Just as another data point.... I'm MCM'd, DJ Stage II, 38 pilot, around 400 feet ASL. I sometimes notice a surge (or even a miss) around 6K. Usually at constant throttle. It doesn't do it enough to worry with, but it is there. Above and below that she pulls steady, and she is smooth rising through 6K. If I remember correctly, it was more pronounced before I did the ground mod... but I was in the middle of monkeying around with my jetting at that point so everything was in flux. Only change I plan on making to the current DJ Stage II is to bump up to a 40 pilot as I have more decel pop than I would like.

 

I'm sorry I cluttered up the thread by provoking KLX678.. Sometimes I can't help myself.. Anyone needs me, I'll be in time out..

 

FWIW:




4: Backfires in Exhaust

Note:

It is normal for many high performance exhaust systems to moderately backfire or pop when the throttle is closed from mid-to-high rpm. In fact, one should expect a well-tuned high performance engine to "pop" and "crackle" when the throttle is closed at high rpm.

The popping is a result of the air/fuel mixture becoming very lean when the throttle is closed and the engine is rotating well above idle speed. It is also necessary that the exhaust system have rather open mufflers.

Why This (normally) Happens:

1) When the throttle valve is in the idle position, fuel does not flow out of the main system (needle, needle jet, main jet). Fuel is only delivered to the engine by the pilot (idle) system.
2) The combined effect of the closed throttle and elevated engine rpm is to create a fairly strong vacuum in the intake manifold. This vacuum, in turn, causes a high air flow rate through the small gap formed by the throttle valve and carburetor throat.
3) Under these conditions the pilot (idle) system cannot deliver enough fuel to create a normal, combustible air/fuel ratio. The mixture becomes too lean to burn reliably in the combustion chamber. It gets sent into the exhaust system unburned and collects there.
4) When the odd firing of the lean mixture does occur, it is sent, still burning, into the exhaust system where it sometimes ignites the raw mixture that has collected ---- the exhaust then pops or backfires.
5) Completely stock Harleys do not do this until open-end mufflers, such as the popular Screamin' Eagle slip-ons, are installed. The exhaust must be both free-flowing and have an open exit for the popping to occur.


Other possible causes:

Air Leaks:

Any source of fresh air into the exhaust system can create or worsen the conditions that bring about exhaust backfiring. The most common entry point is the junction of the header pipes and mufflers. Even a small air leak can dramatically increase the intensity or likelihood of exhaust system backfiring.

A high temperature silicone sealant, as can be found in many auto parts stores, may be used to seal the pipe/muffler junction.

Lean Carburetion:

While exhaust system popping may be considered normal, it is certainly made worse by an overly lean idle circuit.

Be sure that your carburetor's pilot jet is the correct size and that the idle air mixture screw is correctly adjusted before looking for other causes of popping. The procedure for adjusting the pilot circuit is covered in the Tuning Manual.

Ignition:

If exhaust system popping is very loud, irregular and accompanied by loss of power, then you should suspect that the ignition system is not performing as it should. If, for some reason, the ignition sometimes fires at the wrong time, then exhaust popping can become very energetic (loud). Look for failing high tension leads (plug wires), failing ignition coil(s) and especially switches or connectors as possible causes.

The 38 may be a bit lean, I am running a 40 at 1000 ft elevation. Don't know if it pops, because I have stock muffler. My other bikes that idled virtually perfect with 2-2.5 turns out on the mix screw, but with more open exhausts did crackle when throttle was closed.

 

Every bike Ive had with a straight through can has popped on decel when properly carbed.

 

I had the 40 in there when I tried out the lidless recipe, so I have one in my parts collection. It doesn't bother me enough to pull it apart just for that, but next time I'm in the carb (i.e. ...if perhaps I catch up on other obligations and decide to BB351 myself for my birthday *grin) I'll swap in the 40..

 

There is so little air flow required for the idle circuit that lid or no lid wouldn't make a difference. If it worked good with both you're good with either. You might be a shade lean with the 38, but it should be able to be compensated for with the idle mix screw. Turn it in until the idle drops, then while counting the turns, turn it out until the idle drops again. Then divide the turn count by two and turn it to that point. The idle is dropping due to being too rich at one extreme and too lean at the other. If the screw is over around 3 turns from seated and it doesn't drop the jet is too lean since you are increasing fuel input as you open the idle mix screw. I'm betting your 38 is likely fine.

 

"Seems from Partzilla that all the KLX250s from 1994-up run the same cams at least in the U.S. Look for your self. Also looked up 2006 and 2012 KLX250S exhaust cams on Kawasaki site and found same part number 12044-0746 . SnowCity showed the same part too 12044-0746."

Seems to me this "myth" should be put to bed. Maybe a bit more research needed.

klx678. Try a web site that lists original numbers as well as superseded numbers. Fortnine.ca lists both.
Get a pencil and paper and go through all the 300 years and 250 years. You may be surprised at all the different numbers for exh. cams but not convinced and that's OK with me.

I just had my original -1175 exh. cam and shiny the new superseded -0746 in my hands and they are indeed different. Didn't go to the point of "degreeing" both 'cause I have no bone to chew here but I'm pretty good with a mic and more than just understand what I should be measuring.

They're different but believe what you want but from what I measured I hope to see some over the over rev that my buds '09 has in mine now.

 

First heat cycles yesterday and first ever ride at 4000rpm ever later today lol.

I'll PM you when I get to thrash it and trust Chinese.

Maybe west of Monty Lk. Haven't been in there in decades.

 

Since this thread has gone in a few directions I thought i'd ask,,,

To you proponents of the MCM, have any of you had a before and after run with a known quantity? Meaning another KLX 250 you ride with and know from riding yourself to compare differences the day of or after. Have you done it (MCMed and un-MCMed your bike) more than once to confirm your findings using this common standard for comparison?

I come from an era where the proof needs to be in the pudding when it comes to modifications. If the stopwatch or standard is beaten it's not just subjective so your good to go.

So, dyno graphs aside 'cause to me they are like opinions, any tales of 3 to 5 bike lengths from 0 to 60 or trading bikes with something similar and familiar and going from point A to B in dirt, with obstacles lol, a couple of times to compare bottom end?

Just interested in other peoples findings using criteria I am familiar with.

 

I did the MCM and my rear tire vaporized in a cloud of smoke and rubber particles. As it happened Valentino Rossi rode by and was like "Damn Son !! Need me some of that in my MotoGP bike." I am expecting him to show up at Marcelino's door with a fat check and expensive hookers to get him on the engineering team.

On a serious note, the MCM is about as subjective as you get. Some love it, some hate it, others said it basically made no real difference. Trying it out for yourself is really the only way to know if it works for you.