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Discussion Starter · #1 · (Edited)
I have been doing a little planning in terms of adding ignition control to the Megasquirt installation on my 1971 B20E. As part of this, I have been trolling the forums to try and get a sense of what people had been using for ignition control maps on the B20E. While doing this, I came across some posts that suggested that the B20E would run better without the vacuum retard function connected. Just run on pure mechanical advance with a range of about 8 deg to 32 deg (the mechanical advance on the B20E provides about 25 deg of advance on the crankshaft).

I decided to give this a try just because I was curious. I blocked off the vacuum retard line to the distributor and reset the idle down to around 900 RPM and confirmed the advance operation with a timing light. A little test driving with this arrangement showed no perceptible change in performance or drivability. I didn't do enough testing to determine whether there was any change in fuel efficiency with the resulting higher advance at low throttle openings. However, when I went to do a restart on the car with this arrangement I discovered that it had become very difficult to get started. The engine turned over without any problem and there were no misfires; however, I was having a problem getting successful sustained operation. Getting a start generally required wide open throttle like I was doing a flood clear. I reconnected the vacuum line to the distributor and the starting difficulty immediately went away. I repeated all of this twice to confirm that the starting problem was related to the disconnection of the vacuum line. With the current fuel set up that I have on Megasquirt combined with my Bosch SR437X starter motor, the engine is normally a very quick start up hot or cold.

I find all of this a little puzzling. Has anybody else experimented with disconnection of the vacuum retard on the B20E and did you experience similar results?

I understand the theoretical benefits (emissions issues aside) of running more advance at low manifold pressures. However, it seems that based upon my experience, I may need a very low advance setting in the cranking portion of the ignition map before transitioning to more advance in the idle section of the map.
 

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I can't say I ever tried that combination, or if I did I've forgotten.

When you reconnected the vacuum line, did you also reset the timing? Or is the difference just between vacuum line connected or not with no other changes?

I've never measured it, but while cranking I'd expect relatively little vacuum, so little effect on timing. I almost makes be wonder if you have a vacuum leak in your distributor diaphragm.
 

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Discussion Starter · #3 · (Edited)
When I blocked off the vacuum line, I did not fiddle with the ignition timing. I have the timing set to give a maximum advance of around 32-33 deg which, with the 25 deg of mechanical advance would give somewhere around 8 - 10 deg of advance at idle with the vacuum line disconnected.

After some more thought, I think that the problem is related to the amount of fuel being injected during cranking. With the vacuum line disconnected, I had to screw the idle air screw in as far as I could to get the idle speed down to around 850 - 900 RPM. During cranking, Megasquirt injects fixed duration fuel pulses (adjusted for engine temperature) to start the engine. I had my pulse durations set up to give very good starting with the idle air screw in its previous position. After I screwed the idle air screw in, I expect that I am getting less air into the engine during cranking with the result that my cranking pulse widths are too long. I think that is why I had to put the engine into flood clear mode to get it started.

If you pulled the vacuum line with the stock D jet, I am wondering if you would set the same hot starting problem after you adjusted the idle speed down. If you did not adjust the idle speed down, the restart would probably be unaffected

Once I make some decisions around the details of implementing spark control, I will probably do a little test to see whether reducing the cranking pulse widths fixes the hot restart problem with the vacuum line blocked off.
 
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