Motorcycle Repair: Rear pipe backfires and flames when downshifting, screaming eagle pipes, drag pipes

Question
Hi Mike,

Back ground:

I own a 2002 1200custom sportster( about 11,000 miles of normal driving...no burn outs or excessive/hard driving)

As far as I know all motor parts are stock including the CV carb and the air cleaner.( holes were drilled into the back plastic cover of the air cleaner to try to allow for more airflow by the previous owner/I plan on upgrading to a screaming eagle breather or a Force in the near future.)

Upgrades made:

I recently changed my exhaust over to straight/drag pipes called wetdreamz.

At the time of the change over I also added the torque cones the manufactor reccomended for helping to keep the back pressure.

I also replaced the stock gaskets with the thinner screaming eagle gaskets and new chrome flanges.(I'm sure the pipes are on tight and secure)
I relocated my coil to where the horn used to be and am using FXR style plug wires to acheive the desired look and length for the plug wires...Harley dealer said it shouldn't make any difference.
The motor starts great, runs great performance wise at all speeds and idels fine when cold or hot.


Here is my problem:

While reving high and releasing the throttle at a stand still ...or more noteably while down shifting when I'm on the road...I'm backfiring like a "drive by shooting" especially when downshifting into the lower gears while coming to a stop.

At first I thought it was kinda fun and cool... but now I'm growing tired of it and concerned about whether I have some engine problems or tune up issues that need to be addressed.
I also don't want to be damaging my motor.

I used to run standard screaming eagle pipes on the bike and I did back fire on rare occasions with them to on REALLY hard down shifts... but the backfiring was not nearly as bad as what I'm experiencing now...not even CLOSE to the portable 45 magnum I'm riding now...lol

I understand that changing the exhaust to straight/drag pipes can cause the loss of back pressure and horse power.
The horse power doesn't seem too affected so I think the torque cones I installed are working.

I have deduced that the rear pipe (short pipe/only about 2 feet long)is the one that is backfiring and if I am in low light conditions blue flame exits the rear pipe during the backfires. None are visible exiting the front pipe.

I rejetted the carb to 48 & 195 jets to try to help but that had little to no effect on the backfiring or performance that I could tell.

The spark pulgs look tan/brown...not black or white or wet.

1)My question is why would just the one short pipe appear to be backfiring/spitting blue flames and not both?
2)Does it sound like I'm running too lean or too rich on the rear cylinder and if so what is the suggested fix?
3) What can I do to fix or stop the backfiring with straight/drag pipes?


Any suggestions as to how to stop the backfiring or tone it down a bit would be very helpful.

I'm not sure what to do at this point other than live with it if this is a typical side effect of running the shorter straight/drag pipes.

The flame would usually imply the there is unburned fuel that is exiting the motor.

I look forward to your advice,

Skip  

Answer
Without going into alot of tech reasons for your problem, it is caused by the rear pipe being to short. Exhaust pipes have to be a certain length to work properly as well as have the correct back pressure.
You sound like you have the carb properly jetted. You might try retarding the timing and see if that doesnt help the problem.
Good luck and happy riding
Mike