Vacuum retard

Fiddling with my engine, which doesn't run right.

Scenario is 3.9L replacement mill (original engine gone gone) with Crower 50230 (mild boost) and HIF6 carbies. I am using the original distributor on this rig. It is a 41277B. Even though timing at idle is set to 6 deg BTDC (as per manual) the idle is lousy and performance is sluggish.

While looking more closely, I note that the distributor is not the same as pictured in the shop manual. The vacuum can is on the other side. I also tested the vacuum can and found that while it worked, it retarded the spark. Like Ensign Bluebottle I thinks for a bit and then Aha! of course silly, the vacuum can is pushing, not pulling, since it's on the other side. I also have another 41277B from a slightly earlier engine that I have just come into, same story. I looked the number up, has 13 deg distributor advance and either a 2 4 4 or 2 4 5 vacuum can, leading to 8-10 degs of RETARD with even moderate vacuum. The somewhat hotter cam needs more advance at idle to run right, and this thing is doing just the opposite.

These distributors are for US cars (the recently acquired engine is 430, my car is 433 series). I can only presume that the retard was to strangle the engine enough to meet emission requirements.

I am guessing that I should trash both of these distributors and get something that will give me advance instead. Or I suppose I could move the can to the other side, I have a drill... Suggestions?

gbvona

Hi, Vacuum advance or retard units are available, the difference depends which side of the diaphragm the vacuum connection is.

Colin

I am sure you a right about the retard to meet emissions, it may be worth seeing if you can pick up a UK spec vacuum advance mechanism

In this case it is not that the diaphragm is moving in a different direction, rather that the unit is mounted on the other side of the distributor. When I apply vacuum, the rod moves into the housing, like most of the cans I have seen. But the can is screwed onto the distributor in such a location that this motion causes retard, not advance. Another standard vacuum unit would do the same thing. I guess my first try is to drill some holes and mount the vacuum unit on the other side of the distributor, thereby reversing is current action. The 2 5 5 rating seems low both in terms of activation pressure as well as total motion. I am going to need some fairly strong advance at idle, given the lumpy cam. Any suggestions as to what a UK spec (non-strangled) unit's numbers are?
Thanks!
gbvona

I could supply my dizzy's numbers but as its a 3500 and not a 3.9L one would assume you need the numbers from a range rover where they used 3.9l lumps! if nobody comes up with data needed, maybe a check with range rover clubs?

Hi, The vacuum units have little or no effect above tickover when the advance curve is decided by the weights and springs under the base plate. Have you tried disconnecting the vac unit from base plate to see what effect it has?

Colin