I have an SU carbed 240. My Pertronix electronic ignition appears to have died from a voltage increase caused by a faulty voltage regulator. So have gone back to points. But I'm having problems adjusting the RPMs downward.

I've adjusted the SU jet to get the best rpms. And its throttle plate closes almost completely and the cold start screw is clear of the cam.

Adjusted the points to 58 degree dwell (60 is recommended). But can't adjust timing above 10 BTDC else the engine rpms go above 975 at idle. I'd like to increase the timing to around 20 BTDC and drop the rpms to around 900.

What am I missing?
--
1980 245 Canadian B21A with SU carb but electronic ignition and M46 trans in Brampton, Ont.

Your SU should have a decel valve in the throttle plate. It's possible that the spring on it is getting weak and not holding the valve closed. You can remove the carb and solder it shut, although it's tough to get the solder to wet out without removing it completely, which is what I do routinely when rebuilding those carbs.
Write me from www.sucarburetors.com if you want further info on how to do it.
Rhys

Two things come to mind- a vacuum leak, and using the incorrect screw to adjust idle. The proper one is the one on the intake manifold just beside the kill solenoid.

Rob

I'm not using the solenoid. (I think I get better mileage without it.) It is there but disconnected, so the passage should be blocked and rpms would be at a minimum.
--
1980 245 Canadian B21A with SU carb but electronic ignition and M46 trans in Brampton, Ont.

Perhaps there is a vacuum leak somewhere.
Start by plugging off all the vacuum hoses and try it from there.
It is also possible (but I'd say unlikely) that there could be an intake gasket leak.

Are you sure the throttle plate is closing fully?
What rpm are you trying to achieve 20 degrees BTDC?

You are trying to adjust the timing a little, adjust the idle a little, adjust the timing, adjust the idle etc. right?

I'll look for a vacuum leak.

"Are you sure the throttle plate is closing fully?"
The throttle plate has to be left slightly open according to the manufacturer. Besides full closure (throttle stop screw not touching cam) causes the plate to slam closed and stick.

"What rpm are you trying to achieve 20 degrees BTDC?"
I want idle rpms to be around 900 to 950 at 20 degrees BTDC.

"You are trying to adjust the timing a little, adjust the idle a little, adjust the timing, adjust the idle etc. right?"
Increasing timing speeds up the idle. The problem is I'm not getting a low enough idle to allow me to increase timing without ending up with a very fast idle.
--
1980 245 Canadian B21A with SU carb but electronic ignition and M46 trans in Brampton, Ont.

Keep looking for that vacuum leak.

Is the throttle plate centered properly? Has the carb been apart recently?
20 degrees BTDC at idle seems like a lot. Is that a factory setting on a B21?
B18 & B20s are about 22 degrees at 1500 rpm.

I'm at about factory suggested right now, (12 +/-2 BTDC, 900 +/- 50 rpms) but those rpms won't go lower. Haven't looked for vacuum leaks yet, been busy (seem to be in a streak of luck where things aren't going as planned, time wise).
--
1980 245 Canadian B21A with SU carb but electronic ignition and M46 trans in Brampton, Ont.

A quick way to help isolate a vacuum leak--take a pump/spritzer bottle filled with water and spray a mist it around any suspect area with the motor idling. -- Dave