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Help needed with potential carb issue

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mr72

mr72

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Cedar Park, Texas, USA
I have a riddle for the carb gurus. Imagine an unspecified single-cylinder air-cooled motorcycle with a CV carburetor.

It runs fine all the time, but I suspect it is running rich at idle, which I am working out a half turn of the pilot screw at a time. The one problem that I can't solve:

If you let it idle for more than about 30 seconds, like at a stop light, then it runs very badly for a few minutes after. It eventually clears up after a mile or so at mid-high throttle under load. If you don't ride it long enough to clear up the bad running condition, then next time you let it idle it it will idle too low and eventually stall, then it is hard to start.

Ideas?
 
Sounds like may need drop the pilot jet a size or two and I would check the float height.

 
No, but the kickstarter will hold my 215 lb weight, I have to jump on it. So I don't have a reason to suspect poor compression. And it's only got just over 1000 miles on it.
 
In order
  1. too rich at idle,
  2. plug too cold, gap too wide
  3. poor spark at idle,
  4. low compression
 
Just so I learn something, the theory is that the rich idle is fouling the spark plug and it takes a bit of riding off idle to burn the crud off of the plug, correct?
 
U
mr72 said:
Just so I learn something, the theory is that the rich idle is fouling the spark plug and it takes a bit of riding off idle to burn the crud off of the plug, correct?
Click to expand...
yes, common problem on my old drag race cars with dodgy ignitions
 
FE_Rex said:
U

yes, common problem on my old drag race cars with dodgy ignitions
Click to expand...

OK, that completely makes sense, and it jives with my perception of this issue. It's confusing for a number of other reasons, but maybe the big thing is I need a higher heat range plug.
 
mr72 said:
Just so I learn something, the theory is that the rich idle is fouling the spark plug and it takes a bit of riding off idle to burn the crud off of the plug, correct?
Click to expand...
Definitely possible. Usually when I have issues like this the carb needs a deep clean and a rebuild.
 
dash1 said:
Definitely possible. Usually when I have issues like this the carb needs a deep clean and a rebuild.
Click to expand...

Same here. It's been done like 10x. This is why I'm trying to get down to the root cause candidates so it's not just redoing work that I've already done over and over and expecting a different result.
 
Yesterday I tried this with the "air" screw as much as four turns out, which is almost falling out. No change. So I figured, maybe this "air" screw is actually a pilot mixture screw and is metering fuel. So I decided to try it today at one turn out. Still misbehaving in the same way, but a lot less power until you can get the revs up. I stopped a few times and tried to tune it while it's hot, and eventually ended up with 1.5 turns out, which gets the power back for the most part, but still has the won't-accelerate-after-idling problem. Some other issues pop up with what now I'm almost convinced is a much leaner setting, but one way or the other, pilot mixture doesn't seem to affect the running after idling problem.

Now I'm wondering if it's the coil going bad. I have a new one, but it's not at all like the stock coil. I also may try a smaller spark plug gap. I could adjust it to the lower end of the acceptable range for this engine and see how that goes.
 
May want to make sure you got good current going to coil. Not sure what shape your harness/connectors are in. On my KZ, I bypassed decades of crappy wiring/connectors by feeding the coils straight from the battery via a relay activated by the “run” button. Made a BIG difference in how the bike ran.

Be careful though, some coils have a ballast resistor inline to keep them from overheating while the motor is running. If equipped with a ballast resistor, usually the “start” function bypasses the ballast resistor for a hotter spark while cranking. This system is common on both points/condenser and early electronic ignitions (Duraspark)
 
Thanks for the advice, @FE_Rex . I'll have to find time to tinker with it, maybe try a different coil and check out the wiring, along with reducing the plug gap.
 
I pulled the ignition coil and measured it. Primary is in spec but the secondary spec is 17K ohms and this coil tests 21K secondary, which I guess might indicate it's actually got a potential intermittent open when hot.

My generic scooter replacement coil has very different measurements. Primary is like 0.3 ohms and the secondary measures about 2K. The ratio is about right so I am sure it is stepping up the right amount, but I'm a bit worried about the input DCR frying my ECU. Kind of tempted to try it, but mostly tempted to just order a new LML coil.
 
Alright now, the plot thickens.

It appears my new coil probably won't work. Can't work. But I figured maybe the spark plug boot was part of the problem. Then is when I dug into the research about the hard to find spark plug that's been the bane of my existence. The plug I have is a resistor type plug and the stock plug cap has a resistor, so this is probably 95% of my problem.

The replacement plug included a plug wire with a non-resistor cap, so I tried that on the original plug wire/coil and now the it won't start. It won't start with the new cap, or with the old one. It cranks and backfires every few seconds but doesn't really try to start. That's a problem I can probably sort out and fix, but the moral of the story is, I bet the entire problem was resistor plug plus resistor cap equals poor spark and fouled plug when idling.
 
I think I might have sorted this out entirely. If you care, you can read the saga on my latest blog post:

She runs-a! No, she notta-run! Ah! She runs-a!

Imagine that with the Mario accent. That's how I sound in my garage working on the Stellauto. Here I hope to detail my ignition saga. Someon...
joshkarnes.blogspot.com joshkarnes.blogspot.com

Summary is this: I think the problem was actually a loose spark plug cap coupled with running a resistor spark plug when it's supposed to have a non-resistor type, all resulting in a very weak spark at idle. This allowed the plug to foul, when then takes some few minutes of running hot to clear it up. I fixed the dodgy plug cap connection and put in a new resistor-type spark plug and preliminary testing seems to be that the issue is fixed.

Thanks for all the help, especially you guys turning my attention to ignition as possible root cause.
 
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