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2005 KX250F - Popping, wierd starting
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Posted by: AARONA---------------------

Just got a 2005 250F. Strange thing was the 1st starting. Pulled choke "on" and the thing would not start. Once choke was "off", and the throttle was cracked while kicking, she fired up. This has been the only way to start the bike, hot or cold - choke off, throttle cracked on about 1/8.
Runs funny. Idles up and down a little, and backfires when decelerating.
Strange thing is people say backfire is often a lean condition. If its lean, why wouldn't it start better with the choke on?
If I fatten up the pilot jet, wont that make the choke that much less effective?
Anyway, it was cold (high 40's), and I only adjustde the air/fuel screw 1/2 turn out.
Sounds like I need a 42 pilot, raise the needle one, and mess with the fuel screw. The choke thing has me befuddled...like most 4-stroke issues...
Any words of advice from those in the know???

Posted by: biglou---------------------

Sounds like an air leak. Seems lean also (compounded by the air leak). Holding throttle at 1/8 is shooting a stream of fuel down the intake tract. Sharp backfiring can be headpipe leak.

Posted by: AARONA---------------------

I'll check that tonight for sure. Brand new, I hope it's not that. Never know though..
Thanks

Posted by: marcusgunby---------------------

Did you fit the new copper gasket as the header pipe junction with the head?? mine does the exact same thing, choke makes starting worse, i have a 42 pilot and fuel screw 2.75 so i think it needs a 45 pilot, i also run a 180 main but with the different fuels you might find the main too rich.

Posted by: AARONA---------------------

marcus,
haven't pulled the head pipe yet, but I will do.
is the 180 a stock mainjet?
I also hear (internet...) about running a 04crf450 needle for this bike. Supposed to help. I am leery of the internet tuning advice. One guy is a factory wrench, the next reads MXA and slept in a Holidy Inn...
Glad that you have the same rig, hope to pick your brain on this bike as time allows.

Posted by: ochster---------------------

Marcus is right about the pilot jet. I would put a #42 in with the fuel screw near 2-21/2 turns out, you ultimately may end up at a #45 with real low humidity. Your stock needle in clip position #5 from the top should be a "care free" setting at your indicated temps. I have seen several of these carbs with low float levels as well. Make sure the air filter is not over-oiled. Stock exhaust on these is pretty sealed up and a decent performer. The CRF needle (NCYR) is not needed to clean the jetting up. You should also check the valve clearances as you will want to monitor them.

Im just another internet guy, but this forum (DRN) is not just another dirt bike site. The sharpest minds on the internet are right here. If something is posted on here and does not pass the smell test, rest assured you will be called out, for whatever it's worth.

Posted by: AARONA---------------------

thanks for the info. I am a believer in DRN.

Posted by: AARONA---------------------

anyone want to take a crack at why a richer pilot jet is recommended, yet the bike only starts with the choke off? Will not start with the choke on. Its odd.
To me, if I'm lean from idle to 1/4 throttle, then the choke should richen things up, and it could would run better (possibly) with the choke on. But this is not the case at all.
When riding the bike, and just cruising down the road or trail, it does not want to hold a steady throttle, it surges and popps when the throttle is off. When going, if you hold at a steady 1/8 or so, it blubbers and the revs fluctuate.
I have not made the changes stated above as of this post (I will) but wondered for the sake of figuring it out if anyone knew.

Posted by: marcusgunby---------------------

I have a friend with a 05 and had the 04, he says he never uses the choke apart from 1st kick in the morning, says it works for him, i dont try too hard to analyse the whys-as long as it starts after a couple of kicks im happy.

Posted by: jumpingjoe628---------------------

My friend had the same problem this past weekend and he ended up changing the 42 piolet jet to a 45. It worked.

Posted by: AARONA---------------------

plan on getting a 42 and 45, and a few mains (found out 178 is stock) I'll get a 175, 172, and maybe a 168.
I ride in Alaska, even on warm days we have fairly high O2 in our air. Bikes tend to run leaner up here. Most seem to work with stock jetting. I have access to an air density meter, and will get a baseline and begin from there.
Thanks for all the help. Quick responses and good information, what more can a guy ask for?

Posted by: AARONA---------------------

BIGLOU,

checked the pipe fititngs, everything seemed tight and sealed up properly. Popped the ignition cover, just to check, and everything was tight, looked ok.
Valves are just a little tight...but it only has 1.5 hour run time, and they are in spec, just on the tight side.
So, just waiting on the pilot jet. Had to order, not in stock, although I did talk them into getting 10 of each size.
Will update once its in.
I think our 30-40F weather could be part of it too.
Getting warmer, and staying light longer each day though. (gaining about 7 minutes per day)
Can't wait for summer.

Posted by: Chili---------------------

Quote:

Originally Posted by ochster

Im just another internet guy

Yup just another one of those internet guys who's happened to tune bikes that AMA pro's have run in both AMA Nationals and SX in the last couple of seasons.

Now can you get one of your guys to ride a dang 125 2 stroke so I can get more tuning tips for my kids bike!

Posted by: ochster---------------------

Chili...
We were at the amateur Nationals last week and they have a school boy class (125cc). After watching and listening to the fourstrokes all day, it was very cool to hear a full gate of two strokes. For one it was quieter, almost everyones corner speed was faster, and the racing was much closer. Everyone standing along the track was commenting what a blast it was to watch.

The new crop of 125's is a good group of bikes!

Are you still on a 04' RM125?

Posted by: Chili---------------------

We traded Troy's 03 for an 05 RM125, the cylinder is at Eric's right now getting ported with something along the lines of what he setup the last 2 strokes he did for you for SX. oldguy and I went over and talked to Cole's dad at St. Louis (I had asked if you were around), they seem like real nice folks and spoke very highly of you.

AARONA sorry for the thread hijack!

Posted by: ochster---------------------

Our RMs were very potent. Eric's cylinder and head mods made for a great performing lap time bike, Abrigo just found himself starting from midpack against the 4-strokes and I struggled a little getting aquanted with the unleaded fuel early on. That same bike is kicking butt in the off-road races now.

The Sieblers are great people, Cole rides like Windham and has better day's coming.

Thanks for the kind words Chili, please excuse me as well AARONA

Posted by: AARONA---------------------

Your cordulance is not needed, but is appreciated. Is not the purpose of this forum to exchange information?
Damm, I sound like Yoda.
There are still some 125 2-stroke holdouts up here in Alaska, but they are getting fewer and farther between each season. Still a lot of 250 2-strokes though.

Posted by: AARONA---------------------

so I went with a 42 pilot jet and set the fuel screw from 1-1/2 turns out to 2-3/4 turns out, and what a difference. Absolutely no popping. Easier to start (still wont start with the choke on) and no surging at part throttle. The bike is awesome, so happy we got it (its my wifes bike). Now I just have to talk her into letting me race it..
Also, the header pipe bolts coming from the cylinder were not loose, but not tight either. Snugged them up. Also founs a few misc fasteners that were loose, but that is to be expected.
What a great bike.

thanks to all for this forum and the advice. Would have been trying who knows what if this forum was not here. I hope somebody else also can learn from my experiences on this 05kx250F.

Posted by: marcusgunby---------------------

Also i have found my bike actually starts easier with the hot start -when its cold-i use the choke only for the 1st start of the day.Its weird but works.

Posted by: AARONA---------------------

How do you know when you have gone too far (rich) on the fuel screw? I do understand that people tend to say that if you are more then about 3 full turns out that a larger size pilot is needed. Does the bike smoke if it is too rich on the pilot? Or bog? What are the characteristics of a rich fuel screw adjustment?

Posted by: marcusgunby---------------------

I think 4 strokes are tricky as they dont change in feel like a 2 stroke, but 3 turns does mean a larger pilot.

Posted by: AARONA---------------------

So it comes down to plug reading then? That is different on a 4 stroke compared to a two stroke as well.
I'm going to try a 45 pilot and just use the same method I always end up using, trial and error. Usually more error...
I noticed on hot starts that the bike starts best for me whan the throttle is about half open. That confuses me as well. Does it need more fuel, or is it that it need more air? Or is it a mass flow (needing more fuel/air flow) type of situation. Trying to figure it out so it starts easier for my wife. The hot start helps, but it always seems to take several kicks.
I had a YZF426 that I could start like clockwork. Every time, hot or cold. This booger isn't there yet.

Posted by: marcusgunby---------------------

Sorry i cant really answer you questions better but the 4 stroke is very new to me, the 45 works in winter better than the 42 but in summer the 42 should be the one, also you went richer on the needle?

On starting mines now pretty good
1st thing in morning a few kicks with the choke
rest of day no choke and kick-its its been started within a hour previously the hot start seems to help alot, ususally it kicks within 3 kicks using this method.

one thing does bug me is the tickover wont stay the same day to day or even race to race?i have ot adjust it a fiar bit- i get huge engine breaking, or it runs away at the end of the race when riding back to the pits.

Posted by: AARONA---------------------

marcus,
tickover? whassat? do you mean the idle adjustment. Mine seems to fluctuate aas well.
I'm thinking your correct though, I'll run the 45 until it gets above 60F, then switch to the 42 for the rest of the summer. Middle of the summer for our 4th of july race in Fairbanks it can get to 95F, might be using the 400 for that weekend.
yeah, whats a tickover?

Posted by: marcusgunby---------------------

yes idle lol.

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2005 KX250F - Popping, wierd starting

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