I live in Japan, and I recently picked up a 2009 KLX250S with EFI. Later, I found a FMF Powercore 4 going for a good price on auction, so I thought, "What the hell?" and picked it up. I was surprised to see that this FMF pipe had a fitting for an O2 sensor when everything I have read about it says get rid of that thing. So my question is, do others with the FMF Powercore have the O2 sensor installed? I mean, if FMF have made this pipe with a hole for the O2 sensor, is that how I should be using it, and if I remove it and fit the O2 sensor eliminator, will I be causing any damage?

Here is what I have tried so far:-
O2 eliminator - snorkel + stock pipe = runs well, but still strangled
O2 eliminator - snorkel + PC4 = runs well, more top end, but backfires a lot
O2 eliminator + snorkel + PC4 = runs well, good top end, backfires a bit less

For now I've put the stock pipe back because the PC4 is just too deafening for commuting. I'm planning to get a quiet insert for it, but other than quietening it down, is it possible it will also improve the backfiring?

 

I'm guessing you don't have a fuel controller like an EJK or Power Conmander on it? If not, then yeah you'll be flowing a lot more air and need the extra fuel to make up for it. Can try putting the o2 sensor in the fmf and see if the computer makes up for the difference. I never went that route cuz my Q4 didn't have an o2 sensor hole and I didn't want to make one. So bought the EJK.

 

You'll be flowing less exhaust so it might improve it some. But still flowing more than the stock exhaust, so I'd guess there will still be pops there.

Now on the subject of pops...I've got my fueling set up perfectly for my bike with stock header and q4, and I'll still get some occasional pops. Our bike has an air injection system that shoots air into the exhaust to help burn off unburnt fuel. I'm thinking the stock muffler would mask these intended pops but aftermarket one wouldn't. So some pops are "natural" for sure and shouldn't be considered bad. Though in your case, I'm sure the fueling is off which is causing the pops

 

That's pretty much true. When the throttle is closed the mix will be lean. On the carbureted bikes Mikuni says the popping is pretty much normal, not loud bangs, but popping and crackling. Here is another explanation from another source. The stock muffler does exactly that - muffles the pops and crackling, a straight through exhaust will not.

Not sure about how the bike EFI works, if it shuts fuel off completely like cars when decelerating down to a programmed rpm. If not, it may be why an EFI bike may pop and crackle too. Maybe someone else can fill us in about that.

 

Dang it, I can't delete KLX678 's posts.. I'll keep trying..


I live in a universe where decel popping is not acceptable, neither is rhetoric/narratives that try to frame decel popping as "normal" ...

 

Thanks for the replies! I suppose I'll try it with the O2 sensor back in. Does that mean I need to put the snorkel back in, too?

 

Go write Mikuni and tell them they're wrong Mr. Know-It-All.

I didn't say it, Mikuni did. I'm just quoting them. But then again,compared to your infinite wisdom, what do they know about carburetors.

Heck, maybe Mikuni will fire all their engineers, and put you on a stool in the middle of the empty engineering design department to tell them the "right answers" since you clearly know it all. Obviously they're wasting their money, right?


Are we having fun yet?