Author Topic: Bike will only start with choke opened up, then dies  (Read 3944 times)

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Offline nathanielhartig

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Bike will only start with choke opened up, then dies
« on: October 10, 2016, 08:44:51 pm »
Trying to diagnose what's wrong with my '75 CB550. Was riding it for a while last weekend, and after about an hour, the engine started sounding rough and felt like I didn't have as much power taking off from a stop. A minute later the bike died, and wouldn't start up. Bike will start with choke opened up, but only revs high for a moment or two before dying. Any advice on what the issue might be would be really appreciated. This is my first bike, so I wanted some opinions before beginning to take things apart. Thanks!

Offline Scott S

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Re: Bike will only start with choke opened up, then dies
« Reply #1 on: October 11, 2016, 03:53:10 am »
 You wouldn't be the first person to be using the 500/550 choke backwards. Are you sure it's in the correct position?

Choke ON



Choke OFF

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Offline DaveBarbier

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Re: Bike will only start with choke opened up, then dies
« Reply #2 on: October 11, 2016, 04:27:11 am »
True. Choke "open" as you describe means no choke applied. Use "ON" or "OFF" to avoid confusion.

What's your intake/exhaust setup? Any engine performance upgrades?

Offline nathanielhartig

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Re: Bike will only start with choke opened up, then dies
« Reply #3 on: October 11, 2016, 06:44:42 am »
Sorry, probably used the wrong terminology. When I have choke on, engine starts up momentarily (with me giving it some gas in the throttle as well), then dies. With choke off, it won't start up at all.

Offline flybox1

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Re: Bike will only start with choke opened up, then dies
« Reply #4 on: October 11, 2016, 06:52:37 am »
Sorry, probably used the wrong terminology. When I have choke on, engine starts up momentarily (with me giving it some gas in the throttle as well), then dies. Take it off choke before it dies!!!!  With choke off, it won't start up at all. 
Choke is used to richen the mixture for easier starting.  Once started and warming up, the choke needs to be turned off.
« Last Edit: October 11, 2016, 06:54:55 am by flybox1 »
'78 750K (F3 engine) PD42b's, Modified airbox w/K&N  filter, 40/110 jets, 1 needle shim, IMS@ 1 turn out. Kerker + Cone 18" QuietCore

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Offline John Eberly

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Re: Bike will only start with choke opened up, then dies
« Reply #5 on: October 11, 2016, 07:31:14 am »
So you're saying that the choke must be "on" (air to the engine restricted to richen the mixture) for the engine to start?

If so, it could be plugged low speed jets - no gas at low throttle opening, choking the motor and opening the throttle would tend to draw fuel from the "fast" jets. How long since the carbs were cleaned? How old was the gas, and how much do you ride? The fix for this would be to pull the carburetors and carefully clean them. That means tearing them all the way down and making sure every passage is clear.

If you are saying that the choke must be off (wide open to allow more air to the motor) and the throttle cranked open to start I'd look at the air filters and make sure they are not plugged. The only way to really tell is to pull them off and try to start the engine. Plugged filters don't always look bad -

Good luck, and keep asking questions.



Offline nathanielhartig

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Re: Bike will only start with choke opened up, then dies
« Reply #6 on: October 11, 2016, 08:26:24 am »
Yes, choke must be on (rich a/f ratio)  for engine to start at all, and then it only starts up for a moment (reving really high when it does this), then dies. I've only had the bike a couple months and haven't cleaned carbs yet. I ride about 50-100 miles a week on it, so gas was relatively new. I'll try removing the carbs and carefully cleaning them. Any additional advice for this process?

Offline flybox1

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Re: Bike will only start with choke opened up, then dies
« Reply #7 on: October 11, 2016, 08:44:28 am »
Be sure to clean all the through-holes of the main and pilot jets, as well as the side holes of the pilot jet and emulsion tubes.
Spray carb cleaner and compressed air through all passageways.
If there is any sediment in your bowls clean it out, and rinse all old gas/debris out of your tank.  Start with fresh gas.
'78 750K (F3 engine) PD42b's, Modified airbox w/K&N  filter, 40/110 jets, 1 needle shim, IMS@ 1 turn out. Kerker + Cone 18" QuietCore

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Offline DaveBarbier

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Re: Bike will only start with choke opened up, then dies
« Reply #8 on: October 11, 2016, 08:56:04 am »
I'll only add that you should wear eye protection and spray like hell. Some carb cleaner got on my eye and it stings like a mother.

Offline flybox1

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Re: Bike will only start with choke opened up, then dies
« Reply #9 on: October 11, 2016, 08:58:12 am »
I'll only add that you should wear eye protection and spray like hell. Some carb cleaner got on my eye and it stings like a mother.
Yeah +1  It sucks  >:(
'78 750K (F3 engine) PD42b's, Modified airbox w/K&N  filter, 40/110 jets, 1 needle shim, IMS@ 1 turn out. Kerker + Cone 18" QuietCore

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Offline Bankerdanny

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Re: Bike will only start with choke opened up, then dies
« Reply #10 on: October 11, 2016, 10:41:15 am »
Typically you need to immediately back the choke off just a tad once the bike fires, at least that is the case on my 550. My bike generally won't stay running on full choke, at least not in warm weather. I haven't started in in a couple weeks and morning temps are dipping into the 50's.
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Offline flybox1

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Re: Bike will only start with choke opened up, then dies
« Reply #11 on: October 11, 2016, 10:45:40 am »
Typically you need to immediately back the choke off just a tad once the bike fires, at least that is the case on my 550.
My 74 550 was the same.  Full choke for start, and then completely off once it fired or it'd start chugging.  Idle smoothed right out once switched.
'78 750K (F3 engine) PD42b's, Modified airbox w/K&N  filter, 40/110 jets, 1 needle shim, IMS@ 1 turn out. Kerker + Cone 18" QuietCore

Past Bikes
1974 550K0 (stock), 1973 CB350F (stock), 1983 Yamaha XS400K (POS)
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"Knowledge without mileage equals bullsh!t" - Henry Rollins

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Offline BomberMann650

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Re: Bike will only start with choke opened up, then dies
« Reply #12 on: October 11, 2016, 11:21:01 am »
I'll only add that you should wear eye protection and spray like hell. Some carb cleaner got on my eye and it stings like a mother.
Yeah +1  It sucks  >:(

Fark, wear a face shield, the stuff tastes terrible too
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Offline Duanob

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Re: Bike will only start with choke opened up, then dies
« Reply #13 on: October 11, 2016, 12:29:15 pm »
It sounds like a major vacuum leak if it won't run with the choke off and the high revs. Once you get it started try spraying a bit of WD40 or something like that around the base of the carbs and see if it changes the RPMs or runability. There are O-rings between the intakes and the head on these bikes that are a common place for leaks.
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Offline daniel9441

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Re: Bike will only start with choke opened up, then dies
« Reply #14 on: October 11, 2016, 09:52:17 pm »
I'll only add that you should wear eye protection and spray like hell. Some carb cleaner got on my eye and it stings like a mother.
Yeah, I tried shooting some into a float bowl but it ended up doing a 180 and nailing me in the eyes. Let's just say that after that I wasn't sure if I would ever see again, nor was I sure if I wanted to.

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Offline Gene

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Re: Bike will only start with choke opened up, then dies
« Reply #15 on: October 12, 2016, 02:25:18 pm »
Tagging for 550 fun.  Repeating everything you already heard.  Choke on to start (come on, it's a Honda) then off once started.  Also, for fun, make sure your idle screw isn't cranked way the heck up. - I'm thinking of that revving when it starts.
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Offline exf5003

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Re: Bike will only start with choke opened up, then dies
« Reply #16 on: October 13, 2016, 05:26:05 am »
I dont wanna be that guy... but do you have enough gas in the tank.  Ask me how i know... :o ;D

Offline nathanielhartig

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Re: Bike will only start with choke opened up, then dies
« Reply #17 on: October 13, 2016, 09:45:15 am »
Yup, got fresh gas. I'm going to try cleaning the carbs/jets, and see if that does anything. After having removed my carbs and then reinstalled thm, , do they need to be re-synced?

Offline flybox1

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Re: Bike will only start with choke opened up, then dies
« Reply #18 on: October 13, 2016, 09:48:07 am »
Yup, got fresh gas. I'm going to try cleaning the carbs/jets, and see if that does anything. After having removed my carbs and then reinstalled thm, , do they need to be re-synced?
Not necessary.
If you mess with the slides at all, then yes.  Another vacuum sync is needed.
'78 750K (F3 engine) PD42b's, Modified airbox w/K&N  filter, 40/110 jets, 1 needle shim, IMS@ 1 turn out. Kerker + Cone 18" QuietCore

Past Bikes
1974 550K0 (stock), 1973 CB350F (stock), 1983 Yamaha XS400K (POS)
77/78 cool 2 member #3
"Knowledge without mileage equals bullsh!t" - Henry Rollins

"This is my CB. There are many like it, but this one is mine…"

Offline Don R

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Re: Bike will only start with choke opened up, then dies
« Reply #19 on: October 13, 2016, 03:45:25 pm »
 If there is a fuel filter after the tank, replace it or remove it. Even if it appears clean, they mysteriously refuse to flow once in a while.  On my SS I could blow air into the tank but fuel would not come back through when I lowered the hose.
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Offline jonda500

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Re: Bike will only start with choke opened up, then dies
« Reply #20 on: October 13, 2016, 07:52:33 pm »
If there is a fuel filter after the tank, replace it or remove it. Even if it appears clean, they mysteriously refuse to flow once in a while.  On my SS I could blow air into the tank but fuel would not come back through when I lowered the hose.
+1 I have encountered this too, even though I could easily blow bubbles into the tank, fuel refused to flow out!
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Offline nathanielhartig

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Re: Bike will only start with choke opened up, then dies
« Reply #21 on: October 14, 2016, 10:58:54 am »
Took out the carbs and cleaned them and the jets, and drained the old fuel and put in new, and she's back running smooth! Thanks for the help, it saved me a lot of time. Cheers, see y'all out there!