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1979 Honda CX500 Custom
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17 Posts
Discussion Starter · #1 ·
Hello, I am experiencing a high idle issue. Once the bike is warmed up, it's idling near the 2k rpm mark, even though my idle adjustment screw is all the way out.

I did some searching on the forum and found where folks are saying the mechanism between the two carbs may be binding. I took carbs out last night and reset them as level as I can (there's not a lot of leeway, I noticed) following instructions in Larry Cargill's book.

If the carb mechanisms were binding, wouldn't I be able to see the butterfly valves aren't in sync? Everything looks good to my eye. Throttle plates are balanced and move together. Choke plates move together as well.

Is there something else I should look for while I have these carbs out? What else could make my idle so high?
 

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1978 CX500 "The Grub", 1983 GL650I "Nimbus"
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10,885 Posts
High idle can indicate a lean condition. Are your idle circuits clean? How about the emulsion tube under the rubber plug?
 

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Routing of throttle cable, lack of free play in the throttle?
 

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288 Posts
When you set/checked float height, did you place the carbs completely upside down or held at an angle such that the floats barely engaged the valve plunger?

If set when completely upside down, the weight of the float depresses the plunger more than when in normal operating position, which means you might have adjusted the float into a too lean condition.
 

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1979 Honda CX500 Custom
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17 Posts
Discussion Starter · #6 ·
Thank you all for the input. I found a large error on my part, but I'm not sure if that was the cause of the high idle. I had forgotten I was jetted for the pods I used to have (I run the stock airbox now). So I replaced the jets with stock sizes. While I was in there, I re-cleaned all the emulsion tubes and did a general check-up of cleanliness. I rebuilt these carbs not long ago using Larry's book, and everything was still quite clean.

I re-adjusted the throttle plates to get them perfectly in-sync. Then, when I re-installed the carbs, I made sure my cables were routed correctly and not binding anywhere. I also washed and re-oiled my K&N filter.

High idle is gone. I'm not sure what the issue was (would the larger jets have caused it?), but I'm glad to have it running properly now. Now on to check valve clearances and cam chain tensioner, then take a nice celebratory ride.
 

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Thank you all for the input. I found a large error on my part, but I'm not sure if that was the cause of the high idle. I had forgotten I was jetted for the pods I used to have (I run the stock airbox now). So I replaced the jets with stock sizes. While I was in there, I re-cleaned all the emulsion tubes and did a general check-up of cleanliness. I rebuilt these carbs not long ago using Larry's book, and everything was still quite clean.

I re-adjusted the throttle plates to get them perfectly in-sync. Then, when I re-installed the carbs, I made sure my cables were routed correctly and not binding anywhere. I also washed and re-oiled my K&N filter.

High idle is gone. I'm not sure what the issue was (would the larger jets have caused it?), but I'm glad to have it running properly now. Now on to check valve clearances and cam chain tensioner, then take a nice celebratory ride.
After you check/tweak the valves, probably best to re-check carb sync with bike running and warm. Bench sync can get you close, often good enough, but running sync gets them perfect.
 
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