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Should the gasoline come out in a solid stream while the bike is off on a 1981 CX500C, or in sputter

  • Solid Stream while engine off, petcock ON

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  • Thick sputters while engine on, nothing while engine off

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Discussion Starter · #1 ·
I have a 1981 Honda CX500C and it's been a little strange recently. Checked the valve clearance, and it seems perfect (although my only experience is with a Rebel 250).

When I down rev and then pull the clutch and up rev the engine I get a small "pop" or backfire. I think the bike is running lean and I'm trying to pinpoint the problem. I replaced the spark plugs, and have done everything else short of a carb rebuild... Which I really do not want to do. This carb scares me. The Rebel's carb was bad enough and I just had to rebuild a clutch last month.
 

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if i recall on the 81 its a vacuum operated petcock so unless the engine is running then you will get no fuel at all and it does sound like you do need a carb clean i would also take the petcock off and clean the in tank filter screen



also update your profile that way you can put your location and bike type in it you never know some one might live right around the corner and is willing to lend a hand and help you out
 

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Discussion Starter · #3 ·
if i recall on the 81 its a vacuum operated petcock so unless the engine is running then you will get no fuel at all and it does sound like you do need a carb clean i would also take the petcock off and clean the in tank filter screen



also update your profile that way you can put your location and bike type in it you never know some one might live right around the corner and is willing to lend a hand and help you out


Hm.. Alright. Well if anyone can give me any more information about how the vacuum works then that would be great. If I were to plug the petcock vacuum tube incorrectly would it cease to run or would it just stutter the way that it is?
 

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it would run for a few minutes i believe i do know that you can make your petcock a non vac type real easy and real quick all you have to do is plug the line on the right carb (sitting on bike) and then take the petcock apart and switch the spring to the other side of the diaphragm and then run a piece of fuel line to the 2 nipples on the petcock and thats how you make it a non vac petcock just





but any way the vacuum works by the motor when the engine is running its creating a pull from the pistons and then that pulls into the carbs and then the carbs pull from the petcock thru the vac port and i am pretty sure if you do a search for petcock problems you will find alot more info
 

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In more basic wording intake manifold vacuum opens the petcock.
 

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Assuming that it is a vacuum actuated petcock it will have 2 smaller diameter hoses hooked up to the back of it and a larger diameter fuel hose that goes between the petcock and the carbs (if the previous owner installed an inline fuel filter you should change it and if they didn't you should install one). The small hose closest to the front of the bike is the vacuum hose. It needs to be hooked up to the vac nipple on the right carb. The other small hose is just a vent hose, and should just run down in front of the rear wheel with the other drain/vent hoses.



So, to eventually answer your question, when the bike isn't running no fuel should flow and when the motor is cranking a steady stream of fuel should flow. If it flows when it shouldn't you're probably going to want to take the petcock apart and have a look at it. If it is sputtering when the motor is running you'll want to check the condition of the vac hose too. It'll probably at the very least be fitting loosely, if not actually split. Either way you won't be getting good vacuum. This would also explain your sudden lean situation. What colour are the plugs? The carbs are actually pretty reasonable to disassemble and clean, especially with the good advice (there's a book too) available on this forum. They're a little harder to get 'right' but it's worth it.



I don't like this style of petcock and a lot of people here don't either. The fix that Lucky described is one that has worked for many, although I encountered the odd problem that with the two ports joined with a short length of hose, when the bike warmed up, the air in the bypass tube seemed to expand enough to force the diaphragm closed in the other direction. At least, that's the guess I came to when the gas repeatedly shut off shortly after the bike got hot. I think my fix was to keep the vent line in place and use a blocked-up short piece of vac hose to block off the vac port on the petcock. Actually you'd need another of these plugs to block off the port on the right carb too. Since then I haven't had any problems except for when it puked out about a cup of gas onto the freshly surfaced parking lot at the Tim Hortons in Sault St Marie. But that was a stuck float and it hasn't happened since.



If it's a regular petcock valve then nevermind.




Actually, you can clean it and install an inline filter. You should probably get the carbs apart and cleaned too but at least you can see if this makes a difference.



And there's lots of help on this forum for the asking.



Cheers
 

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Discussion Starter · #11 ·
Probably shouldn't run if the petcock was functioning normally. Vac line runs to the forward most nipple on the carb, the middle one is a vent.


I had it plugged into the middle one, and it definitely didn't feel as though it was running normally. And it wouldn't start for a short period of time but then it started running (although poorly. It felt as though it was running extremely lean)



Maybe I've muddled up the petcock now.. We'll see. I'm going to get some riding in during the next few days and see how it behaves.
 

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If you had the vac line plugged into the vent port and your petcock isn't working properly (it sounds like it isn't) I suppose it's possible that you were sucking air into the right carb by way of the vent line, leaning it out. These are stupid petcocks. Stupid stupid stupid.



We seem to have 2 Calebs on the forum now, unless they are two incarnations of the same Caleb. My friend Caleb, out in Vancouver, collects Calebs as friends on Facebook. I think he's got over 30 by now...
 

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Discussion Starter · #13 ·
If you had the vac line plugged into the vent port and your petcock isn't working properly




Weird thing.. I had the line plugged into the vent port. The bike would run, and take me from place to place. I plugged it into the correct place last night and took a ride, and the bike died on me. (I had gone about a mile, maybe two?) My choke never pulls out by the damn knob, so I grabbed the choke wire and pulled it directly until it slid out and the bike started back up rather sluggishly. It got me the rest of the way home, and accelerates/decelerates just fine but at cruising speed it stutters ever so slightly.



Stuttering in a way so that I can feel that the RPMs dropped for a split second. But it doesn't jerk, and it's not the "running out of gas" stutter. It will give me that split second dropped RPM feeling once every 15-30 seconds.

Stupid petcock.
 

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Discussion Starter · #14 ·
Ok well I just went out and converted it to non vacuum. I placed the spring on the opposite side of the rubber seal. I took the small plastic tip that I cut off of my hypoid oil container, shoved a BB of plumbers putty into the cone and wedged that on top of the vacuum.



Still a little of the 'stutter' at super low to low RPM, but I went around the block with the bike heated up and it seemed smoother than it has been.. So hopefully that's the fix and I just need to calibrate the air/fuel mixture screw. Hopefully.
 
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