Jump to content
Sign in to follow this  

Fine tuning carb for idle - 1/4 throttle

Recommended Posts

I have a 02 XR250 and just cleaned up the carbs and did Gordon Mods.  Went to next higher main (135) and pilot (50).  Started great without choke - which I read on this forum meant it was running rich.  When it was hot I could not kick start without letting it sit for 15 minutes.  I figured out I was getting too much fuel and I had pilot screw 3 turns out so I successively turned it in until finally settled on about 1 /14 turns out.  Starts when hot now on first kick.  However, now i have just a slight stumble off of idle.  I am trying to understand the role of the pilot screw, jet and idle screw and how to optimize start up - cold and hot, idle and pick up right off idle.  I have the stock pilot (48?) and wondering if I should go back to this and try adjusting idle screw to compensate.  Any suggestions?

Share this post


Link to post
Share on other sites

I have the same exact bike as you, with practically the same mods.  Only thing that I would say is maybe different is that I have a PC4 instead of the stock pipe.  You also live in the same state as me so our jetting shoud be fairly similar.  What I have done to my bike, and it works great, is to just go to the 48 pilot, not the 50.  I'm at 1 3/4 turns out I believe, or very close to that on the fuel screw and it runs just fine. 

 

Now, my bike still has the "XR bog" straight off idle if you snap it, but that's due to the carb not being a pumper.  If this is what you're talking about when you say "stumble" then you wont get rid of that without going to some brand of pumper.   You will just have to accept it and adapt, or shell out the cash for a pumper.   I've gotten used to it, and as long as I don't ever hop on a bike that has a pumper I am totally oblivious to the bog and just ride around it.    It is disappointing however that Honda did not put a pumper on these bikes, it would have most likely made them even more desireable, but that's a whole different topic. 

 

Anyways. to explain the fuel screw.  It will give you the adjustment equivalent to approx 1 size bigger or 1 size smaller on the pilot jet.  If you have to turn the fuel screw more than 3 turns out to get it to idle right then you need to go one size bigger on the pilot; set the fuel screw to the factory spec, and adjust from there again.  Judging by what you explained if you are only 1 1/4 turns out you are a little on the rich side on the pilot circuit, so I might step back down to the 48, set your fuel screw to 1 3/4 and see if that helps.  

 

P.S.  I'm no carb expert by any means, most of what I learned came from this site, and if you spend some time searching around there are guys on here that are a lot better and more knowledgable than me.  I'm just giving you what worked for me.   

Share this post


Link to post
Share on other sites

Now, my bike still has the "XR bog" straight off idle if you snap it, but that's due to the carb not being a pumper.

 

Bullshit.  Snap the throttle on my stock-carb XR400, and the bike will stand straight up and flip you over backwards before the rear wheel moves.  Same for my wife's stock carbed TTR125.  You need to set up your pilot jetting for throttle response.  Snap the throttle from closed to 1/8-1/4 open - response should be instant.  If not, your pilot isn't set up right.  OP - it sounds like you're very close - you definitely have the right pilot jet in there, but you probably want your fuel screw in the 1 1/2-2 turns range.  Open it up slowly until that stumble goes away.  Jet your needle the same way - snap from 1/8 to 1/2 open.  Twiddle with needle clip until response is instant.  1/2 to WFO snap for the main.

Edited by jayc250x
  • Like 1

Share this post


Link to post
Share on other sites

Thanks for the replies.  I just got a 48 jet in the mail and will try that with a few more turns of the pilot screw.  I guess the question is what is the right balance between pilot jet size and fuel screw turns - meaning, as one goes bigger (jet size) the number of turns out is smaller.  Is a 50 pilot and 1.25 turns equal to a 48 and 1.75 turns?  Is it basically accomplishing the same thing or should I focus on jet first and pilot screw second?Or use the pilot screw to fine tune?  May be a nonsensical question but inquiring minds want to know!

Share this post


Link to post
Share on other sites

nope... you can't get the same result  of a larger pilot by merely adjusting the screw....

 

the pilot is a larger jump than the fuel screw...  if you can run the fuel screw all the way in and the bike still runs, then your pilot would be too fat/large... conversely if you can run it all the way out without the bike stumbling it could indicate a lean/small pilot...  you should be able to both flood the idle with too much gas (fuel screw out) and also starve it of gas (FS all the way in) when you have the proper size pilot... your balance (optimum fuel ratio) is somewhere close to halfway between these extremes... need to figure out the difference between a bog from too much gas and a stumble from not enough....  its jetting like everything else so depends on elevation, bike mods, humidity, etc to know whats best for you, but its close to the middle there... RARELY you might adjust the fuel screw if you are doing a ride that changes several thousands of feet in elevation....

 

your main jet in combination with the needle and clip position work similarly on the rest of throttle position... the higher clip allows the needle taper to sit deeper and more clogging/reducing fuel flow through main creating leaner condition....  lower clip means a higher needle allowing more gas and a fatter condition... confirm golden plug color... white color means you need more gas (bigger main) cause you are running lean... too fat and the bike will stumble at WOT(Wide Open Throttle!!!)

 

Usually start new pilot at 1.5-2 turns out... then find the extremes... then find the middle or wherever it sounds smoothest...

Edited by desertwalker62

Share this post


Link to post
Share on other sites

I put in 48 pilot and went 1 3/4 turns out.  Kicked at full choke - stumbled and stopped.  Put at middle choke level and it started with one kick.  Ran up and down street and slight stumble seemed to be gone right off idle.  Came home and stopped - then kicked it and it started on first kick when warm.  I will give it an extended ride this weekend.  Taking small screwdriver for pilot screw adjustment.

Share this post


Link to post
Share on other sites

 Is a 50 pilot and 1.25 turns equal to a 48 and 1.75 turns?  

 

You're definitely on the right track.  You will find that these combinations are similar.  I found the difference between a 42 and 40 to be one turn vs. three turns and I go back and forth between those two sizes  If you get tiny bits of debris in the jet or fuel screw passage, it will throw the jetting off and make you think you need a larger jet.  You're getting good advise here.  You might eventually try a 45 as well.  Try to tune at the lowest idle setting.  If you are so lean that you get a surging idle, open the screw a half turn or more.  It's a lot of trial and error to get dialed in.

Share this post


Link to post
Share on other sites

Kicked at full choke - stumbled and stopped.  Put at middle choke level and it started with one kick. 

 

 

yeah... shouldn't need full choke unless its actually cold where you are... half choke is a great summer starting position...

 

clean carb is a necessity....

Share this post


Link to post
Share on other sites

Bullshit.  Snap the throttle on my stock-carb XR400, and the bike will stand straight up and flip you over backwards before the rear wheel moves.  Same for my wife's stock carbed TTR125.  You need to set up your pilot jetting for throttle response.  Snap the throttle from closed to 1/8-1/4 open - response should be instant.  If not, your pilot isn't set up right.  OP - it sounds like you're very close - you definitely have the right pilot jet in there, but you probably want your fuel screw in the 1 1/2-2 turns range.  Open it up slowly until that stumble goes away.  Jet your needle the same way - snap from 1/8 to 1/2 open.  Twiddle with needle clip until response is instant.  1/2 to WFO snap for the main.

Maybe I was confused when he said stumble.  I don't have problems when I'm rolling down the road and crack the throttle, but if I'm sitting at idle and slam it from 0 throttle to WFO and hold it there it will bog and die if I don't roll into it first.  What is your idle set at?  Is it higher than normal?  I know that on my bike if you set the idle high it will remove this bog, but that is just because you're getting the bike out of the pilot circuit.  I like a low idle so that I have more engine breaking for riding in the woods.   Maybe I do need to move up a size and see what happens, but I haven't heard of many people on here running a #50 pilot on a 250.   Or like you said in your post it might be my neddle jet is off.  I thought I had it running pretty goodm, but now I'm starting to question if there's still more left in the tank. 

Share this post


Link to post
Share on other sites

I went to the 48 pilot, 1 3/4 turns out.  Started first kick on middle choke. Seemed like the "bog" was gone, but I understand the carb can only go so far.  Probably did something to help it but may just be perception not reality.  Doing 100+ miles tomorrow so it will get the gamut of cold start, warm start and hot start.  I am feeling pretty good about this set up.  Hopefully everything works out tomorrow.

 

Funny, but I was already looking at the bigger bore, cams, etc., to get a few more hp out of the motor.  Price tag for everything came to about $1K.  Put $1K in a $1200 bike???  Sure.  Guessing late one night after a few beers I'll order everything online and then wonder what the hell I just did.

Share this post


Link to post
Share on other sites

Just curious, but do you race any Harescramble series in Missouri?

Share this post


Link to post
Share on other sites

 

 

This.   I've had a lot of XR/XL's and I never gave it a thought until I started reading here and the "go faster" 's  started complaining about the bog. Yes it there, but if you have it all the time you adapt and never give it a thought. 

 But I succumbed to all the talk and got a pumper for the 400. It IS nice, but my stock carb was jetted perfectly, started 1st or 2nd kick and I don't dump it a lot so re-starting isnt' a big deal to me.

 Is it nice and noticeable performance wise? Yes, I'd rate it roughly as noticable as going to a higher compression ratio, but pump gas safe like 10:1. 

 Is it night and day? No, not if the stocker is jetted correctly. 

 Have a really clean 03 stock carb jetted for dave's mods if anyone needs.  ;)

Share this post


Link to post
Share on other sites

Oh - what I'd give for my old XR400 bog!

 

I was riding a super-tight snotty time trial this weekend on my CRF250X.  My bar nicked a tree (which was happening about a half-dozen times per lap) then all hell broke loose. I got knocked off balance for a split second and my bike suddenly rocketed forward and smashed into the next tree.   I found a big footprint on the trail during forensic analysis, so I'm guessing that the bar-tap knocked me off line, but then my throttle-side foot must have slipped off the peg, which caused me to whisky-throttle as I was sliding sideways/backwards, and that instantaneous throttle response that everybody is always looking for shot me forward like a bullet right into the next tree.  Dislocated my thumb - I put it back into place (which was DISGUSTING) and made it off the trail - then had to put it back into place AGAIN.  Ugh.

 

Bike took the impact directly in the throttle-side rad guard - Flatland guards are awesome, by the way - guard is mashed (should be able to beat it back into shape) but rad is completely unscathed - rad shroud is shattered.  I also broke my kickstand in half somehow, even though it was on the other side of the impact.

 

If I was on my XR4 back in the stock jetting days, when I slammed the throttle, it would have just coughed and the crash wouldn't have happened.  Be careful what you wish for - you just might get it. :)

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Reply with:

Sign in to follow this  

×