Zimbob,
You're correct i'm talking about thick carbon deposit on the plugs here.
I'll take note about not to lug the bike. As for choke, i don't even use it. It's already too hot here haha. So OK it's clear that I need to fix the AFR problem. Thanks a lots, i'll ask the dealer to do their best.

By the way, any recommendation for adjustable ignition module and high-performance coil for 2000 Sportster 1200 ?

 

I had the exact same problem with my 05. The plugs would foul every 5K miles. It turned out that oil was leaking through the falve guid seat. I replaced the valve guidee seat and it cured the problem.

 

turn off your petcock, and drop the float bowl, remove the jets and tell us what the numbers on them are.. I would expect 170 main and 45 pilot would be about right, but then the fuel you're buying may be completely different than what I buy. Also , I expect they replaced the diaphragm because it had a burnt/melted spot on it? This is from the flames that go through the circuit when it 'coughs' through the carburetor. The enrichiner plunger gets burned too, so put in a new enrichiner plunger and make sure
that it's closing all the way when your choke **** is pushed in. Only use the choke for 30 seconds or so getting it started. Post your results.

Hogdoc

 

What happened with the diaphragm is that it's weary on the outer side.
Alright, i'll show this thread to the dealer mechanic...

Thanks!

 

As stated before, I'd quit taking the scoot to the dealer and find a new mechanic, if you are not going to do the work yourself this time. It sounds like you need to change the jets in the carb because it's running to rich. Oh, and using the choke, not the throttle lock, is the proper way to warm the bike up.

 

Dana and Crane both make a good adjustable single fire ignition for your scoot. I ran the Crane unit, fully adjustable from front cylender to the rear. worked great.