I have a 2003 Sportster 1200c that is getting a backfire up through the carb. I have had the backfire since I bought the bike and it is starting to burn the ring connected to the rubber booton the carb.

The backfire happens when throttling down, or slowing the engine coming into town zone etc. Otherwise the bike runs great! Sounds perfectly normal and all the power I need accelerating and holding speed.

I am open to any ideas before I take it to a dealer and get a big bill.

Some pertinent information:

-Bike was purchased new and sat for ~3 years before I bought it
-I have put in a dynajet kit
-Installed performance air breather
-There is a lag before the bike throttles down. Not much but noticable. (Likely not related to backfire but just in case: was backfiring before this problem started)
-Running 87 Octane

It has been suggested by others that:
a) Sticky Valve
b) Timing (I thought this was unlikely do to great performance)

At this point I have my shop manual and will try and check timing myself ifI can borrow a timing light, but I would appreciate any pointers please.

Thanks in advance!

 

First, the 1200 needs 91 or higher octane. How did you determine the jet sizes when you installed the Dynojet kit? Your symptoms indicate you may still be running lean.

 

Yea..I always use 91 or higher octane. Dealer installed a 27170 - 89
slow jet........