[Diy_efi] Spike? on tach line.

[herald lamperts]

Hello Marc,

im not shure about your ecm, but at higher RPM you
should see an advance in ignition timing. I would
start checking wiring, all installed speed related
sensors and especially grounding.

Sincerely,

Robert

--- Mark Bacon <mfb9 at comcast.net> wrote:
> Hi,
> 
> I have been calling everyone I know, and searching
> everywhere to find some
> assistance with my problem. Maybe you can help me, I
> hope.
> 
> I have a '94 Ski Centurion boat with a 5.7liter
> chevy 350. I recently
> converted the carb system with a TBI system ( TB,
> sensors, dist and coil &
> #...747 ECM) from a late '80's Chevy Truck. The
> harness and chip were
> supplied by Affordable Fuel Injection. New Vortec
> Heads, Cam, Timing Chain &
> Sprockets, Manifold, Fuel pump, Wires, Plugs, etc.
> The install went
> smoothly, and fired right up, but I have encountered
> a really goofy problem.
> 
> There is a spike on the tach, seen both digitally
> and analog. ie idle at 700
> rpm suddenly jumps to 2400 rpm intermittenly, then
> back to 700 rpm. This
> spike occurs every couple of seconds, regardless of
> the engine speed. The
> resulting problem is that the ECM retards the timing
> when it sees the spike,
> and under load results in a backfire. While watching
> the timing mark at any
> constant speed, I can see the timing mark jumping
> around.
> 
> If I disconnect the timing bypass connector on the
> distributor, the timing
> and rpm's become steady. When I replug the bypass
> and restart the engine, it
> returns. I am not sure if what I am seeing is really
> a spike on the tach
> line, induction of some sort, or if it may be in the
> ECM. And how do I get
> rid of it? Could you please offer some assistance?
> 
> Thanks in advance,
> 
> Mark Bacon
> 
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