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Carburetor operation and jetting explained



Motorcycle carburetor jetting and operation explained!

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    • By IgWannA
      I noticed on my DRZ this carb nipple isn't connected to anything and it spurts fuel every time I twist the throttle, even with the engine off. It's a Kehin FCR carb from the enduro model.
      Photo here
      The bike runs mostly okay but I'm having a possibly-related problem where the engine cuts out when I blip the throttle, especially at low revs. I wondered if this carb port had something to do with it, since it's connected to the accelerator pump (which would explain why I can't blip it). Can anyone tell me what the port is for and why it's squirting petrol?
      Any help appreciated.
    • By Stabby
      Hey All, dug around lightly an didn't quite find what I needed, as I have specific things I'm lookin for. 
      Idealy a place to start with jetting settings, couldn't find a thread with the same mods I got. 
      CW 440 big bore, STOCK header and exhaust that was cut in half by previous owner, hotcams stage 2, 3x3, K&N street filter, stock carb, sea level altitude.
      Just a bit odd having the stock exhaust and BBK, hence a little difficult to find a good starting point from advice.
      Thank you for any and all advice!
    • By John B.
      I hope someone can help me correctly jet(or at least get to a good starting point) my son's 04 CRF250R. It has a Hot Cams Stage 1 cam, stainless valves, FMF PowerBomb header pipe with a FMF Powercore 4 S/A Silencer muffler, Stealthy 31 ounce flywheel and a 53 tooth rear sprocket(stock 13 tooth front sprocket). The FMF PowerBomb header pipe and FMF Powercore 4 S/A Silencer muffler are new this year. We do mostly trail and fire road riding, no track racing, which is the reason for the heavy flywheel and +2 on the rear sprocket. FMF recommended: 50/50 mix of race fuel and super unleaded, main-170, pilot-42, needle- NHGT in 3rd position and 1 1/2 turns on fuel screw. They couldn't give me any specs for just pump gas and race fuel is hard to find near me(not to mention pretty expensive for trail riding) so I'm hesitant to use this as a starting point. I also read on TT to use a NCYQ needle-3rd position, 48 pilot, 50 leak and 2 turns out on the fuel screw. These are only 2 examples of approximately 6 different jetting specs I've received and they are all different from each other. The current specs are:
      Current Jetting- Main  160
                             Pilot    45
                            Leak    70
                            Needle  NCYR  #4
                            2 1/4 turns on fuel screw
      I'm hoping to tap into the years of knowledge possessed by TT members to come up with a good recommendation to get this bike running correctly. It has always run OK but I know it has a lot more in it. Please help me get it right and get my 15 year old off my a**. 
      P.S.- If anyone knows any NEW ways for me to get my 02 XR250R started easier feel free to let me know. I've already tried all that I've read on TT and it's still a bitch to get running most of the time.
    • By Snow Drift
      I have posted a few threads about the problem I am having getting my bike (2007 WR450) running right, and now that I have implemented more modifications and readjusted the settings I thought it would be best to summarise my problem in one thread with all information and work I have done as I still can't solve the problem I am having and I don't know what else to try at this stage. The Kran and others been very helpful on the threads and Guy Graham has provided extensive help in messages. All of which I am grateful for. I just thought though that I would put this out there one more time to see if there is anything I have missed or could try.
      The problem is when twisting the throttle off idle the bike sometimes instantly stalls with a small backfire that kicks back on the starter motor. It often but not always happens when I am blipping the throttle with the clutch in or when I am slipping the clutch. It is almost as if when you rev the throttle just as the revs drop and hit idle you rev it again, that is when the bike stalls and backfires before any suggestion of a bog. It is almost like the sudden drop in revs followed by the sudden rise in revs again is too much or adds too much fuel or too much air to the equation before the idle can balance itself.
      The problem becomes clearest when I want stop or do a tight turn or slip the clutch to try and move slowly. I pull in the clutch, but because the clutch drags slightly it puts a small amount of strain/drag on the engine which then stalls the bike unless I blip the throttle or give off idle revs. But blipping and off idle revs causes it to stall in this mysterious way. It makes it dangerous as it can stall at exactly the wrong moment, like at a road junction or difficult slow crossing/obstacle.
      The problem has been there since I bought the bike, after cleaning the carb and after the modifications I have done. The latest carb modifications have helped slightly but the problem is still there. This is the spec and work I have done to the bike. I should point out that the only thing I have just done but not run the bike since, is clearing the pilot and fuel screw passages with wire to confirm not restricted (the passage was not visibly blocked and this was done to make sure there was no blockage narrowing the passages).
      Airbox lid removed, baffle out of exhaust, shorter throttle stop, grey wire Carb thoroughly cleaned with both carb and brake cleaner and all passages and jets blown out New jets, 168 main, 45 pilot, 50 leak, 110 air, 70 air, 65 starter, JD red needle Adjustable fuel screw fitted GYTR AIS removal kit installed 2008+ AP diaphragm, rod, and cover (with original float bowl check valve removed) Stiffer AP spring fitted and timing on AP set with strong, early, good squirt visible Float height checked and within spec Throttle valve plate seal is present and appears fine The fuel screw seems to run best at half a turn out but is slightly vague and the change is very subtle. When warmed up, I tried opening the choke and the revs increased almost as if someone twisted the throttle. It didn't make the bike stall. If I pull the hot start instead of the choke, the bike does not stall but does run a little rougher. It can also be tricky to start, often wanting a slight bit of throttle twist.
      The previous owner had 170 main and 50 pilot jet and had not fully uncorked the bike. I don't know if he knew something I don't. However, I am completely out of ideas of what to try now and I am getting tired of taking the bike apart and trying to sort this. I have no confidence in the bike with it as it is because it can stall at the most dangerous wrong moments. 
      I made a couple of short videos which show the problem after all new carb parts. The first is with the idle screw set where I initially thought it should be. The second video is with the idle raised slightly by about a sixth of a turn. In the first video the stall is very much what I'm talking about, although I'm blipping the throttle the point at which it stalls does not seem that harsh when actually doing it. The second video, the bike only stalled because I purposely whacked the throttle open at the end to try and get it to bog going to wide open, the bike bogs though before it stalls like you would expect. I also made a video of the AP squirt.