[Gmecm] 7747 stumbling acceleration when cold

Fri Dec 3 16:49:21 UTC 2010

Could be injector response time. I had problems running an old Holly
Pro-Jection TB with  a '7747. Those were big injectors and took a lot of
tuning in almost EVERY fuel related table. You said you're running a set of
Cop Car injectors? There may be other tables that need tweaked. Anything vs.
temp. Even spark advance if your engine has any more mods than just the
intake. Cam? Headers?

From: gmecm-bounces at diy-efi.org [mailto:gmecm-bounces at diy-efi.org] On Behalf
Of Charles McDowell
Sent: Thursday, December 02, 2010 2:00 PM
To: Scott Peitzsch
Cc: gmecm at diy-efi.org
Subject: Re: [Gmecm] 7747 stumbling acceleration when cold

Well, it kind of made sense to me that other stuff might key off of the BPW
besides just the base fuel map.  Thanks for the help.

On Thu, Dec 2, 2010 at 9:18 AM, Scott Peitzsch <jlg-sep at comcast.net> wrote:

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 <mailto:charles at taildragger.info>  

To: Scott Peitzsch <mailto:jlg-sep at comcast.net>  

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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