Erratic RPM Signal

[NMRAcer]

The idle is extremely intermittent due to the RPM dropping off to zero periodically. I assume this is due to a drop in ignition, but it may have something to do with the signal itself. I have no clue. Here is a visual:

rpm.jpg

I am running the v3 board with VR ignition input. I have gone over the troubleshooting guide multiple times and have verified my wiring numerous times as well. Where should I begin to narrow my search for culprits?

[barf]

try going through the VR adjustment procedure http://www.megamanual.com/ms2/vradjust.htm

what are you triggering from exactly?

Using a scope may help determine if the signal on the MS2 board is suitable. VR signals can degrade when shielding or connections are poor, or if fitting of VR sensor itself is slightly out. good luck.

[NMRAcer]

I tried adjusting the pots, beginning with R52, in a wide array of positions but it doesn't seem to fix anything. This is rather frustrating because I had this problem previously but can't recall how I fixed it. In addition, I've installed a stereo noise filter in the switched 12V line going to MS with no obvious effects.

I am using a Mallory distributor which utilizes an optical trigger connected to an MSD 6AL box. The MS board is configured for the high current ignition driver using the VB921 chip. (I should note that someone previously suggested I switch to the Hall/Optical/Points trigger method on the MS board but that only ended with no RPM signal for some reason.)

If you look carefully at the attached picture, the RPM quickly escalates just prior to being cut out. Could this be some sort of malfunctioning rev limiter? If I could figure out why my motor is trying to idle so high and rich, would this fix anything?

I've never used a scope but I think I could probably find one if need be.

[NMRAcer]

I just had a thought which may or may not be related. The MAP value begins at 99.5 atmospheric and then drops as the engine fires and moves inverse to RPM. My boost gauge is reading 12 psi give or take, confirming the boost condition yet the MAP value seems to be running in reverse. Is this correct or do I have my theory confused?

[Matt Cramer]

I just had a thought which may or may not be related. The MAP value begins at 99.5 atmospheric and then drops as the engine fires and moves inverse to RPM. My boost gauge is reading 12 psi give or take, confirming the boost condition yet the MAP value seems to be running in reverse. Is this correct or do I have my theory confused?

That is how MAP sensor readings work; they drop under vacuum and climb under boost.

[NMRAcer]

That is how MAP sensor readings work; they drop under vacuum and climb under boost.

That's what I was getting at. My blower is pushing more than 10psi into the intake which should make the MAP reading increase, not decrease. Right?

[Matt Cramer]

That's right. MAP should increase with boost.