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2002 XR70R Crazy Idle!

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After 5 years, I finally talked the mama into letting the 9 year old get a dirtbike.  We ride red.  Put her on a 50, she's already outgrown it.  Tried a 70 next, perfect.  Brought it home, maintained it with tires, a plug, an air filter, fresh oil, blah blah blah.  Carb looked clean and good, so I left it alone.  Problem I've got is that it has a wildly erratic idle making it nearly unrideable.  With the choke on or off, it'll start right away.  Sometimes it'll idle perfect, but when you give it a rev it will just stay at whatever RPM you revved it to.  Sometimes it'll start and the idle will skyrocket.  Sometimes you can fix it by flipping the choke lever to the other position, sometimes you can't.  The only way to ride it is to start it, then hit the kill switch, then shift into first just before it dies, then un-hit the kill switch and hold the front brake so it doesn't take off on it's own.  This is bad for a beginning rider that's already nervous.  I stink at carbs, where do I start?  I tried using propane to look for a leak, but the idle is so erratic to begin with I can't tell if it's sniffing propane or just idling all over the place because it wants to.

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Unbolt the carb from the head and dab a light coating of wheel bearing grease on the manifold base and o-ring. Do the same thing at the other end of the manifold, a light coating on the carb and heatstop (black plastic spacer) o-ring. Make sure the air filter is clean and oiled and the airbox is firmly attached at the carb. Report the results back here, the carb may need to be readjusted if the air leak goes away.

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Air filter is a new Uni, fresh oil, so that should be good.  I'll double check that the airbox is secure to the carb.  I assume that the o-ring on the phenolic spacer faces forward to seal against the manifold, and the o-ring on the carb obviously faces forward and provides the seal against the back of the phenolic spacer?  I did not check manifold to head seal, didn't occur to me that there would be an o-ring in there, figured it was a gasket.  I'll glop some grease on there as well and see what happens.

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I put some grease on either side of the phenolic spacer, and the bike required it's choke to start.  Ran at a nice, low idle until it warmed up, and then the idle went to the moon again.  During breaks, the bike would cool, run great, and then get crappy as it warmed up again.  My theory is the O rings on either side of the spacer are crap, and that cold, thick grease seals up an intake leak.  Warm, liquidy grease does not, hence the crap idle once warm.  I'm going to replace those O rings to start, and see where that lands me.

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Here's something odd.  Bikebandit doesn't show the intake manifold or phenolic spacer in their diagrams.  I checked both the carb page and the head page, neither even references that they exist.

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So I read this thread 3 times to be sure, and I think I am, but you mention the air filter is a new uni - It appears you got the uni element that goes in the stock airbox, yes?  If its not and its a pod style one, you're jetting might need to be adjusted for the increased airflow.

Also, I believe this sounds like a lean condition (hanging idle, irratic idle, schizophrenic idle).  If it were me, I'd pull that carb, make sure ALL passage ways and jets are clean (carb cleaner and compressed air), I'd also drain the fuel in the tank (maybe its old or once I had a friend add so much Seafoam when winterizing it the bike ran like garbage), pull the petcock to make sure the screen isn't getting plugged and then also check to make sure the main needle is in its stock position along with the pilot adjustment screw.  After all of that, assemble and see if it'll idle and spray some carb cleaner around the intake areas to see if you can notice idle change.  Your theory on the o-rings could be very likely.  But an intake leak like that will show the same symptoms as a plugged jet or lack of fuel as well.

Also, you should check the valve clearances too... they're likely ok, but for peace of mind I'd check them out.

When you got this bike, were you told it ran fine and was ran recently etc etc?

Edited by apbling

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Correct, it's just a pancake of filter media, same size as the factory one that was almost growing weeds in the dirt it contained.

Saw the bike run when we bought it, no test ride, as I'm way too big, but it started immediately and idled fine.  We fired it up again, with no ride, when we got home, and brought it up to temp, all good.

I couldn't figure out how to get the airbox off, so I removed the carb, popped the bowl off, perfect inside, no varnish or green, and it never ran right again.  I'm guessing the o-rings were smooshed and stuck to the spacer, carb, and manifold.  I un-smooshed them, and they never sealed up again.  Already have them on order, so I'll pop them in and see what happens.  If it's a no-go, I'll go through the carb more closely and check the tank out as well.

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Seriously, if you got the carb off, you might as well clean it. Just YouTube it - there's tons of videos I believe. But if you really aren't confident, then maybe let it be, YouTube it and maybe talk to a friend or something. It's really not bad at all. I knew nothing about carbs 20 years ago. Now I actually prefer tinkering w them over other bike activities.

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doubleJ - I did some more thinking... You might be correct with your analysis of what the issue is.  I didn't mean to pressure you into something you aren't comfortable with, but was more of the viewpoint of since you got the carb off, its like an extra 10-20 min to quickly shoot some carb cleaner and air through the jets and passageways.  For example, you already had the bowl off.  Just take the float and needle out.  Then pull the main and pilot jet, hold them up to some light and see if you can see light through them.  Turn the pilot adjustment screw IN (tighter) and count how many turns it took.  Then back it all the way out and remove it.  Then just shoot some carb cleaner in all the passageways you can find to make sure a little spec of crap didn't get in there somewhere.  Then re-assemble.

That aside, irratic idle like this really does sound like a good sized vacuum leak, like you were thinking.  USUALLY if a carb circuit is gummed up, the symptom will be "it only runs half-assed decent with the choke on" - which adds more fuel to compensate for the plugged pilot circuit. 

I know how you feel though chasing this odd issue.  I have an ancient 1975 suzuki 125 2 stroke that was doing something similiar, but mine would redline right NOW for no reason and the kill switch wouldn't even touch it, had to snub it out or pray the motor didn't let loose.  I narrowed my issue down to a leaking crank seal pulling in extra air - this would apply only to 2 strokes, but the idea is similar to what you have going on.

I'm assuming this bike is relatively stock as well, right?

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Yep, the bike is bone stock, don't have any modification plans as she'll outgrow it in the next year or two.

No problem cleaning carbs, been farting with them for almost 15 years now, but before I pull it apart and dip it, I'm using the mindset of "what did I touch in between it running find and running crappy".  Got my third O ring in the mail today, gonna slap em in and see what happens.  If it's still a thing, then I'll break it down to components and give it a 24 hour dip.

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My 2 cents.....you are absolutely correct to retrace your steps. It ran fine, you tinkered, now it doesn't.
I don't know that airbox but my red 450 is a trick to get filter seated properly. You have to develop a feel for it.

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Have you adjusted the valves? This gets over looked often. As years go by and they tighten up we tweak the fuel air little by little. Then one day we wonder why the idle is so high after it warms up when it generally just needs the valves set properly and the idle screw set back to normal. 

 

Just another thing to try. 

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