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Discussion Starter · #1 · (Edited)
successful jetting on 4 different 250r's and what I have learned

1988 - 250r - oem cylinder with work

  • 88 bottom with long long rod
  • 88 cylinder ported for mid to top, hogged out hard core
  • short rod piston so we used a spacer plate
  • cool head with 215 psi compression, .50 squish
  • fmf fatty pipe and silencer
  • uni air filter in aluminum box
  • vforce 3 reeds
  • a/s 38mm carb
  • stock ignition
Max Free Rev: 12,000
Max 6th Gear pull: 10,000+
Carb Jetting: 62P, CEK 3rd clip, 168mj
Fuel: 100LL Air Plane Fuel

1986 - 250r - oem cylinder prox 310 kit

  • factory head with 205 psi compression
  • fmf sst pipe and silencer
  • uni air filter in factory air box
  • vforce 3 reeds
  • a/s 39mm carb
  • stock ignition
Max Free Rev: 8,800
Max 6th Gear pull: 7,600
Carb Jetting: 55P, DEJ 3rd clip, 168mj
Fuel: 100LL Air Plane Fuel

1988 - 250r - ESR310 with 4 Mil Stroker

  • 88 bottom with Stroker Crank
  • ESR310 cylinder ported ESR TRX9.5
  • ESR cool head with 185 psi compression
  • fmf fatty pipe and silencer
  • uni air filter in aluminum box
  • vforce 3 reeds
  • a/s 38mm carb
  • stock ignition
Max Free Rev: 11,000
Max 6th Gear pull: 9,600+
Carb Jetting: 48P, DGH 3rd clip, 172mj
Fuel: 100LL Air Plane Fuel

1986 - 250r - ESR330 with 4 Mil Stroker

  • 86 bottom with Stroker Crank
  • ESR330 cylinder
  • ESR cool head with 170 psi compression
  • fmf fatty pipe and CT silencer
  • uni air filter in aluminum box
  • Boyesen Rad Valve
  • a/s 39mm carb
  • stock ignition
Max Free Rev: ?
Max 6th Gear pull: ?
Carb Jetting: 45P, DGK 3rd clip, 162mj, Air Screw 1 3/4 Turns
Fuel: 100LL Air Plane Fuel

I know there is a ton of reads on here and mostly what it takes is a lot of reads and for the most part i call it all horse crap. it takes a lot of trial and error and a little bit of money. what you need to realize is that no jetting spec is going to be perfect for you without testing and trying. I am just throwing out there what i have learned and it may help someone down the road some day.

Step 1 - Establish these guidelines up front
  • What fuel and oil you plan to run and stick with it.
    • I recommend Bel Ray H1R and 100LL fuel
  • Purchase the plug you want to use and keep it.
    • I recommend a NGK Irridium plug - BR8EIX
  • Choose a foam filter like UNI for Dirt and K & N for road use
    • Clean it before jetting and ride on it for a bit to dry it and have it ready for tuning
  • Gear according to your riding style
    • I suggest a 15t front and a 38t rear
  • Jet on a 75 - 80 degree no humidity day, no rain day.
    • dont jet in the morning or evenings, jet in the afternoon
      • i find jetting in the afternoon works the best. morning temps are leaner usually and evenings can either be cooler or more humid depending on the day. Jet mid day like 1 pm. you will thank me
Step 2 - Buy Your self the builders recommended Needle and set the clip in there recommendations
Step 3 - Take there pilot jet and drop it by one
Step 4 - Turn Air Screw to 1 3/4 turns
Step 5 - Run the main a extra jet size or 2 over there recommendations

Ok so that sounds standard right - well here is a few theries
  • Pilot - Your not going to burn up your machine on low end. A lean pilot is easy to detect. You dont want to flood the engine down low as this will mess all your jetting up quick. You can move your air screw in and get more response enriching it then you do by turning the air screw out and leaning the circuit. what i mean is the more you enrich the less turns it takes to notice a change and more affect it has on the circuit. for every 1/4 turn in makes the same response as taking a 1/2 turn out. also when you lean or turn out the air screw it becomes "loosened" and can back itself out or fall out. i have seen this happen and it messes with your jetting and drive you nuts!! the air screw can basically turn itself whenever it wants because there are less threads the further you screw the air screw out, therefore you have less thread holding the air screw where you set it.
  • Needles and Clip - Your engine builder knows how much fuel they want your machine to get in the mid. if i had to guess your going to be in the E series needles. Something like a DEJ, CEJ, 2nd, 3rd, 4th clip position. Dont wast your time on a G series needle like DGH or a DGJ, the G in the middle is to "lean" and most of the time the G needles are run in the 4th clip which is like a 2nd clip on a E series needle. save your self some money and by e series needles.
  • Main Jet - Melt down city here man, make sure you have all the jet you need in case you get lean for some reason!!! I like run the machine in 6th gear and peg it and have it cough and flood out then take off and go. i also like to have them pull and pull and pull when they do hit. i don't want them screaming now. i want to play it safe. screaming comes later.

Step 6 - Set your idle up by the thumb throttle so it idles nice and low, not killing but not like ping ping ping more like take it for a ride and see how it runs over all on low and mid. I like to start low and work up. Don't go out and start screaming the machine. lets just say take it around the yard and tap the throttle and see how responsive it is off low end in first and second gear. secondly how hard does it pull when it hits power band. don't go out and nail it wide open in this process, just roll on it when your at about 1/4 throttle and roll it to the 3/4 throttle position. if you bar it you will be getting some main jet effect and we don't care about main jet right now. When in 2nd gear and 3rd gear roll on the throttle. i also like to quick shift from second to third like a braaap shift braaap. you can usually make 1 adjustments in this ride and 1 notable pull. we don't need the pilot to be perfect yet but lets close the gap some between the needle and pilot.

  • Air Screw and Pilot Jet - How is it idling?
    • Revving - gun it a tad and see if the idle hangs then comes down
      • turn that air screw in a 1/4 turn, if still hangs, turn it another 1/4, then another and at about 1 turn you need to change pilot to one richer pilot - repeat step 6 from the beginning and remember to set the air screw back to 1 3/4 turns out with the new pilot
    • Bogging and smoking excessively almost choking out and fuel smell burning your eyes?
      • turn the air screw out 1/2 turn, i know most people say 1/4, trust me 1/2 turns work just fine. after each 1/2 turn wait about 6 to 8 seconds for engine to accept the change. remember the idle should be a and as you screw it out it will be more of a remember don't go past 3 turns out!!! this is where i reference from above the air screw can fall out!! when you turn the first 1/2 turn you will notice the most engine change, the next 1/2 turn might not be as noticeable. if the second turn yields the same rpm jump then stop wasting your time and change that pilot out to a leaner pilot jet. repeat step 6 from the beginning
    • Idling perftect - As long you have your air screw around 1 3/4 turns to like 2 1/4 turns your doing good. you should have enough adjustment for 90% of your riding conditions. you know you are right when you turn the air screw from 1 3/4 turns to 2 1/4 turns and revs up but when you tune from 2 1/4 turns to 2 3/4 turns you cant really notice a change. once you hit your peak rpm on the air screw, turn it back in a 1/4 turn for proper tuning. example like 1 3/4 turns and you adjust to 2 1/4 turns and the rpm increase and then you turn from 2 1/4 turns to 2 3/4 turns but get no or little rpm response turn the screw back to 2 1/4 turns since thats the starting point of the air screw being maxed out in its effective range. then turn the air screw back in a 1/4 turn so now you are at 2 turns on the air screw.
      • Side Note - i like to try 1 3/4 then 2 1/4 then 2 3/4 then back to 2 1/4 then back to 2 1/2 then back to 2 and listen each time to find the highest rpm. once you get the hang of this you will understand but you have to try the highest rpm at an idle and going from 1 3/4 to 2 to 2 1/4 and so on seems to not give enough variation for the beginner to hear. that's why i go out on the air screw, then back in, then back out so you start to hear the changes easier.
  • Needle Clip Position - dont make a change yet on the needle, take the machine for a ride after you moved the pilot or air screw adjustment and take this note:
    • Rev like a 125cc dirt bike?
      • if you quick shift from second to third in the power band and you didn't get a massive thrust back and tires picking up but more of a real short power band feel like you were going to rev through it quick and excessive wheel spin then your lean. now is the time to move the clip down the needle which cause the needle to pull up out of the main jet more and let more fuel in to richen the mid. at this time more gas will basically ooz out around the needle into the carb and into your motor. your idle will likely increase due to extra fuel being allowed into the motor from the ooz effect. repeat strep 6 from the beginning to adjust the air screw and possibly the pilot jet.
    • Bogs out - first time running through this step
      • You might be lean believe it or not. this clip is a tricky thing. move the clip one richer. we are playing this safe. if you are to lean and you can burn your motor up. repeat step 6 from the beginning.
    • Bogs out worse then before - use this guideline anytime other then the first time you do step 6
      • if machine bogs more then before move that clip up 2 positions now. 1 for the previous attempt and 1 for a new setting. for example: if your needle was in the 3rd clip from top, then you moved it to the 4th and it runs even worse in the mid, then move to the 2nd and start step 6 over from the beginning.
        • side note: if it runs all zingy and on the edge of you may want to return to the mid clip position or work with that setting. this is totally up to you. when these machines hit in 2nd and 3rd you should get wheel lift and thrust, not just scream and go no where. look for that pull feel, that thrust feel, that wider power not hit and quit.
          • if you prefer this setting you may need to richen the air screw again or in some cases a richer pilot may need to be added since you removed fuel from the mid. clip position does change low end fuel. once you lean the mid you actually allow less fuel into the entire system to ooz out into the carb later causing a leaner condition down low. you should be able to work out this less fuel with the air screw. start by richening procedure from step 6 taking 1/4 turns at a time basically start step 6 from the begging again.
          • If you are running a "G" Series Needle do not attempt to run the second clip position. Only try this 2nd Clip on "E" series needles. You are at risk of destroying your machine if to lean on the clip position!!
            • For example: do not try this on DGH, DGK, CGK etc
            • For example" try on DEJ, DEK, CEJ etc

Step 7 - ok once you get your low side running and the air screw around 1 3/4 turns and you like your mid section pull your ready for the top. usually when these 250rs are rich they have a miss to them on a hard crank out in the upper gears. you can hear a spit or miss in the upper rpms. take down 1 mj at a time until that miss goes away. 5th and 6th gears should pull clean and not stumble. i like to ride in 5th or 6th gear up a slight hill and under a load open the throttle wide open and see if it can pull itself out of a lug or if it falls on its face.
How is it running?

  • [*=1]if it falls, coughs, spits, sputters etc you most likely will drop the main jet down till it pulls out clean but lets error on the side of caution which means lets stay rich to make sure we are rich on the first run

    • [*=1]go 2 main jets richer and repeat this test and see if its better or worse.

    [*=1]if it falls, coughs, spits, sputters etc worse then before then do this on until things run correctly

    • [*=1]drop those 2 main jet sizes and then drop another 1 for testing. for example if you had a 178mj and then you added 2 sizes in the first part of this step then and now there is a 182mj installed, drop to a 175mj at this time. Repeat step 7 from the beginning - only drop 2 sizes when you realize you went the wrong way from the above step!!

      [*=1]after the initial drop only go 1 jet size per time. Repeat step 7 from the beginning
      [*=1]if you drop more then 5 main jet sizes start over from step 6 with new main jet. you can start leaning the whole system with the main jet and harm your engine.

this is the only way i find a effective way to make sure your machine is running the proper way and proper jetting. plug chops work, but they don't tell the whole story.

Ok so that sounds standard right - well here is a few theories

  • Pilot - Your Pilot jet can and will change as you jet your top half of the fuel system. as you lean out the main jet less fuel will ooz out around the end of needle and cause a leaner idle condition. The same is true when you either richen or lean the needle either way. so if you find yourself feeling lean on low end once you get the top jetted in, you may be lean and may need to change the pilot.
  • Needles and Clip - Jet around the best feel on the clip position. the needle clip position, needle clip position, needle clip position is so important!! this gives you all your pull!! find the spot u like and rejet around that clip.
  • Main Jet - As you move the clip the mj will also lean out or richen. if you richen the clip you will most likely lean the main jet and just the opposite if you lean the clip richen the main jet.

More about needles - i dont know if i stressed enough about the needles but here is more knowledge on a needle that seems hard to understand for many
  • the needle is broken up into 3 letters or part: DEG
    • D - This is your richness of the needle on top end and i have found the following to be true.
      • D works good for non ported and non drag style machines
      • C works good for those ported and screaming style machines
      • Change from a D needle to a C needle and effectively lean the top end to scream harder and clean out better in hot weather instead of changing jets and loosing race fuel.
      • C works better in high humidity conditions
      • C Offers higher RPM Output of engines
      • Change from a C to a D when the weather gets cold!!
      • D will keep your machine running cooler in general
      • 168 - 178 main jets very common on big bore engines.
    • E - This is how rich your mid section is. Ive said this before but E needles are the catch all for most of the machines out there.
      • Richen the clip 1 clip for every 20 degrees F cooler to richen the entire carb system
      • When hot lean the clip - this is far less common to do.
      • Most of the time this is set in the 3rd clip position

    • G - This is your low end idle and pilot to needle transition.
      • normally a 45 to 50 pilot meshes the best with a J needle
      • if you have a machine that you just cant lean it enough on low end then change this letter on the needle to like a J or a K or a L and its as effective as changing a pilot jet. I usually have 3 or 4 needles like a DEJ, DEK, CEK and in the fall i use a DEJ in the fall, summer DEK, and CEK when the humidity is so hot and muggy. This way i can change a needle and have to drop the carb off. Needle is quick and easy to change.
      • figure this letter a 1/2 of a pilot size. example: needle is DEJ and 45p is to rich and a a/s turned to 3 turns and a 42p is to lean and the air screw is turned to 1 1/4 turns. Drop in a DEK and and a 45p to feel like 43.5p.
      • Sometime you may need to put in a DEL needle to lean back out that 45p to like a 42p or use a 48p with the DEL to create a 45p. it may seem odd but this blend maybe the only way to get that a/s right in that 1 3/4 range. the 1 3/4 range on the a/s is important to ease of tuning when the weather changes you have enough adjustment without re-jetting all the time.

More Random stuff:
  • There maybe times where no matter what you do you cant get the pilot to blend with the needle. Change the Last letter on the needle and try rejetting. some engines just wont run on the J no matter what config you do. But if you change to a K you will get it to blend properly. what i am saying is
    • Low End - DEJ and a 48 pilot and the idle and air screw is perfect at say 2 turns but no matter how much you try you cant get the dang thing to stop hesitating or stumbling when you wot it or it wants to pipe on you and almost feels like the motor is surging before power band etc. the cure maybe to change to a DEK needle with a 52 pilot or a DEG with a 45 pilot. sometimes that's just the way it is if you want it perfect. you may need to mess around more then you like
    • Top End - DEJ and a 168 main jet and it wants to cut out when you wide open throttle it from 3/4 to full and the machine is just screaming. you may need to change the needle to a CEJ and move the main jet upto a 172 main jet to get that hesitation to go away. See chart at bottom of this article about where the needles mix into this system for better understanding.
  • When changing the Letter on the end of a needle lets say from a DEJ to a DEK, it works very similar to 1 pilot size per letter change. K is Leaner then J and H is richer then J etc
  • If you are rich on the main jet and you switch from a DEJ to a CEJ expect the main to feel even fatter since the needle is actually jumping into the main quicker then the D series needle and it can mess things up for you quick. Only change from D to C in high humidity conditions
  • Engines make there most HP on the lowest octane the motor can run on
  • 100LL i have used on machines with 235psi of compression and have had no issues
  • 100LL is a faster flowing fuel therefore pilot jets usually are leaner then other fuels use
  • 100LL when flowing will actually"cool down" and make your carb sweat on hot days. this fuel will make your machine run cooler!! i have seen this over and over
  • When jetting is correct on pilot jets you should have to choke the machine early in the mornings when you first start them. your pilot is likely correct if the machine needs 5 seconds or so of choke on. if you can start without choke your to rich, if you have to choke for 30 or 30 seconds your to lean
  • your mid is likely correct if you need to clean the machine out a tad on first start up of the day. kinda take a few braaap, braap to clean it out in 2nd and 3rd gear then cleans ups and runs good.
  • for every clip position you raise go 1 pilot jet leaner. example: if your in clip position 3rd and go to position 4th and have a 45p then lean your low end by 1 pilot size to 42p. if you go from 4th clip to 3rd clip then raise your pilot by one size from a 45p to a 48p. only follow this step once you have your machine jetted correctly and your changing the position due to colder or hotter weather.
  • if you can hear your engine having a knocking sound when it idles you either need more octane fuel or richer pilot jet.
  • standard running temp should be around 180 degrees F to 205 degrees F, after 205 degrees F give her a break. lean conditions cause the machine to run hotter. if your road riding and the machine is over 185 degrees F then your to lean. if its running like 170 degree F then thats pretty good to
  • if your machine runs 150 degrees F on a 80 degrees F day, its simple your to rich! rich conditions will make machines run cooler
  • Boyesen Rad valves are richer off low and leaner in the mid
  • VForce valves have leaner low and richer mid to top
  • running pump gas will need richer jetting. if you have to keep throwing more jets at the machine and running like DEG with a 58 pilot and a 4th clip and a 192mj you most likely need 100LL fuel.
  • Slide Numbers range from lets say number 4 to number 8
    • i have found the slide number seems to really have the most affect on the mid range of a machine.
    • traditionally a higher slide number likes a richer clip position
      • because of the richer clip position then the pilot needs to be leaned or the last letter on the needle needs be leaned, one or the other
    • i have found no change in main jet based on slide number. i speculate this is because the slide is all the way up and had no affect on air at this point
    • Chipped Coating or excessive wear on a slide can cause you a nightmare on jetting!!

This isn't meant to be a know all article, but just a good rule of thumb and with my own personal success. take it as a learning tool.


162 Posts
Discussion Starter · #8 ·
added some more notes

Super Moderator
2,410 Posts
He's still here I was asked to fill in since I'm on the board more...

234 Posts
Cool thread.I learned something about jetting today.I have a 38 mill Keihin on my stock 85 250r.I was going to check the main jet size so I could get small eer jets for it.I found no jet when I pulled the bottom cap off.It wasent even in the carb at all.I have riden and raced this trike and it has ran well a little fluttery on top.I went to the Honda shop and bought 3 jets.# 168,#170 and #172.

162 Posts
Discussion Starter · #12 ·

162 Posts
Discussion Starter · #13 · (Edited)
i put a lot of time in the above chart, i hope it goes to good use for someone. if this thread helps anyone please let me know that i helped you. thank you


913 Posts
Awesome job
remember seeing last year, definitely helped me with needles and how they work
pat on the back for u!! Thanks

162 Posts
Discussion Starter · #17 ·
just checking in and seeing how this information has helped people. i hope it continues to be used as a resource. let me know how it helped you. thank you

13 Posts
Okay guys so I really need some help with my jetting on my 86 250r I got a fresh top end, borded 20 over an ported by eric gorr racing with v force 3 reeds has a fmf fatty gold series pipe with silencer uni air filter in stock air box I tried with lid an no lid with a 38mm a/s carb I got a 48 pj an a 175 main with a jd jetting needle on the middle clip it idles an runing good till about 3/4 to all out then it sputters an bogs I got the air fuel screw 1 1/2 out I ride at sea level an it's always about 65 70 temp
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