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Old 04-08-2011, 07:12 PM
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Default Spark Plugs

Hey All,

I ride a 1990 FXSTC EVO.

My cylinders are burning differently.

Front cylinders is burning hot and had detonations. I measured the gap and it was much larger than .040 and the rear is black and sooty.

I had a lot of backfires, then I replaced the plugs

This happens after maybe 60 miles on new plugs.

Could this be a coil or plug wire issues?

I thought maybe the battery, but I would think that both plugs would be black sooty if it was a weak battery.

Any thoughts?

Thanks,
Joe
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Old 04-08-2011, 08:02 PM
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id ck grounds on the battery, then id spray the intake manifold w/wd40 to check for a leak. nice bike, as you can see i ride a 90 fxstc also!!
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Old 04-08-2011, 08:10 PM
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id ck grounds on the battery, then id spray the intake manifold w/wd40 to check for a leak. nice bike, as you can see i ride a 90 fxstc also!!
Ok, inquiring minds wanna know.... What's with spraying wd-40? Where are you spraying it, and do you do it while it's running?
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Old 04-08-2011, 08:33 PM
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yea, spray it while its running, if the rpm changes at idle you have a leak. you can also use propane or carbcleaner also.
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Old 04-08-2011, 09:02 PM
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Quote:
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id ck grounds on the battery, then id spray the intake manifold w/wd40 to check for a leak. nice bike, as you can see i ride a 90 fxstc also!!
Forgive me for sounding stupid, but what does an intake leak have to do with my plugs?

Thanks,
Joe
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Old 04-08-2011, 09:20 PM
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Forgive me for sounding stupid, but what does an intake leak have to do with my plugs?

Thanks,
Joe
I'm no mechanic, but maybe an air leak will cause some issues with your plugs? As simple as these things are, I'd look into the manifold leak and then work your way through; plug wire may be the culprit, but why is one too lean? You have something strange going on. I can see this happening on a EFI bike, but a simple carb'd bike? I'd say you have some air getting in, but again, I'm no mechanic. I hope you figure it out, cause I'm sure it will help us all someday.
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Old 04-09-2011, 04:55 AM
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If it was weak battery they would be black and wet.I ntake may have a leak and that is why front plug detonates, or there may be some rubber hanging down that blocks front cyl and diverts gas to rear making it run rich.

Check your timing, check your compression too.
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Old 04-09-2011, 05:33 AM
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If your intake is leaking it will change the mixture on that particular cylinder, thus the difference in plugs. Don't use carb cleaner, it makes a mess on the black wrinkle of the motor, spray it right where the intake meets the cyclinders, do the back of the carb also.

D
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Old 04-09-2011, 07:53 AM
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Ok, so now I understand the difference between a bad battery and an intake leak. As soon as I know then I will let you guys know what it is.
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Old 04-09-2011, 03:38 PM
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Did you try snugging all the carb mounting bolts and intake bolts? Really sounds like an intake leak on front cylinder. The extra air coming in leans out the mixture ( radically in your case) causing the symptoms you have; preignition, overheating,and plug failure. I would not ride it until it was fixed or you'll burn a hole in that front piston.

On a normally running Twin ( with carb) the rear cylinder plug will always look hotter as that cylinder has a harder time getting air to cool it.
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