Classic Mustangs List Archive

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Mail From: p2kandm2 (p2kandm2

Well, I found 95% of the stumbling miss fire that was happening around and above 3K RPM.

I had an open manifold vacuum port, a 1/4 inch port no less.
It seems that the TBI has a manifold vacuum port (extreemly visually obscured) next to the return fuel outlet and this has been uncovered for quite some time!

My objective today was to hook up vacuum gauges on both types of vacuum sources, ventrui and manifold, so I could check the advance of the distributor at differing speeds and RPMs, while comparing the two vacuum levels at the same time.
...Low and behold I found a vacuum leak!

I put my gauge on the TBI manifold vac-port and plugged the one it previously used, wich is actually on the manifold.
- WOW, it tuned easier, has crisper throttle response off idle (in neutral)!
After the re-tune I drove a few blocks.
The missing is reduced tremendisly.
Acceleration feels (subjectively) a little softer from a stop.
...time will tell on driveability improvements.

Now to isolate the rest of the miss fire. It is probably base/centrifugal/vac timing, or plugs, or distributor cap/rotor.
...this *will be* a weekend project.

Heh, maybe my city MPG will improve, EH?!
11/22 is quite a spread for MPG.

Thanks for listening
KandM

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Mail From: Lance (Lance Robaldo)

You might want to check for an exhaust leak for the cause of what's left of that
backfire. That's what was causing it on mine.

I had a bad donut between the exhaust manifold and the intermediate pipe that
ended up being the source of my backfire.

As the superheated gasses passed the leak, they sucked in fresh air and caused
the fresh oxygen to ignite.

This was especially happening on deceleration.


Lance.