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MG MGB Technical - dizzy vacuum unit

The Vacuum Unit on my '74B is shot and I'm looking for a new one...It's a 25Dizzy stamped 41491...which I believe is the end of the 25's run...and only Moss has it for 83 bucks...(gulp!) and I'm looking to save a few bucks...Anyone have a opinion if I would revert to earlier 41339 unit?...which is available for alot less?...Thx all...
Pete

Pete,

what is the engine no. in your car and is anything stock in this engine?

Ralph
Ralph

Pete - check out the following website http://www.teglerizer.com/mgstuff/advance_curves.htm it has a listing of various advance curves and vacuum module specs. The 2 vacuum capsules you are talking about have very different specs as you will see.

Erick
Erick Vesterback

Ralph...Its totally original...672 engine(I think)...except air pump jettisoned...

Erick...I looked and I concur...Thx
Pete

Pete,

You may want also to check with Jeff Schlemmer. Tell him "Allen sencha".
Jeff Schlemmer <jschlemmer1@mn.rr.com>

' Hope this helps.
Allen
Allen Bachelder

Pete,

here are the specs for the vac cpsule of the 25D4 41491:

Manifold vac. type, start at 10Hg Max. at 15 Hg, total vac advance 10 deg.

You will find 10/15/10 stamped on the body of the capsule. If ordering a new one, it should have the same charateristic!

If it is difficult to spot one of these, you can use a 45D4 41599 with very close figures to those your 25D dizzy.

Ralph
Ralph

I better get a 41491 from Moss...Thx to all for the helpful comments...these darn cars are addicting...
pete

The numbers stamped on the vacuum capsule relate to distributor advance, for crank advance you double it, i.e. if stamped 10/15/10 it would give 20 degrees at the crank. However the 41491 capsule gives 10 degrees at the *crank*, so it would be stamped 10/15/5.

The 41339 is quite a bit different to the 41491 as it was for the high-compression engine. However the original curves are all rather irrelevant now on today's fuels, unless you are going to spend a lot of time and money on a rolling road determining the ideal curve for your engine *now*, then tweaking bits to achieve it, you'll not get much better than setting it for the closest to the original dynamic advance you can get without pinking at any combination of throttle, load and revs (that's if you can get an LC engine to pink anyway), and that will be the case for any distributor. And *any* MGB distributor is going to better than a busted original :o)
Paul Hunt 2

The 41339 unit is no longer avaialble from Moss - sort of. It orginally had a 7-13-5 setting and the ones from Moss now have something like a 5-17-10? Not even close!!! The original 41339 units are GREAT for performance use, and quite rare, although I have 100 coming my way in the near future - custom ordered. Pete, good idea to buy a new one while they're still available!
Jeff Schlemmer

This thread was discussed between 07/03/2007 and 17/03/2007

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