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Typical jetting on 250 trials bike

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What is typical jetting for a 250 2t trials bike? pilot/main

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google spanky's jetting guide , and run through the tests to see if pilot is correct size

main should be ok from factory i would imagine and trials bikes aren't really on the main long enough to warrant a change

maybe a 1/2 clip richer needle so you can run a clip higher (eg: 2 instead of 3) to be half clip leaner if necessary ( hot humid conditions or at elevation )

out of curiousity what bike do you have ?

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On 5/2/2017 at 9:52 PM, ccullins said:

What bike? Kind of carb on it? 

I am a carb fanatic. I can't rest until it's perfect. I tune the best idle with a tach to determine how many turns out from bottom. which points me in the direction I need to go. I also weld a bung on everything I own to tune for target AFR numbers. My present bike, a KTM Freeride is known to have many carb issues. Most have given up on it and replaced with Lectrons and similiar carbs. I have just about put that one to rest. Soon, real soon. I ask because the bike was tuning in at a 55 pilot jet. Which sounds waaaay off for a trials like bike. I countered that by changing the 3.5 slide for a 2.0 and brought my pilot jet down where I think it should be at 45. I also tried a 3.0 slide which tuned well at 48. I was still getting da,da,da, bump bump. I figured that one out today as well. You could see my post leading up to that discovery and all the many carb fixes on another forum. It has a main circuit aneurysm contributing to the pilot circuit. I will lower the needle to fix that. Oh, it's a 28pwk carb. Great because I can buy different slides rather than retrofitting them as I usually do.  I expect to dial it in real soon. One more day and I'll have it. I'm just curious what a typical trials bike has for jetting. It would seem that it would be lower than a typical dirt bike. 230-250 trail 4t bikes are usually ... 42 to 45, 120 to 125. 

Edited by 1gr8bldr

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18 minutes ago, highmarker said:

That's extremely off. The main is so lean that everyone gets the Diesel effect because the plug tip becomes a glow plug. "Lean runaway" .  Maybe it will be rewritten when I'm done. For example, the 122 gave AFR numbers of 16. Scaaary lean. Other than that, some good info there beside the fuel portion 

Edited by 1gr8bldr

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7 hours ago, highmarker said:

you asked for an example ?  sounds like you got it all figured out...

I asked what a typical trials bike has for jetting. So far, just one response

Edited by 1gr8bldr

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7 hours ago, ccullins said:

55 pilot not so far off. 

My beta 250 jetting. 50 pilot 120 main. Keihin 28 mm

Thanks, higher than I expected for a technical bike

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To clarify, I'm not asking for info on my bike. I have more fun and confidence if I verify that myself. I am just wondering about trials type bikes. It would seem to someone like myself with no experience that they would have 26mm or less carbs. And to hear them run on youtube, they sound crisp and not overly rich. I ask a lot of questions, most times to put my assumptions under the microscope

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6 hours ago, 1gr8bldr said:

To clarify, I'm not asking for info on my bike. I have more fun and confidence if I verify that myself. I am just wondering about trials type bikes. It would seem to someone like myself with no experience that they would have 26mm or less carbs. And to hear them run on youtube, they sound crisp and not overly rich. I ask a lot of questions, most times to put my assumptions under the microscope

A 26mm round slide Dellortto used to be the typical Trials carb. It worked well, was easy to tune and reliable. However people found the bikes would run cleaner and rev out more with the Kehien 28mm so people started switching over. Most manufacturers use that as their standard carb now.  Even on the 125's. Jetting we're running now on my son's 125 is 55 pilot and 112 main. Runs very strong and really clean!

KTM did build a motor much like a trials bike, but having ridden one they need to do a little more work on the power IMO. Seems a little weak off the bottom, not smooth enough in the mid range and doesn't rev out like a good trials bike should.  Almost seems like it has the power curve from an early 90's trials bike.  

 

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Gas Gas 250 with a Keihin PWK28...

 

Air screw 2 turns out 

45 pilot

#3 slide (#3.5 is stock -  I might even try a #2 as there still seems to be a lean spot at about 1/8th throttle)

JJH needle clip in the center position

125 main (I might go to a 122 is the mid summer heat / humidity)

 

This is running straight race gas.  I've noticed on the 10% ethanol 93 octane it runs leaner because the fuel has somewhere between 3% - 5% oxygen where as the race gas I'm running has 0% oxygen.  This is most noticeable at low throttle settings where the bike already seems lean.  But I think with the #2 slide it would be fine on 93 octane pump gas.

 

My bike is an odd duck though.  It seems much leaner in the 1/8th -1/4 throttle range than other club members with the same exact bike.  I've never been able to figure out why mine is leaner.  I've swapped carbs and even a CDI with another club member and my bike always runs much leaner in that throttle range.

 

Doc 

Edited by Doc_d
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On 5/4/2017 at 6:00 PM, ccullins said:

A 26mm round slide Dellortto used to be the typical Trials carb

That's what's on my GG 250. Factory settings:

35 pilot

120 main

D36 needle, clip in the middle

60 starter

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