Distributor Vacuum Advance

[Lord Croker]

Hi Guys,
Can anyone help me with some information, is there a difference in the vacuum advance curve on the Lucas 59D distributor, on a 998cc A-plus engine, depending on whether the engine is the normal compression with dished pistons or the high compression HLE with flat top pistons? The reason I ask, my engine is the lower compression version, (or was) & the vacuum advance doesn't become effective until about 2,200 rpm, tested by removing & replacing vacuum pipe with strobe timing light running. A Mini I work on regularly has the high compression HLE engine & the vacuum advance comes right in at an idle speed of 1,000 rpm, In my own mind it's the HC version that's not right, but I would appreciate any advice as I can't find any spec as regards vacuum advance curves.
Thanks.

[Spider]

I haven't looked to see if they are the same or different, however the vacuum pot on the carb for the vac advance doesn't always recieve full manifold vac. It is placed such that at idle, it is pretty well covered over by the butterfly and as the throttle opens, more and more of it does become exposed to manifold vac, so RPMs really don't come in to place, it's all about throttle position. The wrong spec carb can give results similar to what you are seeing as there are a few variations on the vac advance port.

There are different vac can too (which is your original question) that vary as to what In/mg are need to start movement, what In it finishes at and how much advance

[1071 S]

Vacuum varies with load/throttle opening.... So I would think that just increasing revs in the shed may not tell you much. The specs are usually stamped on the can so it may be instructive to compare the ones you have.

Cheers, Ian

[carbon]

From 1984 ARG service manual
99H929P, CR=8.3:1, 59D4 41857, Vac can 4-11-7
99H929P, CR=9.6:1, 59D4 41857, Vac can 4-11-7
99HA06P and 99HAO8P, CR=10.3:1, 59D4 41907, Vac can 3-9-8

[Lord Croker]

Many thanks to you all for some very informative replies, I do appreciate that the vacuum advance is all about throttle position, but I was puzzled about getting such dramatically different effects at idle speed. However, now I have plenty to go on, so thanks again.