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Intermittent one cylinder not running, need help to troubleshoot.

362 Views 6 Replies 4 Participants Last post by  3DdesignsCX500
Hey guys,

I have a 1980 CX500. All original, low mileage bike.
Murray just rebuilt the carbs, dropped them in as is. I have not adjusted anything. The bike fires up, runs, but having issues that I need help to sort out.
  • It idles very high, around 3000
  • It also runs mostly on 1 cylinder, but sometimes, typically at higher RPM's the second cylinder kicks in, then when the engine revvs lower it drops to one cylinder again.
  • The ignition is the original CDI
Pls help me troubleshoot.
Thanks in advance,
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if its idling that high with the enricheners off then you have the cable tweaked somewhere call me tomorrow
Hey Murray, I haven't had time until today, but I pulled the tank, and made sure the cables are not getting pinched. All of them look like they are seated properly and there is no tension, however, I m still getting a high idle, and the left bowl is leaking while the bike is running. Incidentally, it is the left cylinder that is cutting out. I tapped on the bowl, and blew into the hose, but it did not make a difference. I am guessing the float is stuck?

Can we chat about this?
Pls send me a DM with your phone # and best time to call.

Thank you ZK
Hey guys,

Hope you're all having a great Memorial Day weekend!

I had some time to troubleshoot the bike and here are my latest findings.
Please let me know your thoughts. Where do I go from here next?
  • As per @murrayf 's advice I checked and made sure the throttle and choke cables are connected properly and not pinched anywhere making sure the throttle could freely move and seat at the bottom.
  • I tightened the hose from the petcock because I noticed a leak - and not from the left carb bowl (as I originally thought) - note to self: make sure to tighten the fuel hose clamp after installing the tank.
  • I adjusted the idle speed by turning the black knob. (It did not need a lot)
  • I ordered and installed new o-rings for the intake to make sure there is no air leak.
The result was encouraging, the bike idled well with no choke, and it seemed both cylinders were running, when I checked by pulling either plugs althought, the left cylinder ran much weaker. (See video)

I went for a spin, but it immediately felt like it's running only on one cylinder as soon as I started going. The bike had no power and needed to give a lot of gas revving to 4k, just to get moving, and it was only accelerating on one cylinder. I pulled the plugs to see while moving, and found the left cylinder was not running. The bike still idled fine though.

In my original posting I am referring to an intermittent cylinder issue. At first the second cylinder would catch on at higher RPMs, but now it does not anymore.

After this ride, I did the following tests:
  • I checked for spark, by pulling the cables from plugs on both sides while running (as per my video)
  • I pulled the plugs and while the right one looked good, the left one was oily and black indicating it's getting fuel but not running.
  • I swapped the coils to check if one was not working, but it made no difference. The left cylinder was still the one not running. (I swapped them back)
  • I cleaned the connectors from the CDI in all 3 clusters.
  • I checked the stater resistance values against the Haynes diagram and the numbers look good. (See below)
    1/5 - 116 (normal range is 95-116)
    2/5 - 98 (81-99)
    3/5 - 212 (185-225)
    4/5 - (95-116)
    6/5 - 99 (81-99)
    7/5 - 215 (185-225)
    8/9 - 88 (77-95)
    5/9 - 360 (387-473)
Keep in mind the carbs are stock and just got rebuilt by Murray.

Some additional notes.
I also noticed a lot of smoke behind me on the road (probably the unburned fuel from the left cylinder). It still runs on one cylinder but seems to idle well on two.

I have been doing some research, and I have a sinking feeling that this might be a CDI related issue. I have also been researching options, for repairs and it appears the options are limited.

Looks like the pit bike coils are cheap enough and easy to replace, so it might be worth a shot, just in case but at the end it sounds like a CDI thing. ... again, open to your suggestions and comments.

I also measured the coils' resistance.
Left coil: 766
Right coil: 802
It appears they are also within the acceptable range, so no replacement is necessary.

Keep thinking CDI... thoughts?

Thanks in advance,
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Did you replace both spark plugs with new ones as an easy and cheap elimination of the possible problem?
Hey @Speedster. Thanks for the suggestion. My thoughts were the same. Even though the spark plugs were new, they were the iridium ones with the resistor, so I ordered the OEM non resistor version, since the plug caps are original with the resistors. The new plugs are on their way. I guess I'll see but I am not holding my breath.

Also, an update. I went for a spin yesterday (or rather tried to go for a spin) after measuring the coils' resistance. The bike started up fine cold (with no choke), and ran OK just getting down the alley, but as soon as it started warming up it was back to one cylinder, then died. I could not even get it started. So this is getting worse.

I did not try to measure the coil's resistance when warm. I might just get pit bike coils and swap to see if it makes a difference. Other than I am thinking I am going with Ignitech, but it's 250 bucks (give or take).

Any of you with BTDT experience?
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