My quest to stop the "puffs" and backfires....

I have a 1980 KZ250CSR and I've just jetted it it with a Stage III Dynojet kit,a single Keihin (SH23) carb using a Generic pod filter, and the stock exhaust with the muffler gutted and I bored out the exhaust hole to 1.75".


I start'er up and I have to choke it about half-way, then I can drop the lever on the side of the carb after about 2-3 min., and it's fine(somewhat). Every so often at idel and before 1/4 throttle, it "puffs" or backfires air/fuel out the carb thru the air filter. However, when I'm done riding, and I bring her back to idle and shut it off. it backfires louder than a shotgun!

What could be the issue?

Is it possible that the stock header is providing too much back-pressure?

Now, FYI, I've not gotten into this motor yet. I have only cleaned/rebuilt/jetted the carb, and replaced the O-ring on the carb intake boot.

If I missed providing any info, just lemme know. Thanks!

With the pod filter and gutted exhaust you have leaned out the fuel to air mixture.
Welcome to the world of tuning cv carbs with pods.
Do a search on that subject and enjoy.
As for the backfire,check your valve clearence.
You should perform all the general maintenance before you do any modifications. :woohoo:

raw gas in the exhaust pipe will make it back fire like that.

JD

Nebr_Rex wrote: You should perform all the general maintenance before you do any modifications. :woohoo:

Well, not the first time I've had to chase a tune. I've been putting it off for some time. So it looks like I'll do a complete tear-down, since my new Athena gasket set is in. Oh, btw,...I am also leaking oil out the head-gasket, ever so slightly ( a drop of oil each day, roughly)

Well, I tore off the top-end of the motor and cleaned it a bit. Once I got it back together I noticed I was off in my timing by nearly 2 degrees!!. I adjusted it, along with the valves, (which desperatley needed it since they never really closed!) and she runs ALOT better. I also gave my mixture screw a half-turn, with the colder-denser air coming into the motor, and she no longer puffs, backfires or leaks oil at the head gasket.

Go, me!! I love this hands-on learning as I slowly educate myself on bikes.