Confused ATDC and BTDC

[thejinx]

My pulley has two notches TDC and BTDC. Playing with van today and reading how to keep your VW alive it states a 1972 should be timed to 5 ATDC. If I do this it sits just above stall. If I move the dizzy to the 7.5 BTDC (second notch) it sits ok. Any ideas on what is right? Should I open the small fuel screw slightly more to compensate?

Cheers

 

[rlepecha]

My understanding is that 5deg ATDC was used when the dizzys had a vacuum system which pulled it back to BTDC at idle. This was to meet some Californian emissions requirements at the time. Most dizzys on buses in the UK now should be timed to 7.5 degrees BTDC, or 30-32 degrees at maximum mechanical/centrifugal advance.

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[thejinx]

Cheers thought I was going mad for a bit

 

[Tofufi]

As above. I believe the 5 degree ATDC setting was for vehicles with a DVDA (dual (2x) vacuum, dual (vac and mech) advance) distributor - you can identify these as they have two vac hoses going to the vacuum can.

 

[thejinx]

Cheers chaps. My air cooled knowledge is at the point where o sort of know what I am doing but could still get into trouble unless I check in here now and again. :msn4:

 

[matt-me]

I would double check your dizzy code too, here. Maybe the original has been changed??

http://www.oldvolkshome.com/ignition.htm