Off Idle stumble on WUE

[RuneT]

Hi.
I am trying to tune my MS setup. It is a GM TBI (from a 4.3L) on a Ford 351W engine. I am running MSII 2.905

I have a bad stumble problem coming off of idle under load, but only while WUE is on. as soon as WUE is off, the engine runs beautifully.

Looking at a log of when it happens, the RPM on the engine jumps from 1800 to 3000 RPM in one step. First I though t i might be a signal problem, so I redid the grounding, but that did not help.
Since the stumble goes completely away as soon as I reach 160, I'm thinking it has to be something in my tune.

Can anybody help?

In the attached log, look around 80-90 second mark.

[Matt Cramer]

Looks like the real problem is with the RPM input; it's dropping out. What are you using for ignition?

[RuneT]

I'm using the distributor off of a '94 F-150 (hall effect, no vacuum/mechanical advance), connected the hall effect output directly to ms board, triggering on falling edge (rising edge out of dist), and driving the coil directly from MS. (rather than trying to use the stock duraspark II dist).

If the problem was the RPM input I would have thought I'd see the same problem at any time. I don't. As soon as WUE is off, the problem is gone.

[Matt Cramer]

Try jumpering the MS for VR input (note: you'll probably need to flip the Ignition Input Capture to Rising Edge) and adjust the trim pots. See if you can filter out the noise that way.

[RuneT]

I enabled Tach Signal Masking (1ms/50%) and it seems to have removed the noise. At least I didn't see it during some testing yesterday.

Would the VR input even work on the output from a hall-effect sensor?

I am still confused why I don't see the noise after engine is warmed up.

[Matt Cramer]

Some sensors behave differently when cold. The VR input can actually work with a pretty wide variety of sensors. But if you've got it working, you may just want to leave it alone.