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Discussion Starter · #1 ·
Hi to all,

I'm new to this forum and just purchased a CX500 Eurosport to get back to biking after too many years to remember, I'm a proper old fart now but still 30 odd in the head. The bike needed some attention, fuel tap diaphram popped, plug on blink, plug cap cracked, exhaust leaking slightly but now all fixed except one final problem so need your valued help. When the bike's cold and on choke it runs fine but once warmed to running temperature it runs fine and holds revs steady with no misfires but when the throttle is closed fuel pops in the exhaust, seems like fuel is getting into the exhaust after throttle is closed. Would I be right in thinking this could be caused by a blocked air filter? Also what does the air cut of valve do? Could this be a candidate for my problem.



Regards to all and roll on the summer.
 

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Hi there... where yat?



As you mention, there have been a "few" issues already adressed ( sounds like mine ) sounds like you need to go through the air intake syst from the filter, boots, then carbs...



My bike ran like a turd initially. Cleaning the filter(s) in the airbox and un plugging the crankcase breather did wonders ( ran on both cyl ) Fixing the airleaks also made things much better.



The big winner was cleaning my carbs last week... the little decelleration pops I figured were normal are now 99.9% gone.





You could have so many small issues combined or one big one... it's impossible to guess. A good cleaning and maint of air syst. and carbs won't hurt anything and may bring it around. most if not all cx's either have had, or badly need this TLC...



Reading your sparkplugs is a good initial diagnostic.



Added: from the web...





AIR CUTOFF valve - Enrichment Circuit

You're blasting down the road and you see a hairpin coming up, so you close the throttle. This closes the butterfly valve. You and your 400 lb. baby (the bike, not the other love of your life
), are still rolling with inertia, keeping the engine revs high. Now it can't get the mixture it's trying to suck in because the butterfly valve is closed, blocking the air flow.







To compensate for this there's an air jet in the lower diaphragm chamber that transfers ambient pressure to one side of the coasting enricher's spring-loaded cut off valve. The excess vacuum in front of the butterfly valve is transferred to a drilling that leads to the other side of the cut off valve. This sucks it open, allowing the pilot jet to feed more fuel to the engine, preventing an overly lean condition. (Pop. Pop. Pop.)





 

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The ACV valves are exactly for what you describe is happening. You may need to pull them and check that the diaphragm is not pinholed. Just hold it up to the light, you should be able to see the pinholes if they are there. Also check that the O ring is installed properly under the ACV cover. Honda designed these valves to meet EPA guidelines for a leaner burn. They do work when everything is right.



Some have bypassed the valve's function by installing a small rubber disk in lieu of the O ring. You leave all the parts inside intact. But the action of the diaphragm is stopped by the lack of vacuum thru the O ring hole. If this is done, you may need to richen the idle mixture up a bit to compensate for the fix. This can be used indefinitely, altho most replace the valve.
 

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Usual cause are the carburettor Air-Cut-off valves.You can get replacements or bypass them with no ill effects.









You leave the mechanism in and block the small hole where the tiny O-ring would go at about 7pm/8pm on the picture.
 

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Discussion Starter · #5 ·
Many thanks to all for such quick responses.

Nolavox -- at the wrong side of 60 I don't think I'll be blasting down any road towards a hairpin more likely tortoising towards the nearest public convenience as my bike and I will be doing about 3 pisses to the hour :) I'm in the UK incidentally and just realized I'm on a US site. The advice you all give and your knowledge levels are excellent. Look forward to solving my problem, probably after severa hours of cursing

Regards

Kenny W
 

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Many thanks to all for such quick responses.

Nolavox -- at the wrong side of 60 I don't think I'll be blasting down any road towards a hairpin more likely tortoising towards the nearest public convenience as my bike and I will be doing about 3 pisses to the hour :) I'm in the UK incidentally and just realized I'm on a US site. The advice you all give and your knowledge levels are excellent. Look forward to solving my problem, probably after severa hours of cursing

Regards

Kenny W


Although this is a US based site it has international membership.Put where you are in your Signature or under your avatar.I live in Hull,East Riding of Yorkshire
 

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Hi there... where yat?



As you mention, there have been a "few" issues already adressed ( sounds like mine ) sounds like you need to go through the air intake syst from the filter, boots, then carbs...



My bike ran like a turd initially. Cleaning the filter(s) in the airbox and un plugging the crankcase breather did wonders ( ran on both cyl ) Fixing the airleaks also made things much better.



The big winner was cleaning my carbs last week... the little decelleration pops I figured were normal are now 99.9% gone.





You could have so many small issues combined or one big one... it's impossible to guess. A good cleaning and maint of air syst. and carbs won't hurt anything and may bring it around. most if not all cx's either have had, or badly need this TLC...



Reading your sparkplugs is a good initial diagnostic.



Added: from the web...





AIR CUTOFF valve - Enrichment Circuit

You're blasting down the road and you see a hairpin coming up, so you close the throttle. This closes the butterfly valve. You and your 400 lb. baby (the bike, not the other love of your life
), are still rolling with inertia, keeping the engine revs high. Now it can't get the mixture it's trying to suck in because the butterfly valve is closed, blocking the air flow.







To compensate for this there's an air jet in the lower diaphragm chamber that transfers ambient pressure to one side of the coasting enricher's spring-loaded cut off valve. The excess vacuum in front of the butterfly valve is transferred to a drilling that leads to the other side of the cut off valve. This sucks it open, allowing the pilot jet to feed more fuel to the engine, preventing an overly lean condition. (Pop. Pop. Pop.)
brilliant post Nola,iv never seen that before....sweet
 

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Discussion Starter · #8 ·
Well I got lucky,

I thought I had a carb strip down job and rebuild to contend with BUT found a slight leak from the front exhaust pipe where it bolts to the engine, the top stud had come away and lost its grip, a quick re tap and new studding cured the popping exhaust on overun. I could not hear this leak when the bike was outside but picked up on it when starting up indoors, however the missus does not like the new addition to the front room carpet, you never can please 'em no matter how hard you try!
I have learned a lot about the fuel side of my new old Honda though thanks to you guys, cheers to all.

Kenny W
 
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