[Gmecm] 7747 stumbling acceleration when cold

Thu Dec 2 16:18:20 UTC 2010

Just FYI - I didn't ever mean to infer that changing the BPW would help.
Just thought you might like to know what the correct values should be.

-Scott

  ----- Original Message ----- 
  From: Charles McDowell 
  To: Scott Peitzsch 
  Cc: gmecm at diy-efi.org 
  Sent: Thursday, December 02, 2010 10:51 AM
  Subject: Re: [Gmecm] 7747 stumbling acceleration when cold

  Lower BPW did not help.  I decided to install the closed air cleaner with heat riser though and that seems to have helped a lot.  It has warmed up some here, so I'll try it for a while longer before I give my final verdict.  

  The factory air cleaner is an ugly beast on a dressed up engine.  I hate the two part design with the spacer.  My engine didn't originally have this exact setup, so I really have no idea how they routed the hose from the spacer without it interfering with the throttle cable. I got part way down the street and realized I could only open the throttle part way.  I have a hose on the spacer trying to get over to the driver's side valve cover.  I know this isn't EFI related exactly, but can anyone explain how the factory routing goes for the engine breather?

  On Tue, Nov 30, 2010 at 8:22 PM, Charles McDowell <charles at taildragger.info> wrote:

    Well, I loaded up a new bin with BPW=122.  The engine isn't cold so it's not a real big test, but it seems to run fine.  The BLMs were higher, but not super high on my short test drive.  The range was like 123 to 140.  I'm going to let the engine cool off and see how it works in the morning...

    On Tue, Nov 30, 2010 at 6:00 PM, Charles McDowell <charles at taildragger.info> wrote:

      Interesting.  Somehow I got myself to 132.  The standard ASDU is 135, so I'm a little confused,  I'm sure I originally scaled it like 135*(55/65), but maybe I found the BLMs to be running away or something....  I think I'll try again and double check.

      On Tue, Nov 30, 2010 at 3:14 PM, Scott Peitzsch <jlg-sep at comcast.net> wrote:

        The 9C1 Caprice TBI injectors (aka Cop injectors) are nominally
        rated to flow 68#/hr @ (I believe) 12PSI.  The correct BPWs for
        them are:

        BPW on a 305 = 107
        BPW on a 350 = 122

        ----- Original Message -----
        From: "Aaron" <aaron at creativeoverdrive.com>
        To: <gmecm at diy-efi.org>

        Sent: Tuesday, November 30, 2010 3:11 PM
        Subject: Re: [Gmecm] 7747 stumbling acceleration when cold

        I've not seen "cop" injectors, but a quick search shows them to be 65 lbs/h.
        But to calculate a correct BPW you MUST know what they flow.
        And the rated flow is based on a steady fuel pressure, so you'll need that
        as well.

        It's been so long since I've looked at the hack for the 7747, I can't
        remember what you should check next.
        I've always started with a bin that was close.
        Maybe Allen will pipe up soon. He's better at this than me.

        From: cmcdowell12 at gmail.com [mailto:cmcdowell12 at gmail.com] On Behalf Of
        Charles McDowell
        Sent: Tuesday, November 30, 2010 12:02 PM
        To: Aaron
        Cc: gmecm at diy-efi.org
        Subject: Re: [Gmecm] 7747 stumbling acceleration when cold

        I can check in the log file tonight for the items you asked about. I was
        having major grief when I first installed this because I was given 305
        injectors by mistake. I examined every tiny detail of the install so I'm
        pretty sure I don't have any air leaks. Also bear in mind that it all works
        fine when warm. I know things expand when they warm up, but it sure can't
        be anything major. The thing idles and cruises fine when cold too. It
        really seems like an acceleration thing.

        Does anyone know for sure what things need to be changed when using a larger
        injector size? All I changed was the BPW to account for "cop car"
        injectors. Are there other things too?

        Thanks.
        On Tue, Nov 30, 2010 at 9:19 AM, Aaron <aaron at creativeoverdrive.com> wrote:
        We have discussed the adapter plates before.
        They are notorious for creating a vacuum leak were the TB and IAC gasket go
        over one of the bolts.
        The IAC sometimes even hangs over the adapter plate, so there is no seal to
        the gasket.

        What is the IAC position at idle open loop and closed?
        Map voltage?
        TPS voltage?
        Fuel Pressure on the supply side?
        Fuel Pressure on the return side?
        Have you looked at the fuel spray pattern from the injectors?
        I like using a dial back timing light to strobe the spray to see it
        more clearly.

        From: gmecm-bounces at diy-efi.org [mailto:gmecm-bounces at diy-efi.org] On Behalf
        Of Charles McDowell
        Sent: Monday, November 29, 2010 3:09 PM
        To: Eck, Joel
        Cc: gmecm at diy-efi.org
        Subject: Re: [Gmecm] 7747 stumbling acceleration when cold
        I'll double check my logs, but I think my temp is good. Bear in mind that
        it's not like this thing ran well once and stopped. It never worked well
        since I installed it. I have an Edelbrock performer quadrajet carb manifold
        with an adapter for the TBI. Thanks.

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