backfiring?

[95 katana]

my sisters bike (96 kat 600) has stock airbox and stock air filter. her stock header has been modified to turn outs underneath the engine. she called me and said its not running good at 45-60 mph. i test drove it this morning and it sounds like its backfiring at those speeds... could it be her exhaust mod causing this?

the guy who did this is an exhaust man but im not sure he did this right. where he cut the headders off, he welded in a 3/4 flat washer inside it and near the end of the turn outs, he welded in a 1" flatwasher. she said he told her it was to maintain back pressure.

this theory sounds good to me but im thinking its not ENOUGH back pressure causing the bike to run this way (it sounds like its running out of gas). Stock carbs and probably could use new plugs on the bike... can yall chime in with any ideas?

[GSXFJim]

Put it back to stock.. the problem should go away.. if she just wants a loud pipe, get a D&D and jet accordingly...

[Kreylyn]

Well.. if it ran well before, and doesn't now... I think that might point towards the issue.

The carbs either need to be tuned for the new exhaust (if they can be), or you need to change what he did to the exhaust to counter the problem.

krey

[95 katana]

yeah im thinking so. not so sure how it ran beforehand because this is the 1st time ive driven that bike (she got it last weekend and had the exhaust done right after she got it). it sounds wicked but not so sure that its going to work with her setup... maybe i'll chop it and do it like mine (Y piped into the right side with a stock GSXR can)

[Squisher]

It's probably running seriously lean (yes, lean condition can lead to afterfire...it makes the combustion in the chamber go too slow and you still end up with unburned fuel in the exhaust).

My first bike had the stock can, but was gutted, and even with the opening on the end the correct size on the end it flowed enough more that I had to go up in jet sizes a surprising amount -- like from 52 pilot to 62 and from 140 to 150 main -- I probably need to take the main up to 155 actually, but 150 + needle shims seems to be doing fine so far. Thank god for singles for learning how to do this...

The easiest thing would be go back to stock (even temporarily) to make sure it was running OK then. Even stock a lot of bikes are set pretty lean from the factory for emissions, and opening up the airflow (either with exhaust or intake or both) just makes it worse.