Very erratic between 2200 and 2700 RPM

[ajg]

The car is quite smooth upto about 2200 RPM (in any gear), but then as the revs reach about 2200, the rev counter jumps upto about 2600-2700 RPM and the car become very erratic until it gets to about 2900 RPM when it goes smoothly again.
Some History: The car has always been a little bit ‘nervous’ at low throttle openings when holding a steady speed, but not too noticeable.
A couple of weeks ago the petrol hose sprang a leak, to get at one end of the hose I had to remove the plenum chamber cover (as well as heater hoses, bonnet catches etc), while moving the cover out of the way the petrol pump started by itself for a couple of seconds, this was with the ignition off (yes I know I should have disconnected the battery!). I finished the job and sealed the plenum chamber cover.
When I went for a test drive the problem of erratic behavior around 2200 RPM was evident. I have tried disconnecting the battery to reset the ECU and while doing this I found the tune resistor lying on the floor of the passenger footwell, someone had cut off the connector for some reason? I have since purchased a new tune resistor (same value 3900 ohms), I have also fitted a new distributor cap but the problem appears to be getting worse.
Please Help
Andy G

[MatthewYo]

Hi,

Have you cleaned the stepper motor? If this is dirty it can cause a whole load of running problems, could quite easily be as simple as that. I I cleaned mine recently and it's made such a difference

Thanks,

Matt.

[ajg]

Yes, I cleaned that a while ago and again when I took the plenum cover off, it was VERY dirty the first time but quite clean the second time.
I am wondering if the plenum cover seal is not perfect?
I am going to take it off again this weekend and try sealing it again, anybody any suggestions on which compound to use? or any other possible reasons for this problem.
I'm sure I've read somewhere on PH that the ECU goes from closed loop to open loop at around 2200 RPM?
BTW, the HT leads have also been changed about a month ago and the plugs changed last service (6 months ago).
Any responses gratefully received.
Thanks
Andy G

[GreenV8S]

Is the vac advance pipe connected to the right port on the throttle body? Assuming it's all correct, the rev counter behaviour makes me suspect an ignition fault, is it possible there's a poor contact somewhere? This might be sensitive to engine vibration and hence rpm.

[ATG]

sounds to a complete non-expert that the ECU could be playing silly buggers. As you say, that is about the point at which it goes open loop. What other sensors might you have disturbed? Throttle pot?

[ajg]

Vac advance looks like it's in the right place, the engine side of the throttle butterfly.
I did disconnect the throttle pot to move the plenum chamber cover, I'll check that it is securly connected later, anyone know what the resistance should be?
Andy G