MSD 6A tach signal

[flatbill]

I am running MSII, V3 board, 2.891. I have a Chrysler electronic distributor with the stock VR sensor and no distributor advance. I have the distributor output going to the relay board. The output of the relay board goes to the MSD 6A ignition box to run the ignition. I am using the tach output line from the MSD to control my tach. Sometimes, not always, the tach reads too high. It almost seems like its double what it should be. Sometimes restarting the motor will fix it, sometimes not. I would think that if the tach signal from the MSD doubles then wouldn't there be double spark output from the MSD?

I had been having a rough idle problem. I read that I needed to increase the dwell setting. I had it set to 3.2 ms. As I bumped it up to 3.5 ms the motor noticeably smoothed out some. I later bumped it to 3.7 ms. There is still a slight random idle miss. I have the max dwell in the ini file set to 25.5. I think that now that I have increased to 3.7 I should go back and tune the inde VE and spark timing. Any suggestions?

Thanks Billk

[Matt Cramer]

Set both dwell and max spark duration to 25.5. This may affect the timing, so you'll need to reset your base timing as well.

If the MS tach signal doesn't spike, the problem is likely with the MSD box or tach.

[flatbill]

Do you mean that I should set the "ignition set-up, dwell settings, maximum dwell duration (msec) to 25.5? I think that I tried that once and it really didn't work well at all. I did not, however, check the base timing. Maybe that is where I messed up. Thanks, I will try it. Billk

[Matt Cramer]

Yes, set both of these to 25.5. This will change your base timing, so you'll need to fix that once you've made this change.

[flatbill]

OK, I got the engine running and started working the ignition dwell up towards 25.5 and checking the distributor timing with a timing light. I was very surprised at how much the crank timing was changing and how much I had to move the distributor to get the idle timing right. There was a definite change in the motor smoothness though as I got up to about 7msec. I kept bumping the dwell up and changing the distributor timing until I got to about 20 msec. Then the motor started running really rough and died. It would not start at this point. I had to reset the dwell back down and reset the timing again to get the motor to fire.

Is it just a matter of increasing the dwell in small increments and adjusting the distributor timing until I get to 25.5? Maybe I was just moving it in too big of steps. Thanks, Billk

[Matt Cramer]

Is it just a matter of increasing the dwell in small increments and adjusting the distributor timing until I get to 25.5? Maybe I was just moving it in too big of steps. Thanks, Billk

Whatever approach gets you there... Just keep your eyes on the goal.

[bleoh]

If you are having to adjust the timing without changing where the pickup is in the distributor, could the issue be that the rotor is just getting to far out of phase?

[flatbill]

Just to clearify things, when I say I'm adjusting the distributor timing I mean that I am rotating the distributor until the markings on the front pulley match what is on the dashboard gauge in Tuner Studio.

[flatbill]

OK, more info. I worked with the car today and I worked the dwell up to 22 msec. I had reset the distributor position to keep the idle advance where I need it. I shut the motor off and it wouldn't restart. Wouldn't hit a lick! So I reset the dwell back down and got the motor started and reset the distributor timing.

Anyway, When I was trying to get the motor restarted I noticed that when I was cranking the motor I was not getting any rpm on the rpm gauge in Tuner Studio. When at last there was a flutter of rpm on the gauge the motor wanted to start, then did start. My dashboard tach seems to be acting the same way.

A question occurred to me. Does this indicate that I need to readjust my VR pots on the MS board? Am I, maybe, on the edge of operation of this circuit? If so, are there any suggestions on which way to adjust the pots? I don't have an easy access to an O'scope. When I first got the motor running I had to adjust them, but it was just stumbling along, then it started.

Another thing I noticed when I did a datalog was that the rpm trace looked really noisey. I'm using a twisted pair, non shielded wire from the distributor to the relay board. A single wire from the relay board to the msd box for a trigger.

Thanks, Billk

[flatbill]

I found a couple threads on the pot adjusting. I will try it tomorrow. Billk

[flatbill]

Latest info. I pulled the cap on the distributor, set the crank to about 10 degrees BTDC, rotated the distributor until the rotor and the cap contact were in pretty good alignment and clamped the distributor down. Then I set up the ignition dwell up to 12 and reset the advance with the timing wizard. I ran it for a while then upped the dwell to 19 and again used the wizard to set the idle advance. Its running pretty well and I will be bumping it up to the 25.5 as suggested for the MSD box. I have had trouble before, i suppose, by trying to reset the distributor to correct the timing at idle.

I'm slow, but determined. Thanks for all the help. Billk