Thanks as always Bob, for your input and sharing of experience and wisdom. Hopefully I will get closer to getting my MCSS running akin to a Swiss Watch. Maybe!
Anyhow, another fuel up and another update. I didn't do so well with the last tank of gas, though I suspect there's a number of reasons for that. Down to 11.7mpg, though not as bad as the lowest I got. Here's the latest Fuelly result:
A number of reasons for the drop in mpg compared with the last tank - firstly, I was doing a lot of tuning with the car idling in drive, so that likely used extra fuel. Also, I was still trying to get the mixture control solenoid to dwell around 30 degrees and it seemed to be dropping more down in the low to mid twenties at idle. As a result it spend a lot of the last tank running rich. Also, one time I forgot to plug the electric choke connector back in so on one short drive the idle revs were sky high and it was choked like crazy. Oops!
As also mentioned above, I was having a nightmare trying to get the TPS to give a stable reading at idle. Furthermore, I think I discovered another issue with regards to the TCC lock up system. I had noticed that it didn't seem to want to go into lock up as easily as it did, and when it did it would come in and out of it at random if I was below 55mph. At the same time this became apparent, I also noticed that when engaging cruise control the car would surge back and forth, many times and at random, even though the speed had been set. I thought that maybe the cruise control system had a fault in it somewhere, and started reading the Factory Service Manuals to get an idea of where to start looking. Looking at how the system worked, it takes most of its signals from the VSS to tell the vacuum solenoids to accelerate or decelerate. I thought that it was using the TPS data, but from what I can tell, it doesn't. Now, something else had been noticeable recently, and that was that the speedometer needle was getting a lot more jumpy and erratic. Of course, the VSS also takes its data from the speedo cable, so to confirm, I plugged in the Brainmaster and checked the VSS readings. When traveling at a constant speed, i.e. 35mph, the VSS reading could fluctuate by as much as 10 mph (it would spike more up than it would down). It would do the same at higher speeds too. Therefore, unless I'm mistaken, the erratic VSS signal is causing the Cruise Control to be unstable as it is trying to match an at times wildly fluctuating speed. It also means that at low TCC lock up speeds, the TCC controller is seeing a sudden change in speed and drops out of lock up. Thus my mpg will be suffering at speeds below 55mph because the TCC doesn't stay enganged constantly as it should. At least, that's my theory. If I'm correct, then I need to get my speedometer cable oiled so that it stops jumping all over the place, which will also stabilise the cruise control and TCC lock up.
Back to trying to set the Idle Air Bleed for the mixture control dwell. As I had mentioned earlier, I had been using the Brainmaster to set the MC Dwell, with common advice being to use an analogue dwell meter. A little bit of hunting on Ebay and I had another new toy to play with:
It's a surprisingly large meter, though luckily it's in very good condition and in a nice case. Today I tried it out to make sure it worked, and also to have another go at adjusting the MC Dwell. I connected the meter to the green MC Dwell connector on the carb, set it to 6 cylinder dwell scale, had the engine up to operating temperature and in closed loop while idling in drive at approx. 600rpm. I also had the Brainmaster connected simultaneously showing the MC Dwell reading digitally. As per previous attempts, turning the IAB screw didn't seem to get the reading much above 30 degrees,nor did it seem to cause the reading to change much. In a fit of frustration I turned the screw I think a whole turn, and lo and behold, now I was getting readings well above 30 degrees! I also had the analogue meter needle fluctuating between approx. 26 and 35 degrees. Video below shows the meter in action:https://youtu.be/ySbESl4iOQI
Is this the sort of behavior expected of the MC Dwell when being read by an analogue meter?
Interestingly, the Brainmaster scanner readings weren't that far off the analogue meter, and in fact matched it quite well. I guess that I am very used to observing digital devices so it wasn't that hard for me to work out the highs/ lows/ average of the digital read out. I certainly agree that the analogue dwell is a lot easier to comprehend needing just a glance at the needle rather than taking note of the numbers.
And here's a picture of the IAB screw showing approx. 7 threads above it. Hopefully not too far off?
I took the car out tonight and continued reading the MC dwell via the Brainmaster. It now seemed to be on the high side of 30 degrees, so I tweaked the IAB screw by abouyt 1/4 of a turn, and now it's reading around 30 - 36 degrees at idle in drive. I will tweak it a little more on each drive until the number more settles around 30 average, and I will double check it with the analogue meter once I feel it's getting close.
Hopefully I'm on the right track and will get my '86 running better than she has in years. Oh, and if anyone could confirm that an erratic VSS reading will cause unstable cruise control operation and also affect the TCC lockup that would be very helpful. I can then add oiling/ replacing the speeometer cable to my list.
Onwards and upwards hopefully!