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Cold Cylinder and poor MPG: Coil?

Posted: Sun Oct 29, 2017 7:58 am
by Giuan
Hello everyone!
This is basically the coninue of http://ww.xjrider.com/viewtopic.php?f=127&t=8872
After having changed the pilot jet I solved the backfiring issue and the bike was running great and I was doing 38mpg (not great but acceptable).
It only lasted a month or so, then the bike started running rough at low RPM: had to use a lot of clutch to move off and open up the gas with decision. Other symtomps were cold 3rd cylinder, smell of gasoline and a whopping 28mpg :oops:
I thought that the other pilot jets were giving up, so I changed the other 3 and the main jets too.
At this point, I practically rebuilt the carbs (all jets, all gaskets/orings, all needles...) so I am pretty sure it is not a matter of carburation.
I also asked a indie mechanic to check on the valve clearance, but he told me that they were all on specs.
I checked the spark plug and I found it very black and sooty, so I replaced it with a used one I had and the bike ran great. So I started suspecting on the ignition.
I changed the spark plug cap (NGK LD05F, which has 5KOhm of resistence instead of 10 as the stock one, so it delivers more current to the spark plug) and put a brand new spark plug. The cylinder is still cold, but the bike runs fine and the fuel consumption gets closer to the starting 38mpg.
The problem is that this lasts just a week or so, then the spark plug gets fouled again and I go back to square one.
Here's a pic of the spark plug after 60 miles
Bougie.jpg
I am now suspecting on the right coil, which is the one for the spark plugs on cylinder 2 and 3 (the second cylinder's temperature is fine, but its spark plug is dark too).
I measured the R and here's the result:
Primary=3.3 Ohm (the multimeter has an offset of 0.7 when shorting the probes) and it should be [1.92:2.88]Ohm
Secondary= 11.8 kOhm and it should be [9.53:14.28]kOhm
To me it doesn't look tragic, so I don't know if I should replace it or look for another root cause...
Does it seem that I have a fried coil to you? Is it true that these coils don't have replaceable HT leads?