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Discussion Starter · #1 · (Edited)
My bone stock '80 CX Deluxe is behaving weirdly. It seems that the right cyl is not firing at idle, even a fast idle like 3k rpm, but when riding it feels OK. Accelerates fine, 40 mph = 4k rpm, etc.

Problem is that at normal and fast idles (up to 3k) I can pull off the RH spark plug cap and there is no effect. I haven't yet tried pulling the cap while riding at 5k rpm.

Compression is a solid 140psi when cold and the plug is firing when removed and held against the cyl head. Maybe the idle circuit is clogged, but the carbs have been off and cleaned with both ultrasonics and compressed air. I've cleaned many carbs before and know what to look for. I've got some BG44k in the fuel as a first pass in case the idle circuit is partially blocked.

In general, is it dangerous to ride the bike with one cyl potentially not firing at low rpm? Will it cause any engine damage? I may need to do some test runs as I diagnose things.
 

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Riding on 1 cylinder or just running on 1 cylinder for diagnostic purposes??.....both are different...running/riding for extended periods will put a strain on other components.....
 

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Discussion Starter · #3 ·
Riding on 1 cylinder or just running on 1 cylinder for diagnostic purposes??.....both are different...running/riding for extended periods will put a strain on other components.....
I have been riding it in current condition, maybe one hour in total so far. Only noticed the weird "removing cap has no effect" situation within the last 30 minutes of riding.
 

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I re-read your OP its only at low RPM.?
If the engine is not "laboring" risk is reduced.
 

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You will get a lot of unburned fuel bypassing the rings in some instances.

Keep an eye on your oil..

To see if it is throttle position{ idle circuit} or RPM dependent spray a little fuel in through the balance port of the non firing cylinder at idle and see if the cylinder fires up. If it does the idle circuit still isn't clean.

Did you pressure test the idle circuit and check for fuel spraying through all holes in the venturi?
 

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Discussion Starter · #7 ·
You will get a lot of unburned fuel bypassing the rings in some instances.

Keep an eye on your oil..

To see if it is throttle position{ idle circuit} or RPM dependent spray a little fuel in through the balance port of the non firing cylinder at idle and see if the cylinder fires up. If it does the idle circuit still isn't clean.

Did you pressure test the idle circuit and check for fuel spraying through all holes in the venturi?
Thanks for the point on oil. I had changed it Sunday before the ride, so if the volume increases I'll drop it out again. I'll do the balance port test next chance I get. Did not do a pressure test, beyond the normal process of running compressed air thru every carb port
 

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Discussion Starter · #8 ·
stator test to eliminate the stator and pickups
Will add that to the checklist, although electrical systems are my weakness.
 

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Discussion Starter · #9 · (Edited)
UPDATE - Fueling is now top of the list of culprits. Shooting some fuel into the balance port while idling caused a momentary surge. Did another test ride and it still seems fine above 5k rpm, pulled fine and the RH cyl head and exhaust got hot, but not as hot as the LH cyl/exh.

Weirdly, if the bike is idling with the choke on, then pulling the RH plug cap off DOES cause the engine to drop. Does the choke cause fuel to be pulled up through the main jet?

I've got a heavy dose of BG44k to see if that might clear passages, but if not it's time to pull the carbs again.

UPDATE#2 - After pulling the carbs, vacuum leak is now more likely than fuel passages being blocked. The intake manifold was damaged, the diaphragm for the cut off valve had a small tear, and the rubber portion of the intake manifold had an exterior crack, but it didn't seem to go all the way thru. I'll resolve those potential leaks, re-assemble, and re-test. For the cut off valve, I've replaced the diaphragm with one in better condition, but will block the air passage regardless to be extra safe. From what I've read, cut off valves aren't critical, but opinions tend to vary.
 
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