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1978 CX500
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Discussion Starter · #1 ·
I've looked around and tried a bunch of trouble shooting already but I still haven't got the issue sorted out.

78' CX500, stock airbox, aftermarket exhaust w/ muffler

Symptoms: Right cylinder doesn't fire at idle. Inconsistent at 2k and above (misfires and popping on decel)

Things I've done:
  1. Brass rod mod
  2. Swapped and replaced plugs
  3. Rebuilt the Carbs like 4 times (stock jets and slightly larger jets, neither fix it, 78/112 80/120 I think?)
  4. Adjusted mixture screw a full turn in each direction (out 2.5 currently)
  5. Checked for spark on both cyl
  6. Valve adjustment? Idk if this would effect anything but I'm putting it in there
  7. Compression check, both cyl are nearly identical
  8. Checked briefly for vacuum leaks
Other things to note: when I had the idle adjust screw all the way backed off, the bike would only run on full choke but it idled right where it needed to be and both cylinders fired and it seemed to ride pretty good

Help me out guys I'm going crazy over this!
 

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Possibly? this comment has me wondering
Other things to note: when I had the idle adjust screw all the way backed off, the bike would only run on full choke but it idled right where it needed to be and both cylinders fired and it seemed to ride pretty good
Have you run this test yet?

Organism Font Engineering Screenshot Technology
 

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Other things to note: when I had the idle adjust screw all the way backed off, the bike would only run on full choke but it idled right where it needed to be and both cylinders fired and it seemed to ride pretty good
This makes me think that you may have a vacuum leak?
 

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The most likely suspect is the seal at the cylinder head, then at the carb and finally cracks in the rubber. If you have a stethoscope you might be able to hear a vacuum leak.
Did you have a tube on the brake cleaner spray bomb? That would allow for getting the spray right to suspect areas.
 

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1978 CX500
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Discussion Starter · #11 ·
The most likely suspect is the seal at the cylinder head, then at the carb and finally cracks in the rubber. If you have a stethoscope you might be able to hear a vacuum leak.
Did you have a tube on the brake cleaner spray bomb? That would allow for getting the spray right to suspect areas.
I've taken those tubes on and off a bunch of times dealing with the carbs and the seals are in good shape. Would a vacuum leak cause issues with firing? I can see how that would have effected the choke thing but I've got that solved now. And yes I used the straw with the brake cleaner
 

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The low speed jet is pressed in, not threaded, so tricky to remove. Were you able to get them out? A couple of methods are described somewhere here. The several passages beyond that jet are very prone to clogging and near impossible to clean with the jets still in. Larry’s carb book has excellent info and pics about successful cleaning of these carbs. Good luck!
 

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1981 CX500C
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330 Posts
Have you checked the resistance values on the stator, the igniters, the CDI box, coils????

Suggest to run down the simple first, work toward the more complex.

Good Luck. These types of problems can be real buggers to hunt down and solve
 

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1978 CX500
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Discussion Starter · #14 ·
The low speed jet is pressed in, not threaded, so tricky to remove. Were you able to get them out? A couple of methods are described somewhere here. The several passages beyond that jet are very prone to clogging and near impossible to clean with the jets still in. Larry’s carb book has excellent info and pics about successful cleaning of these carbs. Good luck!
I believe I did swap them out with my latest rebuild kit but I don't remember how well I cleaned in there after they were out. I can't say my carbs have ever really been gunked up since I've had them and I've deep cleaned them about 3 times in the last year but it is possible there's still some crud hidden away somewhere. I'll have to tear them apart again
 

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Can you tell how you have "cleaned" the carbs please.? Not wanting to be unkind - but squirting with carb cleaner and poking a piece of piano wire at the passageways does not and never will clean these carb bodies....
 

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.. the choke thing but I've got that solved now.
Sorry, I must have missed the post where you solved the choke thing.
 

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You said you're getting spark, but are you getting a good spark? You want a small blue spark. An orange or yellow color will cause your fuel to not ignite properly.
 

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Since you did the brass rod mod and it helped matters I assume you had corrosion in your caps around where the resistor is. It can be a bugger getting all of the corrosion cleaned up in the cap. You should remove both caps and check the resistance of them. With the brass rod mod you shouldn’t have any resistance to speak of. I finally got tired of fighting a cap I had to keep taking apart and cleaning and bought a new one from DSS. It just showed up in the mail yesterday.
 

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Oem freely available....or check out the ngk 5Kohm straight plug cap range....you can download it for free**
Worth checking your local guzzi supplier for the ngk staight caps...

Edit the catalogue is free you cant download the caps or a 3D map😀😀
 

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CX500EC Eurosport 1982
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There are others who are more expert than me … but what you describe sounds similar to a problem I had and ultimately it was a vacuum leak between carb and cyl head. You can put slightly thicker intake manifold O rings in, I think 2.5mm thickness and buy good viton rubber ones and make sure they don’t pinch or snag; I’d bet the carb rubbers are cracked too so easiest repair is to put a couple of layers of heat shrink over it; and make sure the balance-vacuum port is sealed properly by using a little crush washer under the screw head. Try that. If no look the take a good look at the pilot jet in the carb.
 
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