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Really scratching my head on poor starting on an 83 CB550sc Nighthawk. Choke doesn't seem to be doing anything

1101 Views 17 Replies 5 Participants Last post by  n0fumar, 
n0fumar
Hi all,

I'm having a bit of a tough time figuring this one out, and any advice would be greatly appreciated.

To preface this, the bike has been completely rebuilt, and the carbs have been overhauled probably 3 or 4 times. By that, I mean new o-rings, all passages cleaned and all jets are open and clean. I have NOT changed any jets or tried other configurations, as apparently this carb is not good with rejetting.

I will also say the bike does run, but it takes forever to start. Once it finally starts up, it's happy as can be and has fantastic throttle response and no issue at any RPM throughout the range.

So onto the problem I have;

I feel like the choke does absolutely nothing for this bike in terms of helping me start. This is not a typical choke where it closes the butterfly valves or restricts airflow. It is apparently "a "bystarter" system in which the choke lever opens a valve rather than closing a butterfly in the venturi area as on many carburetors. In the Open position, the slow jet discharges a stream of fuel into the carburetor venturi to richen the mixture when the engine is cold" (right from the manual). I've had more success starting the bike without the choke than with the choke. And when running the bike, when engaging the choke, the bike just starts to sputter and die (make sure audio is on. In this video just be mindful I've since rebuilt the starter, so that odd whining sound is gone). This is a little counter-intuitive to what I'm used to, as on all my other bikes when you engage the choke the bike revs much higher. Again, different choke design so I'm not entirely sure.

Sometimes I wind up having to crank for 2-3 minutes (in 5 second bursts) before I get any sort of sputter out of the engine, and then I have to hold the throttle open a bit to get the RPM high enough for it to be happy. About 10-15 seconds of this and it's finally happy.

So here is what I've done to try and diagnose some of this stuff.

  • Completely stripped and cleaned / rebuilt the carbs. This includes removing the choke parts and cleaning out all passageways. Floats are set to correct levels.
  • Replaced all o-rings for the carb setup
  • Verified each bowl is getting gas
  • Set pilot screws to factory settings (I think it's 2 1/4 turns out from freshly seated)
  • Set sync screws as closely as I could to be equal to each other. I used a caliper and got them probably within 0.1mm of each other
  • Always tried first starts with a freshly charged, 2 year old battery
  • Rebuilt starter motor (thought something was wrong and it wasn't spinning fast enough, can't tell if this helped)
  • Verified each spark plug has good, strong spark
  • Finger tested that each cylinder has good compression (haven't gotten a gauge on, but I don't think this would be an issue anyway)
  • Verified no vacuum leaks by spraying carb cleaner around the boots (no changes in RPM)
  • The fuel petcock is clean and works properly
  • New gas (less than 2 weeks old)
  • Gas tank is clean inside
  • Pulled off and reinstalled the carbs more times than I would like to admit. I hate these boots with a burning passion
Kind of at my witt's end here, so if there are any mechanics with some ideas / thoughts on what might help, I'd love to hear anything!
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D
I've been down this road many times. Loss of hair and sleep, chewed fingernails, etc.

There's always some little thing I overlooked: use PEA-based fuel-system cleaners since spray carb-cleaners no longer work due to removal of chlorinated compounds, poke out all bleed-holes with copper wire since dried petrol varnish turns to plastic and can't be dissolved again, especially choke jet since it's not removable. Had one case where choke jet had to be drilled out with micro-bit since it was so clogged.

Now, if I pulled carb more than 4x, I just send it off to Gordon at https://customcarbservices.com

He's had decades of experience with restoring thousands carbs from all sorts of vehicles from lawnmowers to Army tanks. Has all necessary equipment like ultrasonic cleaner and micro soda-blaster.
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n0fumar
Unfortunately, I've already cleaned out each passageway and confirmed nothing is blocked by debris or buildup. I have also used an ultrasonic cleaner on this and can pretty confidently confirm each nook and cranny is as clean as when it came installed on the motorcycle. Would be extremely surprised if there was a clog somewhere.
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D
Then I suggest sending to Gordon @ https://customcarbservices.com
It's always turned out to be solution to all my problems once I get to where I can't figure out what's wrong.
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S
Hi ,I would have thought that applying the choke while running would have produced the stall (running too rich) , but starting from cold (and not being able to start) points to the adjustment of the choke cable (and its application on the carbs) is it an old cable and has the movement at the carbs been adjusted, sorry if you've already tried this
Font Parallel Paper Engineering Paper product
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n0fumar
spagiratwink said:
Hi ,I would have thought that applying the choke while running would have produced the stall (running too rich) , but starting from cold (and not being able to start) points to the adjustment of the choke cable (and its application on the carbs) is it an old cable and has the movement at the carbs been adjusted, sorry if you've already tried this
Click to expand...
It sort of seems to be operating that way, it just seems so odd that it behaves so differently than what I'm used to.

Also, yes I've verified the choke is operating properly. In that video, you can see it moving up and down. It's doing something just not what I would have figured it would do...

It honestly seems like it's acting like a weed whacker or something, where it needs to get started with the choke on once or twice (but then dies) and then you start it without the choke. Surely Honda couldn't have messed up the carbs that badly, right?

Here's a 3 minute video of me going over the steps I try when trying to start and how it's behaving. Sorry for the vertical video, filmed with my phone.

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S
I think your Pilot Screw Initial setting of 2 1/4 turns out could be wrong , have you got the Service Manual for this bike?
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n0fumar
spagiratwink said:
I think your Pilot Screw Initial setting of 2 1/4 turns out could be wrong , have you got the Service Manual for this bike?
Click to expand...
Yep, I've got the service manual. Can confirm it's supposed to be 2 5/8 turns out, but that's what I had set it to, so should be stock.
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Leming
n0fumar said:
It sort of seems to be operating that way, it just seems so odd that it behaves so differently than what I'm used to. Also, yes I've verified the choke is operating properly. In that video, you can see it moving up and down. It's doing something just not what I would have figured it would do... It honestly seems like it's acting like a weed whacker or something, where it needs to get started with the choke on once or twice (but then dies) and then you start it without the choke. Surely Honda couldn't have messed up the carbs that badly, right? Here's a 3 minute video of me going over the steps I try when trying to start and how it's behaving. Sorry for the vertical video, filmed with my phone.
Click to expand...
n0fumar said:
It sort of seems to be operating that way, it just seems so odd that it behaves so differently than what I'm used to. Also, yes I've verified the choke is operating properly. In that video, you can see it moving up and down. It's doing something just not what I would have figured it would do... It honestly seems like it's acting like a weed whacker or something, where it needs to get started with the choke on once or twice (but then dies) and then you start it without the choke. Surely Honda couldn't have messed up the carbs that badly, right? Here's a 3 minute video of me going over the steps I try when trying to start and how it's behaving. Sorry for the vertical video, filmed with my phone.
Click to expand...
I have identical symptoms with my 83 cb550sc Nighthawk.
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D
Very simple issue, carbs are dirty and clogged.

Not so simple is cleaning carbs. Spray carb "cleaners" no longer work due to removal of chlorinated compounds. You'll have better luck using pee as at least it has some ammonia.

Choke jet is usually not removable and needs to be poked out with soft copper wire while inside carb. Had recent case where dried petrol was so hard (it's usually just soft plastic material), that choke jet needed to be bored out with micro-bit to get through clog. To do proper restoration job on carbs, you need decades of experience with hundreds of different carb models to know where to poke, scrub and drill. You'll need:

  • tools to completely disassemble carb down to every last nut, bolt and individual component
  • scrub-brushes of various sizes to fit down all hidden secret passages in carb-body
  • PEA-based fuel-system cleaner (Red Line SI-1, Techron Concentrate Plus, etc.)
  • ultrasonic cleaner with polar solvents (don't use Simple Green as it dissolves aluminium)
  • micro soda-blaster to clear out chunks and chads of dried petrol
  • manometers or vacuum-gauges to sync carbs

All this equipment is expensive for just one set of carbs with no guarantee you'll get carbs back to factory-fresh OEM brand-new clean condition. I usually just send them to this outfit. Expert workmanship, reasonable fees and quick turnaround.

customcarbservices.com

customcarbservices.com, Professional motorcycle carb cleaning

Proc carb service utilizing modern technology in a shop setting. Featuring truly affordable, fast turnaround to deliver comprehensive service to your carbs. While in my possession, they are treated as I would my own!
customcarbservices.com customcarbservices.com
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n0fumar
I can safely say without a doubt, the carbs are not clogged. I think this is just unfortunately how these carbs want to act. Maybe there is a leak around the boots or something, but I'm still having issues with it not being able to start easily. Such a headscratcher.
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genel
From your description, it seems that there is nothing wrong except the starting problems. Might it be that your choke adjustment is wrong? What I mean is when the choke is opened, all 4 choke plungers should open all the way. I have a '85 650 Nighthawk, and the factory manual, and the manual includes your bike, so they must be similar. I use a nylon guitar string to clean passages, some say that wire or drilling out can change the calibration.

I am just finishing up rebuilding my bikes carbs, my main problem was the slow speed jet, the bike had problems running correctly between 1000 to 2500 rpm. I also have some spotty tank rust which residue was in the float bowels.

Good luck!
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n0fumar
I can confirm that all four of the plungers are going up and down when the choke is operated. To be honest at this point, I'm not sure what else there is to check on it from an operations standpoint.
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D
There are multiple circuits in carbs. Just because idle, low-speed and high-speed circuits are clear, doesn't mean that choke circuit is clear and working. If your front-door is locked, but your back-door is wide-open, can you really say that your house is safe from robbers???

This is main-jet circuit which most people think of when they say "carb cleaning". Rarely used unless you're WOT fulltime on racetrack.


Low-speed circuit with pilot-screws are used most. This is area that cause trouble most of time when people open throttle and bike just falls flat on its face. In addition to jets at beginning, entire length of passage needs to be scrubbed out with brushes. Along with bleed holes at carb-venturi (green dots).


Here's start/choke-circuit. Just because plunger opens passage at end, doesn't mean petrol actually make it through tiny 0.010" jet and flows all way to end. Circuit is not easily cleaned because most choke-jets are not removeable. If you can't remove choke-jet, how are you going to scrub passageway after it???


I've seen and fixed this non-operational choke issue at least 100-times. Seriously, your choke-circuit needs to be cleaned out. Preferably by professional.
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genel
Did you try poking a nylon guitar string all the way through? It worked for me, it might work for you. From your description, my opinion is it is the starter circuit.
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n0fumar
Thanks for the handy images and diagrams. Unfortunately in my case, all these passages are completely clean :(
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D
n0fumar said:
Thanks for the handy images and diagrams. Unfortunately in my case, all these passages are completely clean :(
Click to expand...
How do you know that?
Did you ultrasonic soak everything?
Micro soda-blast everything before re-assembling?
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n0fumar
I would like to think that the many hours spent soaking in solvents, blasting with compressed air, blowing out with carb cleaner, running cleaning wires through, soaking in an ultrasonic cleaner, and inspection with a microscope probably gives me enough confidence to say that every single passage on all four carbs is clean. I even went as far as to use a small flow meter to verify each passage had the same readings as each other carb to make sure none were clogged and I couldn't tell.
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