2 Stroke backfire ?

[Marklx]

I've been fiddling with the jets on my '02 RM250 quite a bit, and it's running pretty well, even without cutting the slide.

But now it's doing something different, and having been off bikes for so many years, not sure if this is normal.

It isn't actually backfiring, but when I shut off, a couple seconds after, it will do little ring a ding, bah bing, bah bing sounds, like I just tapped the throttle. It didn't always do this. Could it be jetting, or a reed thing, or just normal 2 stroke behavior.

When coasting down, it will do this and I can actually feel the bike accelerate a tiny bit. That's the part I don't like. I'm guessing reeds, but the bike is only a few months old, so please enlighten me

Mark

 

[bscottr]

Sounds Lean....

It isn't actually backfiring, but when I shut off, a couple seconds after, it will do little ring a ding, bah bing, bah bing sounds, like I just tapped the throttle.

When coasting down, it will do this and I can actually feel the bike accelerate a tiny bit.

When you say coasting down, does that mean coming off 3/4 to full throttle back to idle? If so, I'd say you've leaned out too far.

Pull your plug and check the coloration. Tan like mocha is good, black is bad and white is even worse (lean, potential engine damage).

Check out Eric Gorr's site at http://ericgorr.com/techarticles/sparkplugs.html
He doesn't show a lean plug, but he does show a perfect plug. Also check out his carb tuning guide while you're there.

Good luck and let us know how it turns out.

Scott

 

[Marklx]

Scott,

Interesting observation. I've recently gone to 1/2 clip leaner on the needle, which "seemed" to help low end performance, but that is when this condition started. But, and this is a big but, the plug is dark brown, so there is definitely a rich condition somewhere in the jetting. And usually an inch or two of spooge at the end of the day. Understand, I'm a novice at best, and the bike is often at less than full throttle. Ok., let's just say I'm slow and not usually screaming the thing.

And the coast down thing is from almost any throttle position, but generally after accelerating down a straight. And while I thought it was rich down low, pulling the choke on doesn't seem to have that horrible of an affect. How could it be lean down low and still dark brown plug? I guess this is the infamous Suzuki fussy jetting.

I'll go a clip richer to see if it still does it, but then I'll have that other damned rich thing down low. Argh.

Mark

 

[canvet]

Hello Mark;
Here is my slightly educated guess. Dont be offended when I say that you probably rarely get above 1/2 throttle.(I dont with my '02 cr.) Your bike has run rich from jetting and your not "cleaning" it out. Also, I bet you run your oil to gas very rich. This has caused carbon to biuld up in the head causing detonation when the bike is revved and then let off. The carbon in the head is glowing and causing the bike to fire after the throttle is let off, this is the acceleration you feel after the throttle is closed. Another clue to too rich an oil/gas mixture is the drool out the pipe. www.maximausa.com has great info on oil/gas ratio's. Proper plug reading must be done at constant throttle for a few minutes for each jet, then bike killed quickly. From my experience plug readings other than main-jet are not reliable. Seat-of-the-pants from some-one with jetting knowledge will get you much closer. Maybe time to de-carbon head even though the bike is almost new. The best 10-minute tune-up for your bike would be to let an expert really clean it out!! If your bike is loading up all the time it is impossible to jet cleanly. Great that you are asking for help, we(this forum) will get your bike running perfectly! If at one needle setting your bike is too lean and the next too rich then cutting the slide is THE answer!!! What jetting did you start with and what have you now??

 

[Marklx]

Canvet,

No offense taken on my riding "skill". Whiie I'm often at full throttle, it is usually followed up with plenty of low speed poor technique, etc etc. And lots of idling waiting for friends to come around the track, etc. I'm running the bike at 32:1, and was using maxima superm. Now running yamalube, which seems a bit cleaner, but not materially better or worse. Haven't had the topend off, and it may very well be time to take a peak. Bike has never loaded up or fouled a plug. Running jetting recommended by all the MX mags, as they all run in California, and that's where I am. Some friends have suggested that my idle could be too high. I'm going to go a clip richer and turn down the idle to see if that cures that hiccup. (the bike will idle about 5 seconds and die. Is that normal?) If the hiccup is then gone, guess it's time to keep working on the jetting. Maybe I'm just being too picky, but I want the thing running perfect, and know I can get it there. It's part of the fun of riding, imho. Thanks for the help, and let me know if you have any other ideas,

Mark

 

[canvet]

Hello Mark;
Oil is cheaper than pistons, so mixing rich makes sense. If you jump doubles then your much faster than me! Getting the bikes to run perfect is part of the fun, good luck. My bike has the idle set high for trail riding without stalling. Also have lowered both ends and dropped spring rates front and back.( I'm not only slow but also short). Will race this next weekend with flywheel weight added. Must be nice to have all the mags jetting for you!!

 

[Marklx]

Canvet,

Guess we have the height thing in common. 5'6" on a good day, with frizzy hair.:)

So if you've got your idle high and yours isn't doing the bahbing thing, that probably isn't the problem. And a lot of good the mag tests have done for me. They got me in the ballpark, but not quite. Think it's time to go back to some baseline jetting, check for air leaks, clean carbon, and recheck.

I just need more time and deaf neighbors. Neither seems likely, and whenever I'm riding at a track, the last thing I want to do is tinker with jets. Want to ride! Need patience.

mg

 

[bscottr]

Mark,
Before you go any further, consider repacking your silencer. If you are making a lot of smooge I'm sure it's time and it does affect the jetting. You may also want to clean your power valve while you are at it.

Canvet is right on concerning reading the plug after an extended run. I try to ensure that the main jet is correct before moving on to the lower circuits. This is personal preference, but that way I feel I'm protecting my engine from "lean" damage.

I like to warm up the engine thoroughly, no high revs until warm (only rev enough to keep it cleaned out just above idle). Once warm, clean it out very well. Then take 3 or 4 high speed runs at WFO on flat ground (no need to be on the track) to ensure it's running clean on top. If you hear pinging at anytime, shut it down! :eek: Then richen up the main. If you don't hear pinging, on the 3rd or 4th high speed run at WFO pull in the clutch, hit the kill button and coast back to the tool box. Hopefully you are heading in the right direction. ;)

Pull the plug, look for the mocha color and make corrections from there. Once you have the main correct continue on to the next circuit and yes, it's a little more seat of the pants at this point. You can still use the plug as a guide by extending your runs at half throttle, etc. then take your readings.

I also ran 32:1 for a while and had the smooge. I went to 40:1 and the smooge cleared up. And while 32:1 is a richer fuel/oil ratio than 40:1, it's a leaner fuel/air ratio, so you can use the mixture to fine tune your jetting as well. Like you, I don't spend much time in the high RPMs, mainly because I ride woods and I'm slow :whiner: . That makes the lower circuits that much more important to have fine-tuned. You'll get there, it just takes a little while and when you get there, the weather will change :think !

IMHO the mags, :uh: , did I say that? If your mag reference gives you the temperature, humidity and elevation they had while achieving their settings and you have very close to the same conditions when setting your jetting, it may be a decent guide. Otherwise it's just a very rough baseline. The best guide is finding someone in your area with the same bike/mods (ensure it runs clean) and ask them what jetting changes they made. You may also post the stock jetting and your current jetting, altitude, humidity, temp. so the RM owners can really help. Again, it'll be a baseline unless their conditions are like yours.

Hope this helps.

Scott

BTW, here's another very good jetting guide: http://www.ecmx.com/bike_tips/jetting.html

 

[los36]

Xtremely responsive throttle

Mark-
I have an 02 RM250. I had been riding a 2001 CR250 before I got this bike. One of the first things that I had to get used to with this bike is the EXTREMELY responsive engine. The first few times that I took it out to the track, I'd get myself into trouble going into rough turns, because I would let my right wrist get sloppy over the bumps and the bike would lurch. Never noticed this on the Honda.
I assume that you probably downshift while you are decelerating and since you say that you are a beginner/novice rider, your elbows are probably low allowing lots of wrist movement. The combination of being in 1st or 2nd gear and the bike's snappy power can get anyone into an awkward situation quickly.
This is just my experience with my bike and I have gotten to where I like it, but it can be unnerving at first.

To test and see if this is the problem, accelerate to full throttle in 2nd or 3rd gear on smooth open ground, then let go of the throttle grip and grab the cross bar or front brake resivoir. If the bike does not decelerate smoothly, then start looking at your jetting.

PS - sounds like you have your idle set correctly.

 

[Marklx]

Los36,

Wish it was my sloppy riding, which is to blame for many things, but it does it coasting down on the street, on the stand, etc. Blip or rev the motor, and it gives its little bahbing a couple seconds after I shut off. My feeling about the extreme resonsiveness is that it is jetted poorly down low, and once it gets out of that rev range, it hits like crazy. That's why I'm trying to get that very bottom of the throttle range dialed in. But obviously I'm going the wrong way. Another '02 RM owner says his will idle for 30 seconds before stalling. How about yours? And if you know your jet setup, I'd love to hear it. More info, the better.

Thanks,

Mark

 

[bigred455]

I know exactly what you are going through.I have a 00 kx 250 that did that not new,but a year after.When i would let off the throttle it would come down ,but right before idle,it would go ting ting ting ting ting,THEN DROP.I wen't richer on the pilot 2 steps,dropped my idle way down to where it would shut off,but i still had this i couldn't figure it out.When i was in say 1st or second gear and i would give it a little gas then let go i would get that ting ting ting ting and the bike would jerk forward each time it did that.Well my Big end rod bearing wen't not to long after that. I did not have a air leak anywhere my jetting was right on the money, my piston showed beautiful even color.I had my bottom end done a while ago with a plated cylinder,I do not have that problem anymore,The only thing that i could think of is the rod bearing was giving me a hint that it was getting ready to let go.I do not know for sure if that ting ting ting sound when idling down was a sign for the rod bearing,but when i had my motor done it did not do it.I am running the same jetting and pipe same plug color same fuel and oil,I am not saying it is your rod bearing ,i wish i knew for sure what happened to my bike,but that is the only thing i can think of.I tried to jet around it i even put the choke on when i was idling and blipped the throttle and it still did it.Wish you luck.

 

[Marklx]

YIKES!:whiner:

I really hope that isn't it. Too new. Can't be. Can't be.

mg:(

 

[whyz]

Do you think that you might be alittle lean on your PJ?
What is it stock?And what do ya have in it now?Or is it still stock?
Is your AS alittle too far out?
Try one turn out.
Just asking,
See Ya.:)

 

[los36]

Jetting

MARKKLX-
Yeah, sounds like it's your jetting. I doubt that anything is wrong with the crank, though.
I don't know where you live, but I'm in Houston (about sea-level) and its been about 60*F around here. My bike's jetting is pretty close, but probably not as clean as it could be on the bottom. With the stock pipe, my jetting was:

Air Screw - STOCK (1.5)
Pilot - STOCK (48)
Needle - STOCK, but 2nd clip position from bottom
Power - #50
Main - STOCK #170

Start playing with the air screw first. If that doesn't help, get back to the stock settings and try cleaning up the bottom agian. I personally would not go as far as cutting the slide unless the off-idle performance is really that important to you.
GOOD LUCK!

 

[Marklx]

los36,

I'm in Los Angeles/Ventura county, which is pretty much at sea level. Ride up as high as 3000', but the problem occurs at both. The diff's in our jetting are: main, 168, 1/2 richer needle on 2nd clip, so clip is at "2 1/2" from the top. Same...Air 1.5 out, pilot 48, power 50.

What oil and ratio are you running. A local shop heard it yesterday, and they are convinced I need to go to 40:1 using yamalube or superM.

Mark

 

[los36]

Oil, ratio

I use Pro Honda (Honda shop is real close to my house) @ 40:1 w/ 100% race gas. I forgot to mention the gas. That kinda screws up our comparison.
I think that for the most part, oil brands don't make that much of a difference (except Maxima Castor927). I have no drool coming out of my exhaust pipe, but I was running at 32:1 before and had no problems with spooge then, either.
I am reluctant to try and give you anymore advice, because I really don't have any ideas about whats wrong. You might try going two clip positions richer, just as a test to see if that's part of the problem. Try playing with the choke knob adjustment (I'm not sure which way is richer/leaner). Perhaps you should check your reeds??? I dunno. If you figure something out, let us know!

 

[Marklx]

los36,

I'm using 50/50 race gas and 91 pump, so don't think we're too far off there. And your input is very helpful, and of course, taken in context. Very much appreciated and helpful to know what others are doing in different situations. I don't think LA jetting would vary much from Texas. A rider friend in Texas feels that things should match up pretty close.

Was going to tinker later today, but now we are supposed to have rain! I'm going back to the exact jetting you referenced, which is completely stock, put in a new plug, fresh gas at 40:1 and see how she goes. Checked the cable and slide returns perfectly, no hangup with a little play. Turned idle way down and it didn't do anything, either. Me thinks I may have a thread soon on which aftermarket reeds to run, but that's just my gut feeling of the problem.

Sounds like you've got yours dialed in perfect, and that's what I'm after.

Thanks again,

Mark

 

[cr250john]

Let's be careful with our terminology

When you start talking about fuel premix, 32:1 will make the bike run leaner than 40:1.
Don't try to change mixing ratios and fiddle with jetting at the same time. You'll be running around in circles trying to get it right.
Pick a mixture ratio, fuel and oil type and stick with it.
Then set your jetting.
Be sure you start with fresh fuel, clean top end and power valve, and properly packed silencer.

Just my suggestion...

 

[Marklx]

cr250john,

I know what you're saying about the fuel mixture, and you're right that I shouldn't change that while fiddling with jetting. But the thing is, any zook rider I've talked with on the issue has better luck getting things right at 40:1. I'll try baseline at 32:1, but once I start adjusting again (assuming I get rid of bah bing problem), I'm going to try it at 40:1. I like running more oil, makes me feel better about my bearings, but haven't come across an '02 rm owner having luck at that.

mg

 

[bscottr]

Mark,
I know your head is probably spinning right now with all the advice, but there is some very good stuff here. CR250John is correct about changing premix ratios, however, just prior to his post you stated that you're going back to stock jetting. If you do, now is a good time to switch to 40:1. Drain the remaining 32:1 prior to filling your tank with 40:1.
Then heed what CRJohn is saying. Once you start down the re-jet road, do not change premix ratios until you have found the proper jetting. Then if you do change premix ratios, realize that you may need to re-jet.

Keep us posted,
Scott

 

[Marklx]

Scott,

The info here is great. Everyones experiences have some good info to add to my brainstore of this jetting stuff. The only reason I'm not going to switch to 40:1 now is that it didn't do the bah bing thing before with stock jetting, so it shouldn't now. If it does, then something else is wrong (reeds,rod, air leak, etc). Assuming I've shaken the backfire thing, the gas gets dumped and fine tuning begins. I hope that is soon.

Mark

 

[Battered Sav]

Just a guess, could it be too rich a pilot, you said the choke made little difference,
I'd be interested in what happens with a size or two smaller p/j.
Be careful, as with all jetting experiments,
but it should be ok so long as you dont go flat out then into an extended (long) period of closed throttle.
If it's still the same, probably best to go back to stock p/j.
I had a 96 RM, ported, pipe, reeds etc. never really got it right, but it still ran real hard.
Sold it to a friend, told him with drn's help, we can get it running even better.
"No thanks, tries to kill me as it is! " he said back. :aj:
Good luck, gunna keep an eye on thread, please keep updating.

 

[Marklx]

Battered Sav,

Maybe I've got the choke operation backwards in my head, but doesn't turning it on make the bike richer? Turning it on did not have a devastating affect on how the bike ran, and per the Zook owners manual, that means I'm probably lean. So if anything, thinking I need to go richer on the pilot, not leaner.

The carb is so hard to get at on the bike; this would be so much easier if access were better....


mg

 

[bigred455]

You could also check your float height

 

[Marklx]

Found 1/2 hour after work 2night, put in the stock needle 3rd clip. lubed the cable, made sure there was play, and snugged it back up. was raining and dark, so didn't ride it, but it sounds better. Still the occassional hiccup, which almost sounds like detonation. This bah bing thing started when I switched to Yamalube and a "fresh" batch of pump 101 octane gas. Starting to think bad gas could be part of it. Pulled the plug and it was soaking wet, and black. But I didn't ride the bike, just idling, and blipping the throttle, so don't think that means anything.

New gas tomorrow, 40:1, new plug already in, and optimistic that it will be fine.

mg