Welcome to Our Community

Wanting to join the rest of our members? Feel free to sign up today.

Sign Up

Thunderjet Tuning

Discussion in 'Fuel / Air' started by alhall88, May 24, 2009.

  1. alhall88

    alhall88 UAW

    Joined:
    Apr 9, 2007
    Messages:
    2,121
    Likes Received:
    63
    Trophy Points:
    48
    Location:
    Petersburg, Michigan
    OK so I have been reading up on this mod and for those of you that have them please chime in and see if I'm way off base on my thinking or info interpretation. First thing I have been reading is once you install a thunderjet the next step is to throw away the S&S tuning instructions because you have a totally different animal now. My first question is the external air bleed, am I correct in saying this is only required on the older carbs due to the new ones already having a internal air bleed thats adjustable. The reason its confusing to me is because the air bleed sizes are totally different between an internal and external air bleed. I'm also seeing where they say that the idle mixture screw may only end up 1/2 turn out or so and hardly any accelerator pump adjustment will be needed. The common idea also seems from my current jets is to go up on the intermediate,down in main and up in air bleed. Also last question maybe hdlarry,LDO could help, just for comparison reference what size intermediate,main,air bleed, and thunderjet are you guys running,and is your air bleed internal or external.

    OK here is a quick edit I'm reading now that maybe the thunderjet kit comes with a screw plug that you block off the internal air bleed and then you drill the carb body for an external air bleed? Sound correct?
     
    #1 alhall88, May 24, 2009
    Last edited: May 24, 2009
    chubs likes this.
  2. Raywood

    Raywood The Pirate
    Staff Member Supporting Member Calendar Participant

    Joined:
    Apr 5, 2007
    Messages:
    17,376
    Likes Received:
    668
    Trophy Points:
    113
    Location:
    Rainbow, CA
    My Bike(s):
    2014 KMW Regulator
    First Name:
    Ray
    Al, when I had mine done I sent it to Zippers for the install. The plugged the internal air bleed and drilled/taped the external one in.

    And as you already stated they recommend you up the intermediate jet, drop the main by two sizes and go from there. I had a hard time tuning mine and then had Speeds Performance Plus do it on there dyno while in Las Vegas.
    Sorry but don't remember what jets I had in it. This was on my 03 Mastiff.

    :cheers: :cheers:
     
    chubs likes this.
  3. alhall88

    alhall88 UAW

    Joined:
    Apr 9, 2007
    Messages:
    2,121
    Likes Received:
    63
    Trophy Points:
    48
    Location:
    Petersburg, Michigan
    Thanks Ray thats what I thought, I have the prints on the 3 mods that zippers do and thats what I want to do. The 3 mods are drill & tap for the thunder jet, drill & tap for the external air bleed, and finally add a cross drill hole at a 45 deg. angle for the bowl vent (this is supposed to work better than simply removing the vent plug method).
     
    chubs likes this.
  4. Gas Man

    Gas Man Cool isn't cheap
    Supporting Member Calendar Participant

    Joined:
    Apr 8, 2007
    Messages:
    15,045
    Likes Received:
    274
    Trophy Points:
    83
    Location:
    Metro Detoilet
    chubs likes this.
  5. alhall88

    alhall88 UAW

    Joined:
    Apr 9, 2007
    Messages:
    2,121
    Likes Received:
    63
    Trophy Points:
    48
    Location:
    Petersburg, Michigan
    Yep thats what it is. I'll find out today if there is any benefit of an external over an internal air bleed. On the older carbs that don't have one I understand drilling the carb out for an external one but I can't seem to grasp the advantage of plugging off the internal air bleed which is on the newer carbs just to redrill again for an external one.
     
    chubs likes this.
  6. MARV

    MARV Well-Known Member

    Joined:
    Aug 14, 2008
    Messages:
    1,937
    Likes Received:
    58
    Trophy Points:
    48
    Location:
    CENTRAL JERSEY
    My Bike(s):
    05 CHOP
    First Name:
    SUL
    i believe its basically easy access for racing applications.

    quicker changes.
     
    chubs likes this.
  7. alhall88

    alhall88 UAW

    Joined:
    Apr 9, 2007
    Messages:
    2,121
    Likes Received:
    63
    Trophy Points:
    48
    Location:
    Petersburg, Michigan
    That was my thinking as well until I started reading, Everywhere i read it says you should always change the air bleed when you change the main jet once you have it tuned. The only fine tuning should be the thunderjet itself which is why you use a screw on cap for that.
     
    chubs likes this.
  8. john sachs

    john sachs Active Member

    Joined:
    Nov 6, 2008
    Messages:
    823
    Likes Received:
    821
    Trophy Points:
    93
    Location:
    Deltona, Fl
    My Bike(s):
    harleys
    The advantage is being able to change the jet without taking off the bowl.Nothing more...Nothing less.It's a tuning "thang".:up:
    John
     
  9. alhall88

    alhall88 UAW

    Joined:
    Apr 9, 2007
    Messages:
    2,121
    Likes Received:
    63
    Trophy Points:
    48
    Location:
    Petersburg, Michigan
    Well I was able to use my buddys bridgeport and install my own thunderjet. I wanted to do it myself so that since my carb is chrome I could be careful and minimize any chipping and I had good luck, used nice sharp tools and it turned out fine. All these instructions are available from Zippers.
    The first thing was to block off the old internal air bleed. There are 2 methods for this and the instructions only will tell you to tap the old bleed hole out and use a screw to plug it. But since the hole is already threaded I used the fill it method with plumbing solder. Its kinda hard to see in the pic.
    [​IMG]

    Then close it up and drill tap for the actual thunderjet.
    [​IMG]
    [​IMG]

    Then drill the bowl for the supply line.
    [​IMG]

    Tomorrow we will start the tuning using my LM-1 air fuel meter.

    Now for the new external air bleed and the vent modification. On the left you see the new air bleed after I drill and tapped the body. The using a 5/16 endmill on a 45 deg. angle mill a hole into the air vent channel on the right.
    [​IMG]

    And finally they recommend plugging the front vent hole and removing the bottom plug.
    [​IMG]
    [​IMG]
     
    chubs likes this.
  10. Chopper Dave

    Chopper Dave SIICK!!!

    Joined:
    Apr 13, 2007
    Messages:
    8,445
    Likes Received:
    83
    Trophy Points:
    48
    Location:
    Michigan
    My Bike(s):
    2011 Ultra Limited
    First Name:
    Dave
    wow Al! good work!:up:
     
    chubs likes this.
  11. LDO

    LDO The Cleaner
    Staff Member Supporting Member

    Joined:
    Apr 8, 2007
    Messages:
    2,756
    Likes Received:
    32
    Trophy Points:
    48
    Location:
    P-Cola FL.
    My Bike(s):
    03 Mastiff
    Al,
    sincerest apologies for not replying sooner to your questions. Been doing some work to the house and haven't been on here much.

    Originally, I had a 70 main, 31 intermediate, 41 air. The carb was stock bore (44).

    After John reworked the carb, he bored it out to 47, installed a 74 main, kept the 31 intermediate, 60 external air jet, and a 100 in the T-jet.

    For my dyno run, the only thing that I changed out was the main to a 76. Everything else was good to go.

    After the dyno, the only thing I did was install some washers in my pipes and that eliminated all my decel popping and made the bike feel like it pulled even harder.

    I have been toying with the idea of dropping the int. jet to a 30 'just to see' but haven't gotten around to it just yet.

    As far as adjustments, I used the S&S tuning guide to get my int and main circuits 'close' and I was right on the money with my adjustments.

    Hope this helps and sorry for the late reply.:cheers:

    EDIT: Dang Al, you and I must have been posting at the same time! Nice work!
     
  12. alhall88

    alhall88 UAW

    Joined:
    Apr 9, 2007
    Messages:
    2,121
    Likes Received:
    63
    Trophy Points:
    48
    Location:
    Petersburg, Michigan
    No problem LDO. One thing I did want to mention was that although some may not agree, when I was at the dyno shop by me that runs drag bikes he put my carb on the flow bench and showed me how much drag that XXX booster caused. He didn't understand why you bore a carb out and then put that big booster in there. I realize it may not effect much but I may remove it and try it.
     
    chubs likes this.
  13. LDO

    LDO The Cleaner
    Staff Member Supporting Member

    Joined:
    Apr 8, 2007
    Messages:
    2,756
    Likes Received:
    32
    Trophy Points:
    48
    Location:
    P-Cola FL.
    My Bike(s):
    03 Mastiff
    John removed mine. He made basically the same exact statement as you and I must agree!!
     
    chubs likes this.
  14. lee

    lee Well-Known Member

    Joined:
    Nov 5, 2007
    Messages:
    4,818
    Likes Received:
    67
    Trophy Points:
    48
    Location:
    Bristol, England
    My Bike(s):
    2005 Big Dog custom build and Buell custom build
    great work Al. As for the drag on the booster I read a while ago in AIM the extra turbulence caused by that thing could hinder the performance of the carb rather than help it.
     
    chubs likes this.
  15. alhall88

    alhall88 UAW

    Joined:
    Apr 9, 2007
    Messages:
    2,121
    Likes Received:
    63
    Trophy Points:
    48
    Location:
    Petersburg, Michigan
    Well this thing is going to take some work. The part that I was having a hard time grasping is that the bike was running great before the thunderjet so why mess with anything other that the thunderjet after all its the high range jet that I'm adding. So I leave my original jets 32 int./76 main/but a external 61 air bleed. Tune the mixture in and go for a ride, well it stumbles terrible at 2500 rpm's but once I get to about 3200 it takes off great. Feels like its running out of fuel so I bump up the int to 33 and no improvement. Little puzzled so like I said before the thunderjet instructions say if your bike was running good at the high range before you will most likely find that you will go down 2 sizes in the main jet. That would mean I need to go to a 74 main which really doesn't make sense. I don't have any mains that small so it may have to wait till Monday. I may try to go up to a 36 int. jet and see if that does anything, because it also says to go up 2 sizes in the int jet.
     
    chubs likes this.
  16. alhall88

    alhall88 UAW

    Joined:
    Apr 9, 2007
    Messages:
    2,121
    Likes Received:
    63
    Trophy Points:
    48
    Location:
    Petersburg, Michigan
    Here's a snippet from another respectable builder:
    Not Ranked
    Throw your s&s instructions away if you wanna tune it like zippers designed it. the instruction are good, only they make NO SENSE if you are still thinking s&s instructions. the two wont jive, cause your carb isnt s&s any more. its zippers.

    go up 2-3 sizes on the intermediate jet, go down 1-2 on the main jet. i have run jets as large as 36 or 38 on 96 inch motors with the idle mix being down around 1/8 or a turn from zero. but this is dangerous unless you wanna keep the thing adjusted...the lower this adjustment becomes(because of massive int jets)..the more sensitive it gets. being out of adjustment a hair can be extremely lean or rich. not so bad if you keep it near thier 1/2 turn mark.(by only going up two sizes or so)

    on the side of your carb, near the top facing rear....the adjustable air bleed is located. most likely a 150-170 here. the larger the jet, the later the main jet will kick in.

    the thunderjet will work off a siphon type effect, caused by the small hole on the aluminum collar where the t-jet threads into your venturi...and the vacuum draw thru the venturi.

    most likely a 120 in the t-jet itself. the t-jet and airbleed are mikuni jets.

    the whole thing only draws fuel at really high rpm. it allows you to use a smaller main jet, covering a narrower range, and a larger intermediate jet covering a larger range.

    ps, like it says, the acc pump is now basically never used. maybe 1/2 turn from off.

    the highlighted parts are what i find to be the most useful. you just gotta approach it differently, to use zippers method. most folks dont. they stay with the s&s method...and dont see too much of a gain. but its safe
     
    chubs likes this.
  17. lee

    lee Well-Known Member

    Joined:
    Nov 5, 2007
    Messages:
    4,818
    Likes Received:
    67
    Trophy Points:
    48
    Location:
    Bristol, England
    My Bike(s):
    2005 Big Dog custom build and Buell custom build
    it seems as though main jet become almost obsolete, which in a funny way actually sounds quite logical. It certainly sounds as though he knows what he's talking about. Best of luck with it Al.
     
  18. stlmikie

    stlmikie I wish I had more money.

    Joined:
    Sep 5, 2007
    Messages:
    4,275
    Likes Received:
    71
    Trophy Points:
    48
    Location:
    St. Peters
    My Bike(s):
    03 Mastif
    First Name:
    Mike
    You'ss get it nailed down. Al is the man.
     
  19. alhall88

    alhall88 UAW

    Joined:
    Apr 9, 2007
    Messages:
    2,121
    Likes Received:
    63
    Trophy Points:
    48
    Location:
    Petersburg, Michigan
    Well got it running pretty good again but went against all the "experts" . I tried the increase slow jet and decrease main jet theory and it got worse. Only thing the larger slow jet did was introduce a bunch of popping. So I went back to my original 32 slow jet and start increasing the main, well sitting on a 80 main now and she is running great. I think at this point all I should maybe need is to play a little with the air bleed which is for the fine tuning of the main jet.
     
  20. Raywood

    Raywood The Pirate
    Staff Member Supporting Member Calendar Participant

    Joined:
    Apr 5, 2007
    Messages:
    17,376
    Likes Received:
    668
    Trophy Points:
    113
    Location:
    Rainbow, CA
    My Bike(s):
    2014 KMW Regulator
    First Name:
    Ray
    Hey Al, seems you just set it back to what is was before the thunderjet. I know I had a hell of a time setting mine up properly but you really need to set it up for all three jets to work in unison as it shows in the graph at the bottom right of this image. Again, they are a pain to get set up properly but once you do it will work its magic. But with that big 80 main jet in there you might fine yourself very rich when the thunderjet comes on at about 4000 rpm hence the reason for dropping it in size.

    [​IMG]
     

Share This Page