    choke plate on that drawing...

    Are you sure you're not looking at the throttle butterflies? The
    throttle butterflies will be on the side of the carb that attaches to
    the engine, if your carbs had choke plates, they'd be closer to the air
    filter box, so you would see two sets of movable round plates, one set
    at each end of the carbs...

    CV stands for "constant vacuum". CV carbs are also called "constant
    velocity" carbs, because the venturi area is variable, according to how
    high the vacuum slide rises in response to the engine's actual demand
    for *air*, instead of the rider's desire for more *power* at any given

    The old style slide valve carb may still be found on some dirt bikes.
    Slide valve carbs pollute the air, because the rider can't judge how
    far to open the throttle for rapidly increasing and decreasing RPM and
    load conditions. If the rider opens the slide too far, engine vacuum at
    the needle jet may be too low, and gas doesn't get sucked out of the
    float bowls. OTOH, maybe the slide isn't opened far enough, there is
    too much vacuum, and an unnecessary amount of fuel is drawn through the
    needle jet, only to be kicked out of the exhaust pipe unburned...

    The CV carb takes direct control of the fuel air mixture out of the
    rider's right wrist. And that helps to keep our planet's air

    Other posters have been talking about ultrasonic cleaning of really
    grungy carbs. I've never had one get that bad...
    The first CV carbs most motorbike riders might be familiar with were
    the old SU carbs found on British cars from the 1930's to the 1960's.
    The SU carb had a butterfly throttle, just like a modern CV carb. When
    the driver pushed the gas pedal, the throttle opened, and a vacuum
    slide would be raised as air flow increased through the engine. The
    vacuum slide would pull a tapered needle out of a brass tube that was
    about 1/8th of an inch in diameter. That was the *only* fuel metering
    circuit in the carb.

    If the mechanic wanted a richer mixture, he could raise the needle a
    small amount by loosening a set screw and moving the needle. If he
    wanted an even richer mixture or wanted to synchronize the carbs, he
    could turn a nut on the bottom of the brass tube, and lower the tube in
    relation to the tapered needle. The brass tube had to slide a bit
    through rubber seals, and the seals would be prone to leakage if the
    owner like to fiddle with his SU carbs a lot. And the choke system was
    really silly, the brass tube would be pulled down through the rubber
    seal, increasing the wear on the rubber seals My Triumph TR-4's nice
    hot exhaust maniforld was right under the carburetors, ready to catch
    the gasoline drippings.

    Later model SU carburetors eliminated the leaky rubber seals and had a
    bypass air passage with a screw for idle speed, richness and
    synchronization adjustment...

    By the late 1960's, the old SU carbs were replaced by Bendix diaphragm
    CV carbs that looked a lot like Bing "constant depression" vergasers...

    Later model CV carbs from the 1970's had more fuel and air controlling
    circuits than simple British Amal slide valve carbs that had only a
    main jet, an idle jet, and a tapered needle (total of three

    Japanese Mikuni carbs added an air emulsion circuit, where air was
    mixed with the gasoline before it was sucked into the venturi. This
    greatly reduced the size of fuel particles and made the engine much
    more responsive to carburetor throttle changes. Mikuni also added the
    starting enrichener circuit that I've been describing, replacing the
    old choke plate sytem. But choke plates could still be found on Keihin
    carbs on Kawasakis well into the 1970's...

    The superior throttle response of Mikuni carburetors encouraged many
    riders to abandon their primitive 3-circuit Amal carbs and find a way
    to bolt or clamp a Mikuni on in its place so they could enjoy a
    motorbike that started easily and accelerated smoothly...
    krusty kritter, May 23, 2005
  8. Dave Best

    Matt Guest

    Yes, upon disassembly I found three jets that I naively thought of as
    idle, main, and power.
    Definitely each carb has two butterflies. The ones closer to the
    airboxes are operated by a lever marked CHOKE. No butterflies are shown
    in the diagram, but you can see the linkage between the chokes as item 9
    Matt, May 23, 2005
  9. Dave Best

    Matt Guest

    I take it that a CV carb is characterized by the diaphragm and the
    sliding needle.

    Those are parts of what I have been thinking of as the power circuit.

    Where is the vacuum supposed to be constant?
    Matt, May 23, 2005
  10. Right under the vacuum operated slide. Instead of having a round
    venturi that tapers down from a large round section to a smaller round
    section and expands back to a large round section again, the throat of
    the venturi is sort of d-shaped and the highest vacuum should be right
    there at the needle so the vacuum can suck fuel past the needle jet/jet
    needle combination orifice at part throttle, then at 3/4th to full
    throttle, the nj/jn combination offers less restriction to fuel flow
    than the main jet...

    If your carb has a power jet, you'll usually see an external brass tube
    running from the float bowl up the side of the carb to a discharge port
    downstream of the throttle butterflies. It can't get enough vacuum to
    overcome the weight of the fuel in the float bowls until the engine is
    running at full throttle...
    krusty kritter, May 23, 2005
  11. Dave Best

    Matt Guest

    Thanks for your informative posts. I'd be obliged if you could try to
    find any improvements or anything wrong in the following plan,
    especially regarding damage to the rubber and plastic parts.

    I would remove the carbs, remove the diaphragm/pistons and bowls, remove
    all the small parts (slide valves, jets, idle-mixture valves). Then I
    would warm the carb bodies, bowls, and small parts in the sun or maybe
    in a toaster oven (hopefully to something like 110 F) and spray B-12
    into every little hole and let it soak. I would repeat with ordinary
    spray carb cleaner (methanol, toluene, methyl chloride) and maybe again
    with B-12, until I could see solvent flowing out of every little hole.
    Then I would reassemble and reinstall and drive with B-12 in the fuel.

    One thing I'm worried about is damage by the solvents to the rubber and
    plastic parts. I don't know where all of those parts are and I don't
    think I can remove all of them. For instance the tip of the float
    needle looks like rubber, and I believe maybe the throttle shafts have
    plastic seals/bearings. I expect the diaphragms would be ruined by the
    solvents, but it's easy to keep those safe off to the side.
    Matt, May 24, 2005
  12. If you push the float pivot pin out, you can remove the float and the
    float valve, which has, as you say, a rubber tip...
    There's no good reason to heat the carburetors. All that would do is
    cook the gum and varnish in the carb bodies and make it even harder.
    Carb cleaner has a low evaporating temperature. You're better off with
    cold carburetors, as the solvent will have more time to work before it
    If you can find a more powerful carb cleaner than B-12 and you can
    squirt it through all the holes, you should be able to get almost all
    the crud out...

    Just ride easy with that stuff in the gasoline, you don't wan't to wash
    the oil film off the cylinder walls and run the piston rings dry. Keep
    the RPM really low and try to ride the motorbike with the engine as
    close to idle RPM as is safe so it will have to draw gasoline mixed
    with B-12 through the idle jets...
    Like I said above, you can remove the float pivot pin and pull out the
    float and the rubber-tipped needle valve. You might even want to remove
    the float valve seat. There may be a filter up under that seat. B-12
    will make the rubber diaphragms swell up, but they will return to their
    original size when they dry. I never noticed any damage to the throttle
    shaft seals from spraying carb cleaner on them...
    krusty kritter, May 24, 2005
  13. Except <pedant mode> plenty of old CV carbs don't have diaphragms.
    The Older Gentleman, May 24, 2005
  14. Dave Best

    Matt Guest

    I don't care whether it's pedantic. Let us identify the essence of CV
    carbs---CVness itself---what it is to be CV---the minimal CV sine qua
    non---and so on.

    I seem to recall working on a couple of passenger-car carbs that each
    had a "power piston" that acuated metering rods depending on air pressure.
    Matt, May 24, 2005
  15. Dave Best

    Matt Guest

    I just closed the chokes all the way and opened the throttles all the
    way and was able to get fire for half a second. That is the most life
    the thing has shown since '92 ... probably I'd never tried that before ...

    Can we say that that is consistent with plugged idle passages?
    Matt, May 24, 2005
  16. Yes, you got some fuel past the jet needle...

    The normal way to start an older engine with a real choke was to close
    the choke all the way and crank the engine until it started blubbering
    like it was going to fire, then open the choke about 1/3rd and give the
    engine a little throttle...

    If you gave the engine too much throttle, you wouldn't have enough
    vacuum downstream of the butterflies to draw fuel through the idle
    mixture outlet ports...

    Four stroke engines have lots of problems with low engine vacuum and
    infrequent intake strokes, as I explained about 2 weeks ago to the
    rider who was having problems starting his Sportster...
    krusty kritter, May 24, 2005
  17. Dave Best

    Matt Guest

    I had cleaned the carbs before in my own stupid way ... I used water and
    liquid wrench and didn't use carb cleaner. I freed up the pneumatic
    pistons on that first go-round, but left some water in the carbs, which
    led to a float valve being stuck open. The bike had a strong gas smell
    whenever the fuel valve was open.

    Did the job right this time, using a can of aerosol carb cleaner to
    clean the carbs inside and out. Sprayed through all the passages, but
    didn't see any gunk or chunks. There was something chalky like lime on
    the bowl valve needle and inside the bowls, so used Lime Away followed
    by carb cleaner on the jets and needle. Made sure the bowl valve and
    floats were moving properly.

    It started on the second kick. I don't know that I did much good with
    the carb cleaner, but at least I could use it to verify that the
    passages were clear and dry. I ran out of my other carb cleaner and
    used the Berryman's B-12 Chemtool from Walmart, but its aerosol
    behaviour was somewhat troublesome, spraying too fast and then too slow.
    Sometimes it shot the nozzle extension tube out of the nozzle onto the
    work. It seemed that I had to turn the can upside down to get any
    liquid to come out. Not saying that it's bad stuff, but I liked my
    other carb cleaner better.

    Now I seem to have a valve tappet that is too tight and making an awful
    racket. I think I had the cam rotated to the wrong position when I
    adjusted the tappet.
    Matt, May 26, 2005
  18. Alcohol added to your pump gas emulsifies water in your gas tank, and
    that's a good thing to a certain extent. The water gets turned to steam
    and goes out the exhaust pipe, maybe taking some carbon with it.
    Alcohol also has the property of evaporating at a lower temperature and
    condenses water in your fuel system under various conditions.
    Eventually though, the alcohol in your old gasoline will evaporate, and
    whatever water is left combines with CO2 from the atmosphere and makes
    the calcium carbonate you found in your float bowls...
    Hooray! Another motorbike brought back to life!
    Maybe shaking the can would help? And I've run into the problem with
    the nozzle extension tube, too. It's just a press fit in the spray
    nozzle. Maybe the tube could be carefully superglued to the nozzle? Who

    The resurrection of old crocks must continue, regardless of the hassle
    and time consumed...
    Valves usually make noise when they are too LOOSE and they clatter...

    When they are tight, the cam lobe suffers in silence or maybe squeaks a
    little if it doesn't have enough oil on it...
    krusty kritter, May 26, 2005
  19. Dave Best

    Matt Guest

    You are right. The left intake tappet was way too loose (maybe 2mm) due
    to being 360 degrees out of phase when I adjusted it.
    Thanks for that warning and for your remarks on the other thread.
    Matt, May 26, 2005
  20. Dave Best

    Matt Guest

    You would still have to explain where the calcium came from, if the
    stuff was actually lime.

    I can say that the chalky stuff was removed by Lime Away. Before The
    LA, the deposits on the walls of the bowls were light colored and
    somewhat rough and dry and were not removed by carb cleaner. After the
    LA, the remaining deposits were darker and smeary and rather easily
    removed by wiping with a carb-cleaner-soaked paper towel.
    Matt, May 30, 2005
    1. Advertisements

Ask a Question

Want to reply to this thread or ask your own question?

You'll need to choose a username for the site, which only take a couple of moments (here). After that, you can post your question and our members will help you out.