Does anyone have info on the differences between the stock KLX250S carb and the KLX300R carb? They're both CVK34 carbs but they're definitely not identical. Specifically, can someone comment on the existence of the Air Cutoff Valve on the 300R carb? Why does the 300R have it and not the 250S? It's basically the same engine. (Note: I'm not referring to the air injection system into the exhaust on the 250S.) And, as silly as this might sound, will a 300R carb bolt right up in place of a 250S stock carb?

The reason I ask...
I've upgraded my 2009 KLX250S to the 300R cylinder/piston. Put on a 300R exhaust. Rejetted with the DynoJet 2182 kit (using the DJ128 main which I gather is about a 120 Mikuni). KDX200 snorkle. Pilot jet #38. Removed CA smog crap (its a CA spec bike despite it living in Maryland) and blocked off air injection reed valve. The bike runs great except for my current problem which is exhaust side backfiring when decelerating from 7k RPMs or higher.

I went richer on the pilot from the #35 to the #38 and it didnt seem to help very much - runs best at 1.5 turns out now. I reinstalled the exhaust a second time using some high temp silicone (new copper header gaskets both times) while checking for no exhaust leaks and the pipe seemed a bit sooty so I think I might be running a bit rich actually. That said, I gather it's still going lean during the throttle off decels since it pops and backfires. As I've been reading, the ACV is used to richen up the mix for exactly that situation. I wonder if it's somehow needed now since I'm running the 300R pipe - currently with the final baffle removed but I can't imagine it being any more freeflowing than some of the aftermarket pipes everyone else is using.

The stock KLX300R design spec'd a carb with the ACV. My question is why'd they need it, how does the 250S carb avoid needing it, and do I now need it?

Hey... after 2 years lurking, my first post! I must be desperate!

 

Good question, the air cutoff just closes an air jet orfice to make the mixuture temporarily richer when suddenly cutting the throttle i think ? this would effectively get rid of any popping on decel but as for why the bike "needs" one i have no idea.

My 331 pops a little on decel, especially at higher rpms but not enough for me to worry about it.

 

I'm assuming we're talking about what is also called a coasting enricher on many of the CV carbs. The KLR600/650 also has one. It stops most of that decel popping. Here's an article on it, down in the section marked "coasting enricher". At least I think that's what the OP may be referring to here. Not sure why the 250S wouldn't have it if the 300 does...and the KLR's do.

Care & Feeding Of The Keihin Carb

 

Try adjusting your needle.

Sounds like you are pretty dialed in on your jetting. I would move the needle down one clip position and I bet it would take most of the popping away.

No matter what you do, it will probably always have a little bit of snap crackle and pop.

 

mentioned this in the megabomb review post...but...simply adding a megabomb header eliminated my decel popping almost completely without any other adjustments...only get a light rumble like it's about to pop on steep engine brake/coasting down in gear....

 

some decel burbling is normal but is usually masked by a stock small outlet pipe. As the pipe is bigger it doesn't mask it as well.

Sometimes a combination or a higher rpm and closed throttle fueling will make a lean enough condition to light off any fuel left in the pipe.

2 things may be the problem, excess fuel in the pipe from the throttle setting you came from, and a lean idle mix. So consider both when chasing the problem.

I'd suspect kaw just cheaped out when they took off device off on the 250? with the stock welded peashooter exhaust and jetting calibration it wasn't needed.
The 300, came jetted for the removable core and snorkel they knew everyone would remove.

 

It sounds to me like your too rich on idle circuit,possibly needle too
If you were too lean there would not be anything left to burn, much less pop.
when the throttle is closed air is cut.
I'd also double check for air leaks in the exhaust.
good luck
marc

 

>> I would move the needle down one clip position and I bet it would take most of the popping away.

Actually, I already tried that and it actually got a bit worse. (We are talking about dropping the needle as in leaner... right?) As far as the pilot, I can turn the Kouba screw all the way in and it will die and I can turn it out far enough that it starts to drop off. The peak idle RPM is about 1.5 turns so according to what I've read that's probably the correct adjustment.

>> 2 things may be the problem, excess fuel in the pipe from the throttle setting you came from, and a lean idle mix. So consider both when chasing the problem.

I've gone back and forth on the lean vs. rich cause of the decel popping backfiring. It's worth noting the popping starts immediately after the throttle roll off but the backfire takes maybe 3 seconds or more. I would think any left over rich fuel mix in the pipe would have cleared out by then. So I'm thinking it's just a momentary lean condition.

I agree with RimBender and I'm starting to think the ACV was put there to work with the uncorked 300 pipe. I'm thinking they got away without it on the stock 250S since it has the plugged up exhaust. Although even that did pop a little when stock. I'll try putting the baffle back in and see what happens. (At least that's easy compared to getting the carb out for a 4th time)

Thanks for the Care and Feeding carb link. It's a good overview.

 

mistakenly posted backward? the dj 120 is close to a 128 K, if you have a 128 dj it's likely too big.



burbles and pops shouldn't be TOO excessive and you shouldn't get big bangs.

Not to alarm you, but had a early klx250r that backfired violently and I couldn't tune it out, Some of the valves were zero;d with no smaller shims available, Ground the shims smaller got it running and gave it back to the guy who gave it to me. The soft valves of the early bikes have concerned me, but so far my 06 has been fine. Point being if your bike is hard to start and back fires flames out the pipe like the 4th of july, check those valves!

 

The main differences between the two carbs are:

1: Slide
2: Needle
3: Needle jet
4: Needle jet holder (emulsion tube)
5: Main jet

Ron