CBR600F3 New Spark Plugs, now it backfires HELP

My Son bought this CBR600 F3 just before the Air Force shipped him to Iraq. It needed new spark plugs every spring. The NGKs were always fouled. We tried out the Champion Racing plugs that match the NGKs. It then needed a battery charge. After 1 hour, it fired right up. Then it started backfiring. Every few minutes. My son was home, but could not keep it running.

We drained the tank (It had 2 gal of gas + Seafoam for the winter). We put in 2 gal of the 93 octane. She started but still backfired.

I rode it about 1/2 hour. No backfire when cruising in 2nd gear, but when I had to stop, BANG BANG, it backfired again.

We parked it and pulled the plugs. They were oily and slightly darkened, but not fouled. Air filter is a 1 year old K&N.

I AM STUMPED? Could low voltage (bad rectifier) be the cause?

Thanks for your support

 

two coils fire four plugs, xj's fire 1-4 from the same coil at the same time, same for 2-3 from the other coil
say #1 should be firing and does but the air/fuel mixture in #4 will still ignite when the plug sparks....poof backfire
i'd say the mixture is very rich... does it start from cold with no choke?
air filter clean?
thats my story, and i'am sticking to it

 

is it backfiring (through the intake) or is it popping in the exhaust? there is an important difference.
backfiring - polock said it, with the wires crossed.
popping in exhaust - very rich mixture

 

Filter is very clean

Will not start cold unless the choke is on.

Backfire is in the exhaust.

Wires could be crossed. I will take the tank off and look,

 

Backfire is a lean condition usually...

I'd be temped to look at the condition of the carbs... you may not even need o do a full clean, the most likely cause is the tiny idle passageways... if you take the carbs off - open the butterflies - you'll see 3 tiny little holes. Those are 99% likely the cause - they gum up first, and would cause a lean condition at idle. At higher rpm's you'll be on the mains and they don't clog up as easily.

 

Ok. I cleaned those little holes ->still backfiring.

Check the sparkplug wires. I had # 1 and 2 switched. It still runs rich though, but it runs fine with no back fire. I put 20 miles on it Saturday and it ran fine.

I just got to get rid of those pig snout mirrors he bought before I get a ticket.