Mulan-Is she starving or over eating?
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Mulan-Is she starving or over eating?

This is a discussion on Mulan-Is she starving or over eating? within the Technical forums, part of the Caferacer.net Forums category; 74 Honda CB 550Four affectionately, although not all that originally, know as Mulan.... This is the bike i purchased off CL and posted as a ...

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Thread: Mulan-Is she starving or over eating?

  1. #1
    Senior Member surfrider70's Avatar
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    Mulan-Is she starving or over eating?

    74 Honda CB 550Four affectionately, although not all that originally, know as Mulan.... This is the bike i purchased off CL and posted as a future build with my son when he gets back from deployment. I posted a pic of a bike that you all mercilessly, and understandably, slaughtered and after licking my wounds and re-evaluating and deciding to keep stock for the time being, i humbly return in need of some of your expertise.

    Idles great-carbs were cleaned and synched by previous owner-purchased halloween of this year and have taken her on 4-5 previous rides with no issues. I was humming along PCH and she started running rough out of the blue. Twisted throttle pretty good in 3rd and she just wouldnt go. Letting off on throttle and she would konk out. Pulled over and scratched my head for a minute-to me it felt like she was starving-maybe sticking float in one of the carbs? Petcock got an overhaul with all new orings and both fuel lines look great with good flow no issues there, shes getting fuel from the petcock and lines. I cleaned out tank before petcock overhaul and even the first of three flushes revealed just a few small flakes of crud-tank was in great shape. New petcock screen as well. Examination of petcock bowl revealed no crud. New points and condenser and all new plugs and coil etc.. so i dont think issue is electrical/spark related.

    Back to PCH....She would run but would not idle and ran rough. Waited a minute then restarted her and she ran absolutely perfect all the way home-did not skip a beat-idled happily at lights etc..

    Asked my brother about it and he thought it sounded like she was starving for fuel in maybe just one of the carbs. Told me a trick i did not know about which i tried the next day. She started doing the same thing so reached down under pretty good amount of throttle and turned choke on-she started running even worse-turned choke back off and she ran a little better but still same problem.

    My thought is that she is starving-like a sticking float in one (or more?) of the carbs because when i gave her more gas through the choke she acted like she was gagging from too much fuel.

    Also the other day she started doing it again after about 30 minutes into a ride then suddenly as i was under a handful of throttle to keep her running she suddenly surged forward and ran perfect all the way home-as if the stuck float suddenly righted itself.?

    What do you think is my diagnosis correct? I have no idea what the previous owner did but a receipt showed "Carbs cleaned/synched" not long ago.

    Also-bike came with a 4 into i RC Pipe and i dont think it was ever re-jetted to compensate. I dont think this has anything to do with the problem but even when she was running great i did notice a bit of a flat spot in the power in all gears about mid throttle-thinking the pipe is the culprit due to less back-pressure and probably has the original jets. Plus i removed a good pound of nasty rusted packing from the pipe when going over her so in theory the pipe is allowing even more exhaust to flow and with the old jets still in it i think this may be the culprit for the flat spot in the powerband? I also wonder if our wonderful CA ethanol gummed up something in the carbs when she sat for a while before i bought her? Remember she ran perfect for the first 4-5 rides so im thinking the ethanol issue is not really an issue?

    Also-i am getting a puddle of fresh gas under her every morning even with petcock off and the tip of the drain tube coming from the carb on the throttle side is wet so the float bowl is filling and is leaking out of drain tube? Is this caused by a bad float?

    THanks for reading and the suggestions/diagnosis-you guys never fail to have a lot of knowledge about these things!

  2. #2
    Senior Member TrialsRider's Avatar
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    Just a couple of thoughts after giving your post the first quick read through; If one cylinder has a problem a 4 cylinder bike will usually just run rough and lack some power, if 2 cylinders drop out that will usually make it quit dead as you lower the revs toward idle and barely run at other times. You have 4 carbs and 2 ignition systems, so if one of those ignition systems drops out you now have 2 cylinders trying to run and 2 carbs that are pouring raw fuel into the other 2 cylinders. :/ This is one of those places where 4 into 4 exhaust pipes are kinda nice, harder to diagnose if you have a 4 into 1.
    Fuel leaking when parked does indeed indicate a possible issue with carburetor float, and bear in mind that a stuck needle and seat can starve that carb for fuel, but a leaking needle and seat or float not floating will drown that carb and overflow fuel. All that said your bike has 2 sets of points and points are high maintenance, so I wouldn't be discounting the possibility that you are encountering a points problem and that is easy enough to check so I'd start there. Maybe when it's running rough, a visual inspection of all 4 plugs for good spark and colour would be a wise start point.
    Last edited by TrialsRider; 12-06-2016 at 06:05 PM.
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    Senior Member TrialsRider's Avatar
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    btw: Don't feel to badly, I had to build a fire under my tractor today to thaw out and bleed a frozen diesel fuel system :/ that's a stinky messy flammable job.
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    Senior Member Geeto67's Avatar
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    I had this problem with a 750, a while back - turned out to be a coil wire shorting against the tank.
    Ever tried. Ever failed. No matter. Try Again. Fail again. Fail better.

  6. #5
    Senior Member hillsy's Avatar
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    Your intermittent problem could be ignition related, but you definitely have carb problems if it's leaving a puddle of gas underneath it every morning.

    You need to take the carbs off, clean, inspect, adjust, etc.

    If the float valves are OK then check the o rings on the seats aren't perished / leaking. Normal o rings + ethanol = leak / flooding. And the fact that putting the choke on makes it worse points to the bike already running rich / flooding, not starving for fuel.

  7. #6
    Senior Member surfrider70's Avatar
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    Quote Originally Posted by TrialsRider View Post
    Just a couple of thoughts after giving your post the first quick read through; If one cylinder has a problem a 4 cylinder bike will usually just run rough and lack some power, if 2 cylinders drop out that will usually make it quit dead as you lower the revs toward idle and barely run at other times. You have 4 carbs and 2 ignition systems, so if one of those ignition systems drops out you now have 2 cylinders trying to run and 2 carbs that are pouring raw fuel into the other 2 cylinders. :/ This is one of those places where 4 into 4 exhaust pipes are kinda nice, harder to diagnose if you have a 4 into 1.
    Fuel leaking when parked does indeed indicate a possible issue with carburetor float, and bear in mind that a stuck needle and seat can starve that carb for fuel, but a leaking needle and seat or float not floating will drown that carb and overflow fuel. All that said your bike has 2 sets of points and points are high maintenance, so I wouldn't be discounting the possibility that you are encountering a points problem and that is easy enough to check so I'd start there. Maybe when it's running rough, a visual inspection of all 4 plugs for good spark and colour would be a wise start point.
    Thanks-do you think it may be a bad needle and seat in just the carb thats dumping? How does one go about figuring out what carb(s) have bad needles/seats? Why is the problem intermittent?-it starts-idles and runs perfect most times...
    I assume the seat and needle should be replaced?-and while in there why not just rebuild the carb?

  8. #7
    Senior Member Stephen J's Avatar
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    Had an issue sounding very similar to this with my old Triumph and it turned out to be a bad coil. Don't know if that translates to a more modern Japanese bike but it sounds just like it. Checking the plugs would be a good start as was already mentioned. If they are fine then have a look at the coil(s)
    surfrider70 likes this.
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    Senior Member Teazer's Avatar
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    Are you sure that's it's not a blockage in the fuel tank filler cap?

  10. #9
    Banned pwalo's Avatar
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    You are on the right track with a float not seating it's needle properly or set wrong. If there is a puddle under it then the fuel in the lines after the tap is shut off is getting past the overflow level. A test before ripping it all to bits is see if it does it again and when it does turn the fuel off instead of pulling the choke. If it picks back up before dying again after the good bowls run dry then you should redo the carbs.

    Nice to see you again btw. Hope all is well.
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  11. #10
    Senior Member surfrider70's Avatar
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    Quote Originally Posted by pwalo View Post
    You are on the right track with a float not seating it's needle properly or set wrong. If there is a puddle under it then the fuel in the lines after the tap is shut off is getting past the overflow level. A test before ripping it all to bits is see if it does it again and when it does turn the fuel off instead of pulling the choke. If it picks back up before dying again after the good bowls run dry then you should redo the carbs.

    Nice to see you again btw. Hope all is well.
    Ahhhhhhhh.... (light just went on) got it-if the float/needle is bad and not letting fuel through it will not improve with the gas turned off because its not filling the bowl adequately, but turning off the gas it will burn through whats left in the bowl showing the float/needle that fills the bowl could be bad?
    THanks thats makes sense-i understand the advise about the electrics but i really dont think theres anything wrong with coil-shorts on tanks etc... Also is it considered a smoking gun as far as which carb(s) has the problem if the bowl drain line tube is dripping? I mean it seems obvious but just checking. Big puddle under her again this morning and wet tips of 2 of the drain tubes.

    And yes all is well thanks, ive been lurking about and trying to figure things out on my own before i come back to the Council of Elders...
    I did manage to successfully replace the clutch case gasket-the kick start O ring-the shift shaft oring-replace both fork seals and other odds-ends so feeling more confident as i go.
    pwalo likes this.

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