engine wont stop reving high

Discussion in 'Motorbike Technical Discussion' started by gabe2004, Aug 11, 2006.

  1. gabe2004

    gabe2004 Guest

    I have a honda cl350. At first it wouldn't run but i cleaned the carbs
    and it now runs. It has no problem accelerating up till about 3000
    rpm. If if give it more throttle, it hesitates for about 4 or 5
    seconds before suddenly revving up to 5000 rpm. I can then accelerate
    smoothly above 3000. Once the engine is going fast, above 3000, it has
    trouble slowing down and even has a tedency to speed up. For example,
    say It's going 5000 and i Come to a stop. It's still revving at 5000.
    So what i do is hold the brake and let the clutch out a bit to slow it
    down. This usually works, but sometimes it will then slowly creep back
    up again.

    One thing that might be a clue is that when i had the carbs out, i
    noticed that when i pushed up the pistons, they didnt imiedietly fall
    down all the way

    Besides this problem, nothing else is wrong. It runs fine at high and
    low rpms, just has trouble transitiong around 3000 rpm.
    Does anyone have any idea what could be going on?
    gabe2004, Aug 11, 2006
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  2. gabe2004

    FB Guest

    You have dirty idle mixture passages and you've compensated for that by
    setting the throttle butterflies too far open. I've gone through this a
    bazillion times in this NG, so I won't write it all up again.

    Just google for "transition ports".
    FB, Aug 11, 2006
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  3. Either that or the carbs are way out of sync, to the point that when
    the butterfly on one is shut, the other is wedged pretty far open.

    Ted Mittelstaedt, Aug 11, 2006
  4. gabe2004

    FB Guest

    If the slides aren't corroded, slow return to closed is a normal
    feature of "vacuum" slides.

    Actually, it isn't vacuum that raises the slide, it's air pressure from
    underneath that lifts the slide.

    And, the air has to be pumped out of the vacuum chamber on top of the
    carburetor through a hole in the bottom of the slide.

    The area and length of that hole controls how rapidly the slide will
    lift and fall.

    The whole concept of the constant vacuum carburetor is to reduce
    throttle responsiveness so a rider with a frisky wrist can't pollute
    the atmosphere so easily as he could with the far more responsive slide
    valve carburetors.

    I blew by a co-worker who was driving his Honda Civic to a biker bar in
    the mountains because he knew he'd be having several drinks there and
    would be weaving home...

    I passed him and accelerated away so rapidly that when he finally
    arrived at the hangout, the first thing he said was, "You've got
    carburetors on that bike, don't you?"

    I admitted that I did have carburetors, meaning smooth bore slide valve

    To the mind of many performance enthusiasts, constant vacuum
    carburetors might be good for door stops...
    FB, Aug 11, 2006
  5. gabe2004

    MotorcycleMD Guest

    Hey Gabe,
    What yr. is the bike and are they stock carbs?
    Did the jet needle retaining clip get reinstalled in their original
    Did you check to see if the diaphragms had any holes or tears and did
    they get seated properly in their lip groove upon reinstallation under
    the carb top/cap?
    You didn't happen to get any carb cleaner on the rubber parts? That
    could be bad.
    Did you ream/drill/pick the brass jets? Hope not.
    Throttle cable: How's the throttle freeplay, cable routing, adjustment,
    What condition were the springs in?
    I could go on with the questions. Answer these then we'll proceed.

    Honda specifies that after performing a carb cleaning one needs to sync
    the carbs and then perform the idle drop. This procedure can be found
    in the Common Service Manual.
    The idle drop adjustment procedure makes sure that the carb is
    providing the correct amount of air/fuel mixture to the cooresponding
    cylinder at idle and low speeds. This facilitates smooth operation. Too
    lean an air-fuel mixture can cause surging during low speed operation,
    which is annoying. A lean air/fuel mixture may also cause long warm-up
    times and engine overheating due to insufficient fuel quantity from the
    idle circuit. The air/fuel mixture can be adjusted for smooth operation
    at idle and low speed by adjusting the pilot screw or air screw.

    This should get ya started,
    MotorcycleMD, Aug 11, 2006
  6. gabe2004

    gabe2004 Guest

    Thanks for responding. It's a 1972. Don't know if the carbs are stock,
    but they sure look it, I'll check the number on them. I didnt touch
    the retaining clips when i dismantled the carbs but I did notice that
    the holes in the piston and the needle jet top didnt line up. Don't
    know if thats normal. Checked the diaphrams in a bright light and
    couldnt find any holes and they seated perfectly. Didn't get any carb
    cleaner on the diaphrams. Didn't do anything to the jets except spray
    carb cleaner through them. There is a bit of freeplay in the thottle.
    At first, the right carb was being pulled first so I tightened up the
    left one and now they both pull at the same time. The throttle feels
    very smooth, the cables aren't sticking at all. Springs seem in great
    condition, I think they may be new.

    Also, I got some B-12 carb cleaner and decided to give it another
    cleaning. It's significantly better. For one, there is now NO lag
    when in neutral. I open up the throttle and it goes up to 6000 and
    higher and comes back down to 1000-2000 with no problem. The
    transistion problem only happens in gear under load. The air filters
    are pretty dirty. I'm gonna get some new ones. How much would they
    affect it?
    gabe2004, Aug 12, 2006
  7. gabe2004

    gabe2004 Guest

    An update:

    I started it up this morning and it sounded like it was running on one
    cylinder while idling. I pulled the left sparkplug and that was it.
    The left cylinder wasn't running at all. What i think is happening is
    that the lag I'm experincing around 3000 is when the left cylinder
    suddenly kicks in, making the RPM jump up to 5000. I guess I am gonna
    have to take out the left carb again and have a look. Any ideas before
    i do it?
    gabe2004, Aug 12, 2006
  8. gabe2004

    FB Guest

    That sort of problem is caused by the idle mixture being too lean, or
    by the two carbs being out of synch.

    Your carburetors have pilot air screws on the side. When you turn a
    pilot air screw clockwise, it makes the idle mixture richer.

    Before trying to adjust them, you might try removing them completely
    and squirting
    B-12 through the holes they came out of.

    The B-12 will come out the idle jet into the float bowl, back through
    the pilot jet in the carburetor inlet, and out through the idle mixture
    ports downstream of the butterfly.

    If you have the carburetors off the head, you can make the B-12 flow
    out the plugged up port or jet by by putting two fingers over the other
    two holes that show good flow.

    Then, when you put the idle mixture screws back in, turn them until
    they lightly seat and then back them out about 1.5 full turns.
    FB, Aug 12, 2006
  9. gabe2004

    MotorcycleMD Guest

    Hey, what did that spark plug look like? Dry? Wet? Dark? Light? Black?
    Brown? Cracked? Worn down or same shape as new??

    Yes. You need to do a compression test and a leak-down test on the
    cylinders to test the sealing quality of the intake and exhaust valves,
    piston rings, head and base gaskets. In other words, these tests check
    the general quality of the engine. The carbs may not be the entire
    problem; only part of the problem.

    A compression test should be performed if the engine is loosing power,
    especially loosing power during acceleration. One cause of low
    compression test results could be that the piston rings aren't sealing
    like they should allowing the air/fuel intake mixture to blowby the
    piston rings on the compression stroke, for example. On the other
    hand, high compression could be due to the accumulation of carbon
    deposits on the piston crown and within the combustion chamber. There
    are other reasons as well but the compression test should be conducted

    A leakdown test should be completed as well. The leakdown test tells
    you if the combustion chamber is sealing like it is supposed to. It is
    a much more accurate test than the compression test in that you are
    actually pressurizing the combustion chamber and measuring the rate
    that the air is escaping. During the leakdown test, if you hear air
    "hissing" at the airbox or carbs, you know you have a leaking intake
    valve. Conversely, if you hear "hissing" at the exhaust pipes, you know
    you have leaking exhaust valves. The sound of air escaping at the
    crankcase indicates leaking gaskets or rings. The testing
    procedures/steps need to be followed to receive accurate results for
    troubleshooting purposes. During the leakdown test, soapy water can be
    sprayed/brushed at the gaskets/engine mating surfaces and if bubbles
    appear, well, you know you have a leak.

    Be warned: this is not the testing procedures. This is just a simple
    and brief explanation of two tests. An engine ought to have these two
    tests performed so one may determine the condition of the engine.
    Compression gauges attach to hoses that are threaded into the spark
    plug holes and measure the compression of the engine as it is cranked.
    The engine is not running for this test and simply: the engine pumps up
    the gauge.
    Likewise, the leakdown test uses similar gauges and hoses threaded into
    the spark plug holes and when pressure from an air compressor is
    applied, after calibration of the gauges, the amount of leakage is
    measured. Amount of air pressure supplied to the engine, through the
    gauges, is specified in the Honda Service Manual. Man, buy a manual if
    you haven't already. Not some Climer manual, but the original/authentic
    Honda Service Manual.

    The service manual can also tell you what the stock carbs were for that

    Thanks for replying with the supplied info. That helps.

    What was the condition of the float valve tip in the carbs? The floats
    are connected to a float arm, which is connected to a float valve
    "needle" that rises and lowers with the level of the fuel in the float
    bowl. This needle usually, but not always, has a rubber tip. Another
    thing to check, and this may require another special tool, is the level
    of the fuel in the float bowls.

    You say, "The transition problem only happens in gear under load."
    Describe this better. What gear? From what speed? At what RPM? Let me
    clarify what I'm asking. Are you talking about taking off and
    accelerating from a stop in first gear? Or are you talking about in 4th
    gear when trying to pass someone on the road?

    You also said, " I didn't touch the retaining clips when i dismantled
    the carbs but I did notice that the holes in the piston [lets call this
    a slide] and the needle jet top didn't line up." Did the carbs go back
    together okay? If the needle and the jet aren't lining up...I mean if
    the needle doesn't cover the hole...that's not good. Did you remove
    the jet needle? (Apparently not if you said you didn't touch the
    retaining clip) Did you take the needle out? If you didn't take the
    needle out you may want to make sure the needle is being secured inside
    the slide (piston) of the carb. Double check this please. This clip is
    a teeny-tiny clip that fits in one of the grooves of the jet needle.

    Did you unscrew and remove the jets from the carb or just spray and
    wipe? It is usually a good idea, if one cleans a carb, to remove the
    jets (if possible without damaging them) and allow them to soak in carb
    cleaner for some time. (15 min, an hour?, over night?)

    That's enough for now. I'll think about this some more and refer to
    some of my reference material. Do you plan on performing these test
    yourself or have a someone else do it?
    One last question...did you just buy this bike or how did you get it?
    Just curious as this could lead to more questions for troubleshooting.

    Hey, if you have a digital camera, you could shoot me some pictures of
    the carbs.

    MotorcycleMD, Aug 12, 2006
  10. gabe2004

    MotorcycleMD Guest


    Are these the same carbs that you soldered together the cracks? I was
    reading some of your older postings to see if I could help you some
    more and came across some interesting things.

    You write:
    July 21:
    " just got a 1972 honda cl350. Both carbs leak from the overflow
    tubes. I finnaly discoverd a small hariline crack in the brass
    overflow tube that is connected to the float bowl in each carb. Does
    anyone know of anyway to path it? Also, has anyone else had this
    happen before? Thanks."

    July 22:
    "I'm absolutly sure it's the cracks. The valves work perfectly and the

    floats move freely. I was stumped untill i decided to take out the
    floatbowls and fill them with gas just to see what happened. At about
    halfway full, way before the overflow hole, they started leaking.
    I was thinking about coating them somehow. Recomend any particaulr
    epoxy? Thanks. "

    July 24:
    "Well, I cleaned and fluxed the brass and it soldered beautifully. I
    tested by blowing throught the tube with water in the bowl and there
    were no bubbles so I think it's gonna work. I'll tell you how it goes
    tomorow. "

    July 25:
    "Well, the soldering workd great. No leaks at all. I was able to get
    it running but I have a strange problem. Engine seems to idle ok, but
    when i give it gas, it stalls at around 2500 RPM. Any ideas? "

    1.) In one post you say you have cracks in the brass "overflow tubes"
    of your carbs and this causes fuel to leak. Am I correct here?

    2.) Later, you stated that you blew through the brass tubes and they
    leaked no longer. Are these the same brass tubes in which you soldered
    the cracks?

    Here is The Doctors diagnosis (and it's likely that I could be totally
    incorrect since I haven't seen the unit and know no more than what
    appears here):

    If the float arm is bent in such a way that the fuel level is allowed
    to rise in the float bowl, preventing the float from engaging the float
    valve to a seated position, stopping the flow of fuel to the carb, the
    "overflow" tube will be engaged.

    I don't think there is an "overflow tube". That is actually a brass
    tube used for venting the carb float bowl to atmospheric pressure.

    When the float bowl was taken off, "someone" may not have been paying
    attention to the installation direction/orientation and started
    tightening screws causing that brass tube to get bent and cracked. When
    the screw holes didn't line up between the carb and bowl that was when
    the error was discovered and corrected.

    When your gas filled the float bowl, fuel started coming out of the
    atmospheric vent tube. This is how you discovered the crack in the
    brass vent tube. If you look at the top of this tube you will see that
    it rises well above the top of the float bowl. Fuel doesn't go above
    the upper rim of the float bowl. This tube rises above this level so
    that it can vent.

    By soldering the crack in this tube you sealed it. This changed the
    atmospheric venting properties of the carb.

    According to the Honda Common Service Manual I quote:
    Float System Vents
    Float system vents are necessary to ensure a
    smooth flow of fuel through the carburetor. These
    vents may be external or internal. In addition,
    they may or may not have control (air vent and
    purge) valves.
    Externally Vented Without Control Valves
    The externally vented float bowl has the vent
    tube(s) routed to atmosphere. This system utilizes
    atmospheric pressure to maintain pressure on the
    fuel inside the float bowl. The system was widely
    used (because of its simplicity) until emission laws
    came into effect. It is possible for hydrocarbon pollutants
    to escape from this type of float bowl venting
    system (because the float bowl is allowed to
    breathe to the atmosphere). [PAY ATTENTION HERE GABE]
    The vent tube routing is
    critical. Besides the possibility of pinching shut,
    incorrectly routed tubes may be exposed to low
    pressure when the motorcycle is in motion, which
    could change the pressure in the float bowl and
    alter fuel delivery characteristics.

    Here is what you do:
    Throw away your float bowls. They are no longer able to vent properly
    to atmospheric pressure thus changing the venturi properties. Find
    yourself some more float bowls just like yours because the ones you
    have are no good any longer. Or, just get some more carbs for your year
    and model.

    I think I've been watching too much CSI too.

    No charge,
    MotorcycleMD, Aug 12, 2006
  11. gabe2004

    gabe2004 Guest

    Wow. That was a lot. I really apreciate it.
    First off, I havn't done a compression test yet. I'm gonna try to
    borrow a tester soon. About the float bowls. I took both out and
    sprayed carb cleaner through the outside hole at the bottom of the
    bowls and it came right out the hole near the top of the vent tube, so
    I know solder isn't blocking the vent. When the engine is running and
    I close my finger over the right carbs vent hole, the engine imidietly
    stops. The engine also stops if I pull the right spark plug. I am
    pretty sure now that the left cylinder never comes on. One thing that
    I did notice is that when the float bowls go back on, it feels as if
    something is pushing against them inside. I am going to take them both
    out again and measure how far the tubes stick up from the bowl.

    I know it's not a problem with the throttle cables being synced. They
    are almost perfetly synced. When the engine is running and I use my
    finger to open the throttle on the right carb, the engine imidietly
    revs up. Nothing happens when I do it to the left carb. The spark
    plug on the right looks dry and kinda light. The left plug looks
    darker and wetter.

    The spark on both sides looks strong.
    I think the problem is pretty much that the left cylinder isn't
    running. Before I would feel it kick in once in a while at certain
    RPM. Now I don't feel it. Earlier I mentioned that the lag problem
    happened in gear. I guess I should say under load. In neutral, there
    was no lag around 3000 RPM. The RPM increased smoothly. I only
    noticed the problem while in gear, more noticbly in first or second.
    It felt as if the engine didn't have power at the around 3000. But on
    the highway at 5500, there was plenty of power to accelerate and no

    And the retaining clip. I don't have a diagram so bear with me if I
    get the terms wrong. When I take off the top of the carb, I can take
    out the slide, diaphram and needle. The slide is open at the top. At
    the bottom there are a few holes, 2 or 3 i think. There are
    corosponding holes in a disc that I think is a atachted to the needle.
    A clip holds the disc to the slide. On both carbs, the holes did not
    line up. 2 holes were about halfway covered. I am gonna try to get a
    camera to take some pictures. It will be much clearer.

    I didn't remove and soak the jets. I'm all out of carb cleaner so I'll
    have to get some more.
    I also did remove the mixture screws, sprayed the holes with carb
    cleaner and put them back in and backed them off 1.5 turns. Didn't
    seem to change anything.

    And about the bike: I bought it a couple weeks ago for $150. It had
    been sitting in his garage for about a year. Tires are in really great
    condition. The bike has 12,000 miles on it.

    Also, is there anyway to check if both cylinders are running when I'm
    on the highway. As I said, when I'm in 5th gear on the highway, the
    bikes runs great, accelerations is great. I don't know if it's a good
    idea to just pull out a spark plug while going 60.

    Again, thanks for the help. I'll try to get back with some pictures.

    gabe2004, Aug 13, 2006
  12. gabe2004

    FB Guest

    Your carburetor should look like this if it's a diaphragm type:


    # 7, is the "slow jet". It is in series with # 12, the pilot jet

    The drawing seems to show that the slow jet is an air emulsion type. It
    appears to have cross-drilled air holes to allow gasoline to mix with
    air as it is sucked out of the float bowl.

    You need to remove the slow jet and clean it thoroughly.

    The fuel/air mixture which has been sucked out of the float bowl mixes
    with MORE AIR
    that comes though a passage controlled by # 9, the idle mixture screw.

    The inlet for the air passage for the idle mixture screw may be in the
    mouth of the carburetor, or it may be under the rubber diaphragm.

    Whichever position it is in, when you spray B-12 through that hole, it
    should come out of the slow jet and the pilot jet and it should squirt
    freely out of a hole just downstream of the throttle butterfly.

    If you have the idle mixture screw removed and you spray B-12 down that
    hole, the
    B-12 should come out of all the other holes mentioned. You may have to
    cover some of the holes with your fingers and keep spraying B-12 into
    one hole until it freely squirts out of each of the other three holes.

    I don't know how to explain it to you any clearer. If you don't get
    your idle mixture circuit cleaned out properly, no amount of carburetor
    synching or idle mixture screw tweaking will have any effect.

    Item # 16 is the so-called "needle jet". It may have have cross-drilled
    air emulsion holes for mixing air with the gasoline before it reaches
    the carburetor throat. This breaks the fuel droplets into a finer mist
    so it is more burnable.

    It is also possible that item # 16 is a primary type needle jet which
    does not mix air with the gasoline.

    Item # 8 is the main jet holder which can probably be pushed down until
    it falls out of the carburetor body. This drawing seems to show that it
    is an air emulsion type jet holder which would need to have its air
    holes cleaned.

    You can just barely see the brass standpipe in the float bowl. It seems
    to be an overflow tube, like the overflow in your toilet water tank.

    Float bowl fuel level is critical. It has to be set to within plus or
    minus 1 millimeter.

    Float levels are set with the carburetor on the bench, upside down.

    If the service manual specifies the float level as 14 mm, plus or minus
    1 millimeter,
    13 millimeters would actually be a high fuel level when the carburetor
    is rightside up, and 15 millimeters would be a low fuel level.

    When the fuel level is low, the carburetor has a harder time of sucking
    gasoline out of the float bowl.
    FB, Aug 13, 2006
  13. gabe2004

    gabe2004 Guest

    I did exactly what you said and It's running great. Left cylinder is
    running all the time now. When I took the jets out, the rubber gaskets
    fell apart so I put the jets back in without them. I might get a
    rebuild kit. It's really running great though. Thanks for all the
    gabe2004, Aug 17, 2006
  14. gabe2004

    FB Guest

    If you're talking about little rubber o-rings, you can probably get new
    ones at a decent hardware store that has a selection of o-rings in a
    compartmented box.

    The problem with hardware store o-rings is that they might be made of a
    rubber that isn't gasoline resistant like neoprene and viton are.

    Buying a rebuild kit might result in you buying a bunch of parts you
    don't need to get the few parts you do need.

    www.oldbikebarn.com has aftermarket carburetor rebuild kits for about
    $25.00 each, but they would include the rubber diaphragms. Another guy
    on this NG with a CB or CL-350 had to order them from a place in
    FB, Aug 17, 2006
  15. NRP Carburettors


    Absolutely *brilliant* company. You have to send them your slides and
    diaphragms complete, and they affix new aftermarket diaphragms to the
    sides and post them back. UK turnaround time is about a week - add a
    week for air mail to the Colonies.

    They did a Kawasaki 400 twin for me a few years ago: something like £60
    (about $110 at current exchange rates) for both carbs, whereas Kawasaki
    wanted £70 for a single unit.

    This is one of those lovely little artisanal companies that has seen a
    niche, and exploited it for all it's worth with a top quality product
    and good service. Very, very strongly recommended.
    The Older Gentleman, Aug 17, 2006
  16. gabe2004

    B-12 Guest

    Whoops. I meant to say "but they would NOT include the rubber

    BTW. Berryman's B-12 Chemtool Choke and Carburetor Cleaner will not
    hurt the
    rubber diaphragms. Seriously. I've used it for 30 years and it doesn't
    damage rubber
    B-12, Aug 17, 2006
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