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  • 4bbl TBI high idle

    I have a home made billet 4bbl tbi I'm testing. It has 1.75" bores and an integrated spacer with 8 injectors spraying beneath the throttle blades. It's on a 350 with phase II Bowtie heads, a mild cam, and Performer intake. I'm having trouble getting it to idle below 1000 rpm. I've tried 6 and 14 degree throttle blades with similar results. The blades are sealed up way tighter than the 600 cfm 4160 I'm replacing. The primaries on the Holley are slightly open. This thing is closed tight. If I plug any two of the bores it drops to 600rpm. If I plug three, it dies. I can get it to idle nicely at 700 rpm by leaning it way out. (19:1+) I can still let out the clutch and it tries to climb over my wheel chocks. Both the carb and injector are pulling down to around 40 kPa at 700, but the carb afr is in the low 12's. I can lean it out until it barely idles and shoot carb cleaner at it. I get a good rise at the throttles of course and a slight leak at one of the throttle shaft bores. I brought it inside bolted to another Performer and pulled a vacuum with the central vac. Listening with a hose, the only audible leak is small and at that throttle shaft bore. I completely drained it out but of course I've already been asked why the "whole house smells like gas." I don't smell anything. I'm going to o-ring the throttle shafts tomorrow. What do you guys think?

    Thanks,
    Kevin

  • #2

    Re: 4bbl TBI high idle

    Pictures can say a thousand words. Have any pictures? The Carb afr of low 12's sounds way to rich, for sure.
    Escaped on a technicality.

    Comment


    • #3

      Re: 4bbl TBI high idle

      The part is kinda rough. I'm still learning how to program and operate my machine so there's some work to be done to the code. It also was dimpled up in my tumbler by a star washer I didn't know was in there. I figured I should make sure it works before I perfect the looks. I know the 42 lb injectors are overkill. I borrowed them from my turbocharged 302 Ranger. Pics are here link:http://www.bangshift.com/forum/album.php?albumid=22

      Thanks,
      Kevin

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      • #4

        Re: 4bbl TBI high idle

        Hmm, that's a neat set up. The only thing that comes to mind is a vacuum leak somewhere, which you've thought of, since there is no IAC option and looks pretty well made. Any chance one of the lower o-rings on the injectors might not be sealing up?
        Escaped on a technicality.

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        • #5

          Re: 4bbl TBI high idle

          A CNC in your shop - SWEEET! nice work!

          I agree with Randal - got to be a vacuum leak somewhere.

          Being a machinist I'm sure you've already checked this - but just in case - is your base plate flat across the sealing surface?
          There's always something new to learn.

          Comment


          • #6

            Re: 4bbl TBI high idle

            Kevin,

            Your project is intriguing. DG can attest, I have built my own TBI setup before as well. I have long thought that there should be a way to make them work well. I don't think anyone has gone about it right. My thought process has always been VERY similar to your description.
            "A cross thread is better than a lock washer." Earl Lanning...My Grandpa

            Comment


            • #7

              Re: 4bbl TBI high idle

              something else

              Very nice looking TBI, not rough at all with sweet ball end milling!
              I dont think Id o-ring the shafts at this point. That can lead to a lot of other problems in the long run (no Nitrous is 1). We run a couple more blades than you do and idle down fine 98% of the time... Hmm... questions : What are you using to run the injectors? How is the injection timed to the motor? Not sure if thats the issue but its interesting!
              www.FBthrottlebodies.com
              Bruce K Bridges

              Comment


              • #8

                Re: 4bbl TBI high idle

                Thanks for the input. I wouldn't dare call myself a machinist. Other than what I've done at home, I've maybe spent an hour of my entire life in a real machine shop. I bought this machine last year. If you want to know who your friends are, buy a 10,000 lb machine in Feb, that you have to disassemble to fit through the garage door and have no way to get off the trailer or move other than rolling it on pipe. The trailer belonged to a friend of a friend, the F600 belonged to a friend, but the big guy in my pics is Neal. He's the only one that actually braved the cold, snow and possible crushing death.

                The injector o-rings are just barely in the holes but aren't leaking air. The base is as flat as I can measure. I'm running MS-II, but for fuel only, using an old HEI. I've tried 2 and 4 squirts alternating. I won't groove anything but I bought O-rings on the way home. I'll try sliding them over the shafts and seal them against the body. I'll keep looking for leaks. I'm testing this on my 59 Chevy pickup. Even lowered it's tall. Without taking two currently non running projects out, my garage door won't open high enough to get it in and it's raining. So I probably won't be able to do any testing till Thurs.

                Thanks,
                Kevin

                Comment


                • #9

                  Re: 4bbl TBI high idle

                  big mill, little shop

                  Most of my CNC machines rolled in on 10mm 17-4 throttle shaft remnants! When we moved the the Mazak in it busted the corner of the slab, so we moved it out, covered it up (the Mazak) and dug a new pad for a good 10" slab with lots of steel. Put the Mazak on the slab and built a new "annex" around it! Big machine moving is a real art, or not...
                  You could put some modelling clay around the shafts to try to get it to idle down if you really suspect thats the problem. If thats it, and they are now journal style bearings, then you might be able to enlarge the bores for sealed rollers, but I like the sealed ball bearings the best (but they require more realestate). A small platten under the injector nozzles will help atomization in the manifold (like the EZ throttlebody design) which also may throw a bit of fuel around the shafts (not good for sealed ball bearings though).Tthe pic is master machinist Mike at "the pod" (Milltronics VM16)
                  Attached Files
                  www.FBthrottlebodies.com
                  Bruce K Bridges

                  Comment


                  • #10

                    Re: 4bbl TBI high idle

                    I had considered placing my machine at the end of my driveway and building around it also, but we figured out how to barely get it in and I was out of cash. I haven't had problems with my concrete, but he manual calls for 2' of concrete with the machine anchored 1' deep. I have some INA sce610 pp bearings I was planning on using if the brass on aluminum didn't work. I have room for them. I hadn't thought about clay. I have some slightly greasy Play-Doh in the garage. I'll give that a try.

                    The Interceptor is sweet. I was looking at one on Ebay recently. My only import is a 76 XJ12 with a turbo small block.

                    Thanks
                    Kevin
                    Last edited by ka67_72; May 3rd, 2011, 05:36 PM.

                    Comment


                    • #11

                      Re: 4bbl TBI high idle

                      I think all of my trouble is in my throttle blades. Using my makeshift central vac evacuated manifold again I get the following results. Everything was dry so no complaints today.

                      Absolute manifold vac 79.25 kPa
                      Holley 750 4160 86.8 kPa at idle position
                      (it was clean, dry and I know it works)
                      TBI 88.2 kPa fully closed
                      84.0 kPa with two barrels plugged
                      80.0 kPa with 4 barrels plugged

                      I cut my throttle plates on my series 1 Bridgeport with a fly cutter. The head was tilted at 6 and 14 degrees. The current blades are .002" narrower at the shaft than the bore. Should I try to make it tighter? Can I make them big and somehow lap them in place?

                      Thanks,
                      Kevin

                      Comment


                      • #12

                        Re: 4bbl TBI high idle

                        no idea on the vacuum leak issue (or whatever it is) but out of curiosity, why did you put the injectors up by the throttle plates and not lower in the runners?
                        www.realtuners.com - catch the RealTuners Radio Podcast on Youtube, Facebook, iTunes, and anywhere else podcasts are distributed!

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                        • #13

                          Re: 4bbl TBI high idle

                          I was just trying to make something simple to bolt on mild motors that wouldn't require a port injected manifold. For port injection I won't have the spacer or injectors.

                          Thanks,
                          Kevin

                          Comment


                          • #14

                            Re: 4bbl TBI high idle

                            I was just trying to make something simple to bolt on mild motors that wouldn't require a port injected manifold. For port injection I won't have the spacer or injectors.

                            Thanks,
                            Kevin

                            Let us know how it progresses! I worked on this with the guys at AED - they are competent carb builders - and we just had the damndest time trying to run this on anything over 500hp. But that doesn't mean that we were doing it right, so don't let that discourage!!
                            www.realtuners.com - catch the RealTuners Radio Podcast on Youtube, Facebook, iTunes, and anywhere else podcasts are distributed!

                            Comment


                            • #15

                              Re: 4bbl TBI high idle

                              in picture 7 it looks like the plates do not seat in the bores the same looking at the spaces where they close
                              2007 SBN/A Drag Week Winner & First only SBN/A Car in the 9's Till 2012
                              First to run in the .90s .80s and .70's in SBN/A
                              2012 SSBN/A Drag Week Winner First in the 9.60's/ 9.67 @ 139 1.42 60'
                              2013 SSBN/A Drag Week, Lets quit sand bagging, and let it rip!

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