328 microplex timing "test"

There was a discussion a while back re the advance curves in the microplex computer and how they are selected. I suggested that the curve for the car seemed less aggressive than it should be. Rifledriver pointed out that these engines don't need/want the kind of advance that I am used to with US engines but he also noted that the US curve is retarded around 4 degrees from the "base" curve. He said that (if he remembered correctly) disconnecting the computer's pin 17 ground would cause it to default to the base curve. I checked the wiring diagram and confirmed that his memory was, in fact, correct.

So yesterday I rigged up a wire/toggle switch so I could make/break that connection from the driving position. I figured that would be the easiest way to tell if there is any difference. If the car is accelerating in 3rd gear and you flip the switch, it should be easy to feel any change in the car's acceleration rate as the timing curve changes.

Anyway - I'll be trying it later today to see if the 4 degree change actually makes any "feel-able" difference. I also have the "Dynolicious" program on my Iphone and will see if that registers anything different.

 

OK, Done!

There is a slight difference in the engine sound when I flipped the switch, most noticeable around 4kRPM and I THOUGHT I felt a barely noticeable difference in acceleration rate. Frankly, I'm not absolutely sure I felt anything at all. But I could hear the change in the engine. So whether it's worth the bother...I don't know. But it's a simple thing to do if anyone is interested.

The Microplex is under the left side of the "trunk" behind the engine. There are 4 bolts holding it down. If you remove it you will see a bolt at the rear that secures a ring terminal with two wires to a ground point. One wire (the thicker one) is solid black, the other is Purple/Black. The purple/black ground wire is the one that goes to terminal 17. Both wires are on the same ring terminal. If you are confident you want to do this and leave it you could just clip the purple wire and tape the ends.

The downside is that when you make the the change, cold start idle starts out around 2k and it never falls below 1600 or thereabouts, even when fully warm. I'm sure the idle can be adjusted to spec RPM but I didn't have time to fool with it any more today - I have some other commitments but I wanted to post what I found.

I'll probably leave it as is (purple disconnected) and adjust the idle back to normal speed. If the idle can't be adjusted to 1000RPM for some reason, then I'll reconnect the purple wire.

 

Yeah - at idle just connecting/disconnecting the purple/black wire jumps/drops the idle by nearly 1k RPM. That's a considerable timing change.

Re that: I fooled with it a bit a few minutes ago and I couldn't get the idle below 1500 because the linkage on the throttle body hit the manifold before the idle could drop any lower. Looks wrong to me - almost as if there should be a spacer between the manifold and throttle body that isn't there. I'll check out what's on the car as opposed to what the parts book shows. The throttle body has some markings on it in magic marker that in my mind indicates it might be a replacement and maybe something was left out...I don't know.



Here's a pic showing that the linkage is hitting the manifold, preventing the idle from dropping any lower.

 

Thanks Brad, I'll take a look at the manual and make the adjustment. In the middle of the night I woke up remembering something about the idle adjustment and the big nut you are referring to as opposed to the adjustment screw on the throttle body.

 

OK, I reset the throttle plate per Kjet specs and I set the idle correctly (big screw) and it idles fine now at 1k RPM.

I tried something that I hadn't thought about yesterday when I was fooling with the toggle switch/purple/blk wire. Noting the considerable increase in idle speed (advance) at idle with the wire disconnected, a few minutes ago I re-ran the same test I did yesterday but instead of doing it cruising speed, I did it off idle. There seems to be an increase in initial acceleration when idling along in 1st gear and then adding throttle if the wire is disconnected. This makes sense since there is more advance. Of course, I had to reset the idle each time I connected/disconnected the wire or the test would have been meaningless. Having to do that means there was a couple of minutes between tests so the ability to instantly switch the wire in/out was gone.

But I did have a chance to employ some pseudo-science... There was a patch of water here in the parking lot and I found that if I ran over the patch at idle with the wire connected and floored the pedal, the RPM came up a bit/car moved forward before the tires started spinning. With the wire disconnected, the car started spinning tires almost immediately. Makes sense with the increased advance at that RPM but whether it would translate into any measurable difference in normal acceleration - which is not normally done while idling along - I don't know.

 

The very scientific setting (!) per the Bosch Fuel Injection and Engine Management book (Charles O. Probst) is to unscrew the adjustment screw until it is clear of the linkage, then insert a thin piece of paper between the screw and linkage, then turn the screw until it just contacts the paper. Remove the paper and turn an additional 1/2 turn. That's it! I was expecting some very precise instructions...

No, I don't know what constitutes a "thin" piece of paper. Seems like a specific feeler gauge would be a bit more precise but apparently it doesn't matter enough to worry about.

 

I forgot to answer the second part - per the book, the setting of the throttle plate is simply to ensure it does not "bottom" against the wall of the throttle body and cause wear. The TPS always know where the plate is regardless of where it is set.

 

I certainly don't know anything about Bosch K-jet, my 328 is the first car I've ever owned with it. But I think the Bosch Fuel Injection book by Pabst is thorough and easily understood. That being said, there may be folks here with a lot of experience with Kjet who might feel that the book is not as good as it could/should be. But for a neophyte K-jetter like myself, it seems pretty good.

 

Just thought I'd add...the idle, when turned down to 1000RPM (eng fully warm) is decidedly less smooth compared to the same idle speed with the 'plex using the standard US timing. OTOH, the rougher 1000 RPM idle with the base timing setting sounds kind of cool!

I'm wondering if the idle speed setting for the euro version was different than the US? Also, even with the idle set to 1k fully warm, on startup it will immediately fast idle at 1600 RPM or so.