1. Some members were not receiving emails sent from XJbikes.com. For example: "Forgot your password?" function to reset your password would not send email to some members. I believe this has been resolved now. Please use "Contact Us" form (see page footer link) if you still have email issues. SnoSheriff

    Hello Guest. You have limited privileges and you can't "SEARCH" the forums. Please "Log In" or "Sign Up" for additional functionality. Click HERE to proceed.

Popping on deceleration with pods = pilot jet +1 size?

Discussion in 'XJ Technical Chat' started by Conraderb, Mar 18, 2019.

  1. Conraderb

    Conraderb New Member

    Messages:
    11
    Likes Received:
    4
    Trophy Points:
    3
    Location:
    Philly
    Hi friends,

    First, thanks for this incredible forum. It has been infinitely helpful!

    I have a question about whether or not I have a lean idle circuit, and if I should +1 my pilot jet size.

    First, my bike: 1981 XJ750 Seca, with pods. The pilot jets are the stock #40 size. The previous owner did the conversion to pods, but did NOT change the pilot jets.

    The problem: I am having a noticeable popping sound when I close the throttle at moderate to high revs to slow down. It pops all the way down from 8k to under 4k consistently, and is not subtle. I have had other carbed motorcycles in my life that occasionally did this, but this is much louder.

    I have noticed that adding a touch of THROTTLE helps to stop this popping.

    Also, adding about 1/3-1/2 CHOKE will also help.

    My diagnosis: I believe that my pilot circuit is too LEAN (caused by the change from regular airbox over to pods).

    My proposed SOLUTION: Replace the current #40 pilot jet with a #41 jet, and see what happens.

    Am I on the right track? Combined with the fact that the PO did not rejet the pilot after switching to pods, the pilot jet seems to be the prime suspect. Any thoughts? Should I even go for #42 instead of #41?

    Related info: Yes, I just did a full carb cleaning, and valve adjustment to spec, and carb sync using the BB method.

    Related info that seems to confirm my suspicious: the bike is still a bit reluctant to start on cool-but-not-cold days (<50 degrees), even with full choke. However, giving it a fair amount of throttle seems to help (making me think that bypassing the lean idle circuit entirely and getting into the richer midrange is helping start up).

    This also leads me to think that the idle circuit is too lean, and replacing the current #40 pilot jet with a #41 would be a good idea.

    Any input would be welcome.

    Thanks! Conrad
    West Philadelphia PA
     
    Last edited: Mar 18, 2019
  2. sybe

    sybe Active Member

    Messages:
    576
    Likes Received:
    138
    Trophy Points:
    43
    Location:
    NYC - Brooklyn
    you are running rich. the popping is gas igniting in your exhaust. did you colortune the carbs after you put the jets in?
     
  3. Colin 85 700

    Colin 85 700 Active Member

    Messages:
    487
    Likes Received:
    131
    Trophy Points:
    43
    Location:
    Thunder Bay, ON
    Lean can cause this as well, a misfire too lean can cause fuel to build in the exhaust as well because its simply not being burnt in the cylinder. With pods you are more likley lean than rich.
    Try one size bigger, if it gets better your lean, worse your rich... That simple. (You likley need 2 sizes however) 5% extra
     
  4. Chitwood

    Chitwood Well-Known Member

    Messages:
    866
    Likes Received:
    375
    Trophy Points:
    63
    Location:
    Fort Atkinson, Wisconsin
    While I agree with sybe that unburned fuel in the exhaust can also make a pop, if your jets are stock with pods theres a good chance you are lean. You will likely have to go up a size or two on the main jet as well as 2+ sizes on the pilot depending how the bike acts. It is a trial and error type of thing. What works on one bike may not on another etc. In the fuel section of xj4ever there are some helpful hints on where to start with your jetting for pods based on stock jetting. I'd have a look there. The colortune plug helps a great deal getting you in the ballpark but reading the plugs will also be necessary
     
    Jetfixer likes this.
  5. Colin 85 700

    Colin 85 700 Active Member

    Messages:
    487
    Likes Received:
    131
    Trophy Points:
    43
    Location:
    Thunder Bay, ON
    My bike has pods n a headder 4:1
    If im correct stock is 40/107
    Took carbs apart today, it has 42.5/118 and runs fine.
    Thats roughly 6% bottom and 10% top.
    With factory exhaust i would expect closer to 5% on both, as exhaust affects top end more.
     
  6. XJ550H

    XJ550H Well-Known Member Premium Member

    Messages:
    13,199
    Likes Received:
    3,861
    Trophy Points:
    113
    Location:
    Great North Woods
    lean is typicaly why you get the popping on decelaration. pilot jets tend to run lean even when bike is fully stock
    when using a little choke to stop the pop that's telling you its lean.

    start by checking your wet set fuel levels if low you will run lean
    Setting the fuel levels

    rejetting is covered in the information over load your main jet is where to start increasing size just because you release the throttle does not mean the main jet closes
    The Information Overload Hour

    EXAMPLE:

    A 1982 XJ750RJ Seca using an aftermarket Supertrapp 4-into-1 exhaust and a single K&N air filter in the stock, unmodified airbox. Bike is primarily operated at an altitude of 2600 feet above sea level.
    XJ750 Seca Stock Hitachi HSC32 Carb Jetting:
    #120 Main Fuel Jet
    #40 Pilot Fuel Jet
    #50 Main Air Jet
    #225 Pilot Air Jet
    Y-13 Needle


    MAIN FUEL JET SIZE CHANGES NEEDED PER TYPICAL MODIFICATION:


    Typical Exhaust Changes:
    +2 main fuel jet size for custom 4-into-2 exhaust
    or
    +4 main fuel jet sizes for 4-into-1 exhaust
    or
    +4 main jet sizes for no muffler (open headers)

    Typical Intake Changes:
    +2 main fuel jet sizes for single K&N filter (inside a stock airbox)
    or
    +2 main fuel jet size for drilling holes in the airbox with stock filter

    or
    +4 main fuel jet sizes for individual pod filters (no airbox)

    Additional changes:
    - Add up all the main fuel jet size increases and subtract 2 sizes.
    - Decrease main fuel jet size by 2 sizes per every 2000' above sea level.
    - Under a mis-match condition, such as when using pod filters with a 100% stock exhaust, or 4-into-1 header with stock filter and air box, then subtract 2 main fuel jet sizes.
    PILOT FUEL JET SIZE CALCULATIONS:
    The formula is: +1 pilot jet size increase for every +3 main jet sizes increased.
    Stock pilot fuel jet size is: #40
    + 2 additional jet sizes (since we went up +6 main fuel jet sizes before the altitude compensation was factored in):
    = a #42 pilot fuel jet size.
    Note that no altitude compensation is needed on the pilot fuel jet since our elevation is less than 6000' a-s-l.

    ------------------------------
    RESULT:

    A #124 Main and #42 Pilot is A GOOD STARTING POINT.

    there is more info to read and I suggest you start by reading the section on rejetting


    http://xjbikes.wikidot.com/carb-specs

    read this too
    Good information on Pods vs Airbox :Thanks sybe
     
    Last edited: Mar 18, 2019
    Toomanybikes likes this.
  7. k-moe

    k-moe Pie, Bacon, Bourbon. Moderator Premium Member

    Messages:
    19,647
    Likes Received:
    6,755
    Trophy Points:
    113
    Location:
    The City of Seven Hills
    She is lean, and not just lean at idle.
    Easy fix is to get an airbox and install it.
    Otherwise start with Colin's suggestion about jet sizes.
     
    Colin 85 700 and Jetfixer like this.
  8. Jetfixer

    Jetfixer Well-Known Member

    Messages:
    3,191
    Likes Received:
    1,501
    Trophy Points:
    113
    Location:
    Nothern Indiana
    Stock on 750 main 120 and pilot 40 ,go up to either 122 or 124 main and 41 pilot, this is down side to running PODS you have to experiment with jetting , I would suggest buying a size up , JETS R US has various sizes of jets . Personally I would not drill out jets , and would resist the " stage 1,2,3" or what ever they want to call them unless you intend to race the bike. I had a friend try this stage 3 in his Suzuki it ran awful at slow speed would foul plugs etc , would scream at WOT , he was running pods , after about drowning his engine in a rain storm, he swapped out for stock jets and airbox no more fouled plugs and no more drowned engine .
     
  9. XJ550H

    XJ550H Well-Known Member Premium Member

    Messages:
    13,199
    Likes Received:
    3,861
    Trophy Points:
    113
    Location:
    Great North Woods
    buy step up sizes all at once for the main jets. save you time if you have to get the next size up.
    try the jets tune the bike ride if popping goes away closer to done. check the plug color.

    at this time you should do a few plug chops to see if the bike is running lean or rich at higher rpms.
     
  10. Conraderb

    Conraderb New Member

    Messages:
    11
    Likes Received:
    4
    Trophy Points:
    3
    Location:
    Philly
    Thanks to everyone for your reply! I hadn't thought about the idea that I was running RICH, but I'll spend some time with the carbs and see if I can tune in the issue.

    Thanks again.
     
  11. k-moe

    k-moe Pie, Bacon, Bourbon. Moderator Premium Member

    Messages:
    19,647
    Likes Received:
    6,755
    Trophy Points:
    113
    Location:
    The City of Seven Hills
    You are NOT running rich. The person who posted that assertion is incorrect. When the stock air box is removed the intake volume is increased, leaning the fuel to air mixture.
     
    Jetfixer likes this.
  12. Conraderb

    Conraderb New Member

    Messages:
    11
    Likes Received:
    4
    Trophy Points:
    3
    Location:
    Philly
    Thanks K-moe. I should have been clearer - I am still convinced that I'm running lean, based on the airbox. I was just surprised to hear someone suggest the opposite :)
     
  13. Ryengoth

    Ryengoth Active Member

    Messages:
    720
    Likes Received:
    194
    Trophy Points:
    43
    Location:
    Wilmington, NC
    Anyone ever use a wideband on a port to see stoich ratio when tuning a specific cylinder?
     
  14. Colin 85 700

    Colin 85 700 Active Member

    Messages:
    487
    Likes Received:
    131
    Trophy Points:
    43
    Location:
    Thunder Bay, ON
    And, just to note... If your running lean (you are) due to increased airflow, then you need to add fuel, if you can add fuel without running too rich, you develop more horsepower!
    Horsepower! Oh ho ho! yeah horsepower! :)
     
  15. Colin 85 700

    Colin 85 700 Active Member

    Messages:
    487
    Likes Received:
    131
    Trophy Points:
    43
    Location:
    Thunder Bay, ON
    My bike with headder and pods runs appriximatly 10-11% richer in the top end, theoretically adding 10% horsepower in the top (reality isnt as perfect as math but...)
    My bike stock is 86 HP@ 9200 rpm theoretically adding 8.6+ hp... Grand total 95 HP!
    For a roughly 450lb bike that is an impressive improvement with a ratio of roughly 5lb per horse.
    A gutted out 3000 lb muscle car drag racer needs 600 Hp to compare in performance.
    A stock 3400 lb car need over 700HP! WTF? That sounds expensive!
    This is why bikes kick ass!
     
  16. XJ550H

    XJ550H Well-Known Member Premium Member

    Messages:
    13,199
    Likes Received:
    3,861
    Trophy Points:
    113
    Location:
    Great North Woods
    whats your top end on the bike?
     
  17. Colin 85 700

    Colin 85 700 Active Member

    Messages:
    487
    Likes Received:
    131
    Trophy Points:
    43
    Location:
    Thunder Bay, ON
    Fastes ive had it 167kph, 105 mph, but i shifted too early and it fell a bit flat in 5th
     
  18. k-moe

    k-moe Pie, Bacon, Bourbon. Moderator Premium Member

    Messages:
    19,647
    Likes Received:
    6,755
    Trophy Points:
    113
    Location:
    The City of Seven Hills
    A stock setup is able to reach ~120 MPH with the engine still pullng quite-well up to (and past) redline. An early shift would cause it to fall a bit flat, but not enough to keep you from hitting 120 . Sounds like the carbs are set up well, but the choice of intake setup may be limiting power production slightly at high speed (not an issue for daily use though).

    For reference the 120 top speed I quoted is from a somewhat reliable source, but I think it's a bit low. I've had my 750 to a true 125 at redline, and it makes a few (around 6) less HP than the 700. All other conditions being the same, the 700 should do 125 MPH just as easily as the 750.
     
    Last edited: Apr 6, 2019
    Chitwood likes this.
  19. tabaka45

    tabaka45 Well-Known Member

    Messages:
    2,397
    Likes Received:
    513
    Trophy Points:
    113
    Location:
    Brunswick, Georgia
    Well, that makes me feel good about not staining my stock XJ700 when cruising at a nice 65 mph!
     
  20. Colin 85 700

    Colin 85 700 Active Member

    Messages:
    487
    Likes Received:
    131
    Trophy Points:
    43
    Location:
    Thunder Bay, ON
    Good to know K, however i let off too, it was getting pretty gnarly, especially cause i was wearing a motocross helmet with goggles at the time, had to keep my head down cause of the beak on my lid. It was causing some rough head wobble at 100 mph!
    Maybe ill try for a top speed run some day, (not that thats why i bought the bike, i prefer the 0-60! ) ill wear my full face! Lol!
     
    k-moe likes this.
  21. Colin 85 700

    Colin 85 700 Active Member

    Messages:
    487
    Likes Received:
    131
    Trophy Points:
    43
    Location:
    Thunder Bay, ON
    BTW tabaka, i allways cruise 65Mph/105k:)
     

Share This Page