Author Topic: '76 CB550F: Idle holds consistently for a few seconds before dropping suddenly  (Read 155 times)

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Offline magner

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My bike seems to be running great otherwise, but it seems like this started happening after I replaced the clutch plates and had some issues dialing in the clutch adjustment (not sure if its related). When the bike is cold upon startup it will idle for maybe 3 seconds at 1000rpm, and then drop below 400 and eventually die. It gets better once its warm: it will maybe idle very consistently at 1000rpm for 12 seconds before suddenly dropping off. I have to keep my hand on the throttle at all times and listen so I can baby the throttle to keep it from dying. I suspect this could be related to the pilot jet, however I have not yet ventured inside of my carbs over the year I've had the bike. Thanks for any help as always y'all!

Offline scottly

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As long as it's idling normally when fully warmed up, it's just the nature of the carburated beast. Idle adjustments should always be done with a fully warmed engine.
Don't fix it if it ain't broke!
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Offline Deltarider

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1. Realise your bike is not a car. Read the Owner's Manual CB550F 1976 on 'starting the engine and warming up' p.46-8 in: http://www.honda4fun.com/pdf_libretti/550/550F_76/librettoCB550F_76_2.pdf
2. Realise that the on board tacho can be inaccurate.
3. For CB500/550s I can recommend an idle of no lower than a true 1100 rpm.
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Offline 1976cb750f836

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  • 76 cb750f, 836, 78 wheels
Adjust your air screws 1/2 turn at a time, trial and error. Im guessing out?

Offline magner

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As long as it's idling normally when fully warmed up, it's just the nature of the carburated beast. Idle adjustments should always be done with a fully warmed engine.

That's the thing; it gets better when warm (will hold idle for maybe 12 seconds tops), but will still drop off and die even when fully hot.

Offline maxheadflow

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Back in the day, if the idle hangs like that then falls off, it's typically the advance mechanism. It's hanging up.  Clean it lube it. Make sure the springs are not loose.  Servicing bikes with some miles, I'd pinch the hook on the advance springs with a pair of short needle nosed pliers. 

For me, one of the ways to adjust the idle mixture screws was to set them all the same, blip the throttle, if the idle was slow to return, richen the mixture. If the idle dropped below set RPM, lean it out. Advance mech has to be working properly.

 

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