Fired up the 1979 coupe with HE engine today, and plugged my laptop into the Motec EFI.
The Motec uses an external MAP sensor ( manifold air pressure ) and at 750 RPM idle with engine around 50deg C it registers 40 to 50% atmospheric which equates to 15 to 18 inches of vacuum.
Nothing special about this engine.
Idling at 750 RPM the injector pulse width should be about 1 to 1.5mS. I have ignition set to 6.5deg BTDC. The ignition map for the engine was based on a table I got from a forum member 20 years ago, and is in the ball park but may not track the usual Lucas dizzy characteristic exactly.
I once had a problem very similar to Greg’s. I rebuilt the 2nd hand HE engine and put a 5 speed box behind it. At the time the coupe had the original pre HE engine with the Motec EFI system. I changed over the engine, and had to change a few sensors and modify a bit of wiring. I rejigged the Motec fuel and ignition maps based on information from a member and then came the day to start it up.
I did not have the air cleaners on to give more access while getting it all sorted. First mistake was forgetting about those air cleaner bolt holes that let air into the manifold. No trouble getting the engine going first time with a bang at 3000RPM idle. One minor heart attack later plugged the holes. At one stage in the procedure of getting everything sorted I had badly fouled plugs, obviously a very rich mixture. Much time was spent figuring out if I had any Motec variables set incorrectly, any sensors malfunctioning, any wiring errors. The 12 plugs were in and out 3 or 4 times to clean and refit.
I could make no sense of the problem. Then I just started from square one as if the engine had been given to me and nothing could be assumed to be correct. Within minutes I found the plug leads were out by one, I had put the lead for cylinder 1A into the next dizzy post at 60deg delay, and all the others followed in sequence. No idea why that happened.
The odd thing is the Motec does all the ignition advance adjustment, meaning the rotor is locked to the shaft and there is no mechanical or vacuum adjustment. On a V12 dizzy with posts spaced at only 30deg ( and a consequent narrow rotor ) it is critical to get the rotor alignment exactly right so as the Motec adjusts advance you do not have the rotor missing the post at any stage.
The conventional dizzy adjusts the rotor with the timing so self compensates.
I had the old Crane Cams LED pickup which makes it easy to do static timing and a fair bit of time was spent getting the rotor adjusted to be exactly positioned when the LED signal triggered. I must have spent a lot of time on this before firing up the engine and never noticed the plug leads were wrong.
In the 35 years of owning an XJ-S I must have refitted plug leads at least half a dozen times.
Back to Greg’s situation. As mentioned twice before pulling the CTS on my stock convertible makes no difference at idle, although have not tried that while revving the engine. Looks like the CTS could be a blind alley. Don’t forget the idle RPM is really dependent on how much air is getting into the engine, and the A/F ratio must go beyond a critical limit before RPM will be affected.
The suggestion of a $40 adjustable regulator is reasonable, hoping it is easy to plumb into existing fuel rail and fittings. Small money compared with what has been spent so far. If the A/F can be fixed to give a good mixture and steady idle then ignition and timing can be tackled.
On this forum it has been quite rare for anybody to report a definitely dud ECU in comparison to so many finding other things giving grief when engines not behaving.