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Discussion Starter · #1 ·
I go from thinking it's a fuel problem to thinking it's an electrical problem and back again but, in the end, I don't know what it is. As of late, my '81 CX500 (74K) will tolerate only MODERATE acceleration. If I try to "punch her" from a standstill, she'll try to respond as she always has in the past, but soon stumbles at maybe 3000-4000 rpms. She'll accept a moderate request for acceleration and I've gotten to know just how much she can deal with before she rebels. If I treat her gently she'll eventually climb to pretty much whatever I ask but trying to force her through this bad stumbling is futile. I have no choice but to back off the throttle and work her up to speed gently. She's an eager starter, idles smoothly, never stalls, and I have no fear of getting stuck far from home. It's just that the brisk acceleration that she always had has turned to sad stuttering.



I realize it could probably be one of a dozen things, but any input or stabs at the problem would be appreciated....thanks....gweric
 

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Seriously sounds like gummed up carb jets, and likely the carbs in general needing a good work over. Get your hands on Larry's (LrCxed) book, or find many of the great carb cleaning threads, starting in the General Discussion section under Quick References.



It doesn't sound electrical right off the bat to me, since you can work your way up the ladder.



Joel in the Couve
 

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i to am leaning towards carbs can you rev in neutral or does it still stumble? if you can rev past 5k under load and in 5th gear (i believe) it would most likely be a carb issue if not then most likely its a stator can you give us any history of the bike? maintenance records or of the sort? there should be some more people coming in soon that can answer a lot better than me
 

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reduce the gap in your plugs and/or replace them both if they have not been done recently.

how old is your air cleaner - is it restricted?
 

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Discussion Starter · #5 ·
i to am leaning towards carbs can you rev in neutral or does it still stumble? if you can rev past 5k under load and in 5th gear (i believe) it would most likely be a carb issue if not then most likely its a stator can you give us any history of the bike? maintenance records or of the sort? there should be some more people coming in soon that can answer a lot better than me


Thanks, Lucky, and to all who have a suggestion here. What makes me feel that I could be dealing with an ignition or timing problem here is the fact that I have two sets of carbs which I've swapped in and out on the bike over the years and this haunting problem seems to in time occur no matter what set of carbs I'm running. Fresh plugs seem to cure the problem, but only temporarily. I'm not an ace mechanic but I do feel that I clean both these sets of carbs thoroughly when the others are on the bike, following several "step by steps" offered here meticulously. With the other carbs on the bike and it up and running, it's never a "rush job." I must admit, when it comes to the pilot screw settings, I start with the recommended starting point and then kinda tweek them by ear and feel until the bike seems happy, but never drastically far from the initial setting. I synch them and enjoy a smooth idle at 1100rpms, and could in fact back the idle down to maybe 900 if I chose. I just don't feel the mixtures could be so out of whack to give me the problem I'm suffering from.



On the electrical side of things, I must say that it's all a rather dark realm to me. I do have a multimeter and noted a while back that my charging rate at idle was pretty weak, but I gave my gound connection on the frame a good cleaning (no doubt for the first since the bike rolled off the assembly line) and used a new screw and found the charge to perk up considerably, in fact within the acceptable range. My battery always seems eager. I try to follow the electrical diagnoses that are offered here but I soon get kinda lost. Maybe I'll go back and try again.



Anyway, I've gotten a little long winded here, but, once again, any suggestions are appreciated. This bike ('81 Custom 74K) has served me so well since I bought her second hand maybe ten years ago that it pains me when she's not right. I've put time, energy and more ebay parts than I can count into her but she's always paid me back in spades. Thanks again, gweric
 

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Discussion Starter · #6 ·
i to am leaning towards carbs can you rev in neutral or does it still stumble? if you can rev past 5k under load and in 5th gear (i believe) it would most likely be a carb issue if not then most likely its a stator can you give us any history of the bike? maintenance records or of the sort? there should be some more people coming in soon that can answer a lot better than me


Thanks, Lucky, and to all who have a suggestion here. What makes me feel that I could be dealing with an ignition or timing problem here is the fact that I have two sets of carbs which I've swapped in and out on the bike over the years and this haunting problem seems to in time occur no matter what set of carbs I'm running. Fresh plugs seem to cure the problem, but only temporarily. I'm not an ace mechanic but I do feel that I clean both these sets of carbs thoroughly when the others are on the bike, following several "step by steps" offered here meticulously. With the other carbs on the bike and it up and running, it's never a "rush job." I must admit, when it comes to the pilot screw settings, I start with the recommended starting point and then kinda tweek them by ear and feel until the bike seems happy, but never drastically far from the initial setting. I synch them and enjoy a smooth idle at 1100rpms, and could in fact back the idle down to maybe 900 if I chose. I just don't feel the mixtures could be so out of whack to give me the problem I'm suffering from.



On the electrical side of things, I must say that it's all a rather dark realm to me. I do have a multimeter and noted a while back that my charging rate at idle was pretty weak, but I gave my gound connection on the frame a good cleaning (no doubt for the first since the bike rolled off the assembly line) and used a new screw and found the charge to perk up considerably, in fact within the acceptable range. My battery always seems eager. I try to follow the electrical diagnoses that are offered here but I soon get kinda lost. Maybe I'll go back and try again.



Anyway, I've gotten a little long winded here, but, once again, any suggestions are appreciated. This bike ('81 Custom 74K) has served me so well since I bought her second hand maybe ten years ago that it pains me when she's not right. I've put time, energy and more ebay parts than I can count into her but she's always paid me back in spades. Thanks again, gweric
 

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it at times can take more than one time to get the carbs clean enough i know that once in a while larry aka the carb god sells his work on here it wont hurt to ask what he thinks it could be but the more your talking about the issue and sense you put on more than one set of carbs i am thinking stator is on its last legs but i can be wrong
 

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You should definately do the stator check detailed in the sticky thread. Its possible your stator is starting to go maybe in the high coils. There is a temp fix if I remember correctly.



Mike
 

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Changed the plugs and it got better for a while? What do the new plugs look like if you remove them after it starts runing poorly again?



Could be they're getting fouled out from a rich condition, could be that your spark isn't hot enough. Looking at the plugs will tell you this.



Iridium plugs may help it run a bit longer but you best find the root of the problem.



My PO, with the straight pipes he put on (no crossover or anything) tried to compensate for with the mixture screws and my plugs were black but still fired just fine, does tend to make you wonder if your coils or ignition are weak.



What about the battery? Still crank up good and strong?
 

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Discussion Starter · #10 ·
Changed the plugs and it got better for a while? What do the new plugs look like if you remove them after it starts runing poorly again?



Could be they're getting fouled out from a rich condition, could be that your spark isn't hot enough. Looking at the plugs will tell you this.



Iridium plugs may help it run a bit longer but you best find the root of the problem.



My PO, with the straight pipes he put on (no crossover or anything) tried to compensate for with the mixture screws and my plugs were black but still fired just fine, does tend to make you wonder if your coils or ignition are weak.



What about the battery? Still crank up good and strong?




Thanks, Marshall, and to all who've had an opinion here. I'll start at the top as to your inquiries:



New plugs cure the problem immediately but not for long. After maybe a few hundred miles, the stumbling starts again and when I pull the plugs, they're black and grimey. In an effort to save a buck, I've tried cleaning the plugs but that doesn't give the results new plugs do. My plugs need replacing WAY sooner than they should. I could try new plugs again and turn the mixture screws in a half turn or so. By the way, both cylinders seem to foul the plugs at about the same rate. (I've never done a compression check. worn valves??)



As mentioned above, I'm no stranger to Ebay and, maybe two years ago, scored a set of coils on the cheap which I swapped out with the originals. All the symptoms stayed pretty much the same. I've removed the resistors from the plug caps and have the brass rod system in them.



My battery is never sluggish and the bike cranks energetically every time.



A year and a half or so ago, I did something for which I'm now kicking myself. I had the engine out and the rear cover off to replace the cam chain and mechanical seal but left the stator untouched. I know, I know.... but this problem had not yet arisen and I guess I just went by the "if it ain't broke, don't fix it" theory. If I knew then what I know now....or at least suspect.



Anyway, my best bet now seems to dig up that "stator check procedure" and try to plod my way through it. "They fear what they don't understand"...lol



Thanks again, to all. I've learned a ton on this forum and hope to continue...gweric
 

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Discussion Starter · #12 ·

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to be honest i have no idea but i would have the power on
 

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to be honest i have no idea but i would have the power on


Not necessary as far as I know, you are checking the resistance across different pins on the wirinf harnes, which you will be unplugging, which will disconnect it from the electrical system, so havig the key on will not matter. Just check the different pns as indicated in the diagram and let us know what you find out.



If one of your CDI coils is bad, you can replace the stator, or get an ignitech




Mike
 

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I still don't think plugs would get black just because of a weak ignition, sounds more like a stuck coke or something else related to fuel delivery.
 

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New plugs cure the problem immediately but not for long. After maybe a few hundred miles, the stumbling starts again and when I pull the plugs, they're black and grimey.




What is your compression? Getting any smoke from the tailpipes? IMO sounds maybe like a rich condition or rings are worn.



Good Luck
 

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Rings rarely wear out unless the thing was run out of oil on some occasion.



What plugs are you using? If it is burning oil you could go one heat range hotter.
 

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Discussion Starter · #18 ·
Rings rarely wear out unless the thing was run out of oil on some occasion.



What plugs are you using? If it is burning oil you could go one heat range hotter.




Thanks again for all the input. All I need is a free afternoon to check out the many suggestions that have come my way:



Lucky, I haven't had a chance to do the stator check but that method you gave me seems pretty straightforward. Gonna go after it soon.



Marshall, my choke is ok. The cable is well lubed and I can feel with my finger that when the choke knob is fully depressed the choke linkage is in the fully open position. The plugs I'm using are the standard NGK D8EA's. I'm gonna lean out my mixture screws this week and see if things improve.



Trickster, I get no smoke out the back, but I do get a little "popping" when decelerating and the bike is overrunning the engine. Could that be an air cut off valve snafu?



Anyway, I'm on the bike every day and it's as reliable as ever. It's just one of those small mysteries that's bugging me. Sooner or later I'll hit on the cause of her hesitant response to a good twist of the throttle...thanks, everyone...gweric
 

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There you go, air cutout valves. They're available from several sellers but some people just block them off. Persoally I'd inspect one first, and if it looks bad I'd block them off to see if things change. If that cures it I'd go ahead and replace them. Some people just keep the things blocked off but as picky as these carbs are I think they do more than just tweaking the mixture on deceleration so I'd feel better replacing mine when they finally go.
 

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Poor spark CAN blacken plugs as per an over-rich mixture due to an incomplete burn caused by poor spark and misfiring.

I recently had problems with my 81 custom which turned out to be a fault in the killswitch causing poor firing and instead of the bike's usual light grey plug colour- you guessed it, sooty and black.
 
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