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Howdy folks,

I have a 79 CX500 custon that stutters as I twist the throttle and back fires when I release the throttle. I had the carbs rebuilt but it didn't fix the problem. It seems to get worse when the bike warms up after an hour of riding. Please help I am new to the forum and to motorcycle repair. Any suggestions would be greatly appreciated
 

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One area you might check is the ACV valves. These are on the side of the carb under a flatish plate. The left side is easy to get to even with the carbs attached. The right side is a bugger even with the carbs off.



These valves actually enrich the mixture a bit during deceleration. They contain a diaphragm, a spring, and a tiny O ring that goes under the leg of the cover. All need to be there or you will experience backfiring on throttle closing. Hold the diaphragm up to the light and see if there are any pinholes. Replacements are available, but they are $25 or so. A work-around is to cut a circular piece of solid rubber to replace the O ring. Leave everything else inside. You may have to open up the idle mixture screws a bit to compensate for the change, but no harm will occur. And this may solve the backfiring issue.



These ACV valves were originally developed to help keep the mixture as lean as possible for EPA emissions, but now the EPA police will only put you in jail for less than 6 months for messing with them. I am posting this from cell #54623, Leavenworth, KS.
 
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