I picked up a 2002 cr125 in boxes. I went all the way through it and put in a new crank and piston, replaced all bearings and seals in the engine. rebuilt the brakes, cleaned carb, replaced all chassis bearings, rebuilt forks, ect. Anyway i finally finished up the other day and fired it up. bike has an fmf fatty and what I assume is a fmf shorty. Put a 05+ intake boot on it and They’ve cut the sides of the air box out. Honda crank and pro x piston, all Honda gaskets and seals.
It pulls great and seems to run really well. The only issue is at idle it randomly fluctuates between idling high and normal. When I first start it with choke on it’s super high, turn the choke off and it comes down but still idling high. Every once in a while idle goes down so low it’s barely running. Sometimes after revving and clearing it out. When it’s idling high I can idle it down and for a minute while turning it nothing happens then all of sudden the idle goes way down and will usually try to die. Start idling it back up and nothing happens till all of a sudden the idle shoots back up. There’s not really an in between. Doesn’t make a difference if I blip the throttle between adjustments.
Main is 400
Pilot is 37.5
Air screw doesn’t seem to have any effect.
altitude is about 700ft above sea level.
Not sure what needle is in it as I didn’t think to check while it is apart.
Honda and fmf chart both say it should have a 32.5 pilot in it.
Being as the air screw seems to have little to no effect on anything I’m thinking it’s got the wrong pilot in it but not sure if I should try bigger or smaller. I see a lot of guys with the cr on here running much bigger pilots than the 32.5 Honda recommends.
Any help is appreciated
Here is the bike a week or two ago. Still has the wrong seat on it for now. 
Nothing?
37.5 pilot sounds to big for that altitude...
Have you checked the float height, and checked the float valve seals?
Do you have an air leak, have you done a pressure test?
I haven’t done a pressure test. I’m a bike mechanic and work on 4 strokes every day but I’m new to 2 strokes and Someone else recommended It earlier but I haven’t had a chance to get the stuff I need to do it yet. And I was thinking the same thing about the pilot jet. Fmf and Honda both say it should be running a 32.5 so I was pretty curious why they bumped it to a 37.5. It does have the 05+ air boot and they’ve cut the air box sides out so it may be flowing more air but I feel like 37 is a pretty big jump. I did visually check the float height before I put the carb on but did not measure. And float seals do you mean the o ring around the needle seat? If so it was fine.
The 05+ air boot and air box cut out will give more air flow through the air box at high rpm, when the engine is sucking the most air, it won't make a difference at idle.
Does the float valve stop fuel flowing when the float is raised (float valve closed)?
Honda recommends using a special tool to measure float height...
Most people use a vernier, if float height is higher, the carb will deliver a richer mixture, if float height is lower, the carb will deliver a leaner mixture.
If the float is sticking mixture will vary.
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Are you sure the throttle idle screw is in a correct position?
Go down one on your pilot and see if it helps. If that does not work I have a pretty good idea what it is.
It never seems to run out of fuel and fuel never comes out of the overflow so I would have to assume it’s not sticking. Thank you for info on the float height
My 04,s float sticks probably from sitting. I tap on the float bowl with a spark plug wrench. And it’s fixed. It will run like crap when it’s stuck even shut off at high rpm.When The float bowl is probably empty.
I messed with it some today. I’ve been sick so didn’t get to mess with it much. I figured out if I let it idle for a minute the idle would drop. If I revved it up a good bit and cleared it out it would then idle high again and slowly go back down. So I kept backing the air screw out until about 3 turns the idle would then stay high all the time. So idled it down and it seems to hold a consistent idle now.
So the question now is should I put a smaller pilot in it or do you think I may still have an air leak
You may still have an air leak, the only way to know you don't have an air leak is a pressure test.
I'd go down a pilot jet size.
Okay I’ll order a jet and in the mean time try to find some stuff for a pressure test. Thank you for the advice
I wouldn't waste any time or money on carb parts until you verify 100% that you don't have an air leak.
$10 for a pilot jet!
https://www.rockymountainatvmc.com/parts/pro-x-4-042-series-mikuni-hex-main-jet-p
If it was air leaking it would be spinning up and or bogging.
That's a main jet, how much does Rocky Mountain charge for shipping...
https://www.rockymountainatvmc.com/parts/pro-x-n224.103-series-mikuni-pilot-jet-p?v=1754
LOL.
https://www.rockymountainatvmc.com/parts/pro-x-n224.103-series-mikuni-pilot-jet-p?v=1754
It's free with a min. order. It's $8 if you just order the jet.You will spend that much in petrol running to the dealer.
I work at a dealer actually. Can just get it with the next order they send out at cost for no shipping. It’s pretty nice😂can get pretty much whatever I want oem or aftermarket at cost which is generally somewhere around 2/3 of the price or less that you’d find something on the internet.
Rocky Mountain is good too. They have a warehouse like an hour or two from my house so as long as I order in the morning i get it the next day, at least the few times I’ve used them. So if I need something quick I just get it from them
Start the bike.
While it is running spray brake cleaner around your carb, and reed block. If the rpm rises when spraying a particular spot, you have found your air leak
Pit Row
That won't find an air leak between the crankcases.
Will it find an air leak in the intake tract?
Yes and No, it will find a leak around the reed block or the top of the intake tract, but not the bottom between the in take tract and gearbox, which is a common leakage area for case reed bikes...
Yeah I tried that. Soaked it all around it and all around the cases and topend. Couldn’t get any changes out of it
My last few CR 125s (and 134) I ran 37.5 pilot in Southern California (sometimes around 1,000 ft elevation) and they idled fine. I'd look for leaks as mentioned. What about the reed valve? Broken/warped/cracked reeds at all?
He is in Big Clitty and only 1 size off.
What’s your alls opinion on the pro x jets. Are they worth getting? I’ve heard a lot of people have problems with non mikuni jets
They are fine. Use them plenty and never an issue. I was hesitant at first but zero regrets.
Okay thank you
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