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Discussion Starter · #1 ·
Hello, I am new to the world of motorcycles and purchased a 1981 Honda CX500C. Bike ran great for the first month and a half. Two days ago I found that when I gave anymore then half throttle the bike would bog down and would not rev past the 5k mark.



My friend and I searched online and found this useful forum to try and diagnose the problem.



We checked the valves(within spec), we check the carbs (spotless) and we even did the resistance checks on the CDI. The resistances pretty much all came back approximately 30-40 ohms higher on all except the 5/9 (i think it was 392 ohms). I am really hoping someone might be able to help or direct me as to what i might be able to do to correct this problem I am having.
 

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First, welcome to the forum. And kudos for putting your location and bike's model in your profile. We like you already.



As to your problem, there are a couple of things I would check. One is if the jets are in their proper towers. The higher number jet goes into the brass hex fitting. And they can fit either way. Second would be the resistors in the plug caps. There are posts on how to remove and clean and repair this. Third could be the carb boots that are on both sides of the carbs and the O rings at the head. Use some starting fluid to spray around the area in puffs to see if the idle speed changes. If it does, you have a leak at that area. And the CDI and the stator can also be going out. Lets hope it is the first couple of areas I mentioned.
 

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Discussion Starter · #3 ·
Yes we cleaned out the plug caps also. They were initially reading in the 20-30 million ohm range. After we cleaned them out we got them back down to roughly 13k ohm. Might look at checking the jets tonight. The carb boots were replaced with new about a month ago so i would sure as hope that they aren't the problem. The more I read about the staor / cdi unit issues the more i think it is going to be that...
 

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I had similar issues and went through a whole battery of tests ruling out one item after another. Mine eventually pointed to the CDI. The test was a no load test of the output voltages from the CDI to the Coils. Each voltage should read 140 to 150V.



Disconnect the coils, remove the spark plugs from the cylinders, set multimeter to 200VDC, connect the multimeter "+" Lead to the CDI output, probe the multimeter lead to ground terminal on the battery. Ignition on, roll the bike over/hit the start button noting the highest voltages on the meter. These values will fluctuate, so just watch for the highest value.



Mine measured 140V on one side, and 37 on the other--a significant imbalance. The bike would sound as you described when the voltage imbalance occured--like a drag bike a the start line rev'ing poppopopopopopopopop...green and then go except my bike had no go, sounded like it was on one cylinder etc.



I now have an Ignitech and the difference is amazing.



[EDIT] You will want to find that table for checking the stator resistance values...not sure cause I dont have the table in front of me but the one that you suspect might have somethign to do with your ability to go above 5k.
 

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Discussion Starter · #5 ·
Bike is bogging down when we spray around the left side carb.



Tried the voltage test, but i must not be doing something right, not getting any reading. Pulled plugs and coils and testing from output to battery ground.



Just removed the air cutoff valve cover on the carb and found a tear in the diaphragm, could this cause it to bog down when i give half throttle or more and not allow to rev past 5k rpm?
 

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Bike is bogging down when we spray around the left side carb.



Just removed the air cutoff valve cover on the carb and found a tear in the diaphragm, could this cause it to bog down when i give half throttle or more and not allow to rev past 5k rpm?


Find out where the air is leaking and fix it. Then you can block the air cut-off valve with no ill effect. There is a tiny O ring that goes under the tab or ear on the cover, replace it with a solid piece of rubber the same thickness as the o ring, put the old valve and spring back in and replace the cover.



See how it work with these things fixed,,,



Let us know what happens,,if you do need to do more testing we can walk you through it.
 

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Discussion Starter · #7 ·
Carbs are all good now, unfortunately that did not fix my issue. Picked up an inline spark plug tester and hooked it up. Right side had continuous spark while giving throttle, hooked up to the left side and when I got the rpms up to around 5000 rpm the light in the tester would cut out.



Going to try and get the numbers from the resistance checks on the stator and try and post them some time today (at work right now...shhhh).



I am thinking the CDI box is my culprit.
 

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...I am thinking the CDI box is my culprit.


In addition to the Stator Resistance and Continuity Tests, you will want to measure the CDI voltage outputs to each one of the coils then. Reg from Bristol introduced me to this check, and it is what left me to conclude my CDI was failing.



I would start with the stator resistance checks, the continuity tests between the windings and ground, and then move to the CDI output voltages to each one of the coils.



I think you are heading down the right path for sure.
 

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Find out where the air is leaking and fix it. Then you can block the air cut-off valve with no ill effect. There is a tiny O ring that goes under the tab or ear on the cover, replace it with a solid piece of rubber the same thickness as the o ring, put the old valve and spring back in and replace the cover.


Requesting a sidebar...



The ACV that I can get to on my bike (the left one) has a small tear in the diaphragm also. Are you saying I can just plug that off and be ok, or is that only a temporary fix until I get a new one?



Fib
 

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Requesting a sidebar...



The ACV that I can get to on my bike (the left one) has a small tear in the diaphragm also. Are you saying I can just plug that off and be ok, or is that only a temporary fix until I get a new one?



Fib


Yes.You can plug the small hole but leave the rest of the old assembly in place.One of my carb sides has been done this way and then just adjust mixture if required




Also see this thread,



http://cx500forum.com/index.php?/topic/8684-need-cdi-coilmodule/
 

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There is a test on here somewhere by Reg that tells how to test CDI outputs on the pink and yellow wires. Should be at least 100 v from memory, while cranking.



Try swapping the coils to rule them out, but the stator fault usually interferes with the running of both cylinders.
 

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I'm going to have to hunt down that test and try it.


I posted the test early in this thread...



I had similar issues and went through a whole battery of tests ruling out one item after another. Mine eventually pointed to the CDI. The test was a no load test of the output voltages from the CDI to the Coils. Each voltage should read 140 to 150V.



Disconnect the coils, remove the spark plugs from the cylinders, set multimeter to 200VDC, connect the multimeter "+" Lead to the CDI output, probe the multimeter lead to ground terminal on the battery. Ignition on, roll the bike over/hit the start button noting the highest voltages on the meter. These values will fluctuate, so just watch for the highest value.



Mine measured 140V on one side, and 37 on the other--a significant imbalance.
 

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Discussion Starter · #14 ·
I posted the test early in this thread...



I had similar issues and went through a whole battery of tests ruling out one item after another. Mine eventually pointed to the CDI. The test was a no load test of the output voltages from the CDI to the Coils. Each voltage should read 140 to 150V.



Disconnect the coils, remove the spark plugs from the cylinders, set multimeter to 200VDC, connect the multimeter "+" Lead to the CDI output, probe the multimeter lead to ground terminal on the battery. Ignition on, roll the bike over/hit the start button noting the highest voltages on the meter. These values will fluctuate, so just watch for the highest value.



Mine measured 140V on one side, and 37 on the other--a significant imbalance.


I tried doing this test but with no success, I removed the coils and plugs from the cylinder, i attached my "+" to either the yellow or pink wire depending on what side i was testing and probed to the battery ground. Set meter to Vdc and cranked the start, didn't get any reading. I'll see about doing this test again tonight, I'm sure there was some user error somewhere.
 

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The CDIs are dropping like flies this year. My low range coil just went.
 
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