British-Cars.org

Welcome to the DMR Site for British Car Information.

Recommendations

Parts

MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG MGB Technical - Distributor 25D4 40775B Specs

Does anyone have the advance curve and dwell angle for this distributor. I currently have this in my 72B. I am rebuilding the orig.I can not seem to get more than 45dgrs dwell without very poor performance. Vac. 10-11-24
Thanks, Greg G
g e granstrom

Greg. The following website has distributor information on it: http://www.starchak.ca/tech/pdfs/lucas.pdf .

It shows the 40775 dizzy as having the following advance curve:

2,800 rpm 17 deg
1,250 rpm 8 deg
800 rpm 1.5 deg

Be aware that these are distributor degrees, not crankshaft degrees. Both the crankshaft rpms and the crankshaft degrees of advance are twice those shown on a distributor test machine.

This distributor is not listed as to application and does not sound like a reasonable choice for an MGB as the engine would have to be turning 5,600 rpm before the full advance of 34 degrees came into play.

As to the vacuum advance figures you quote, 10-11-24, this does not seem to be even in the ball park. The figures, if quoted accurately, indicate that the vacuum advance begins at a vacuum of 10" of mercury (Hg), is fully on at 11" Hg and the total vacuum advance is 48 degrees at the crankshaft.

Small wonder this distributor does not seem to be working out well for you.

By the way, most dwell/tachs have only a 6 cylinder and an 8 cylinder scale. When using the 8 cylinder scale, the readings are doubled to indicate what is happening with a four cylinder engine. Does your dwell/tach have a four cylinder reading or are you using the eight cylinder scale? If using the eight cylinder scale, a reading of 45 deg, doubled, would make for a 90 degree actual reading and, yes, the four cylinder engine does not run very well with 90 degrees of dwell--i.e. the points never open.

What points gap are you showing?

Les
Les Bengtson

FWIW this is not an MGB distributor. Dwell is purely a function of points gap and nothing to do with the vacuum and centrifugal advance characteristics. AFAIK (for MGB distributors at least) all 25D4 should be 14 to 16 thou which gives a dwell of 60 degrees +- 3 degrees and is a function of the cam profile, nothing else. The above site doesn't quote that, nor the characteristics of the vacuum capsule. Bear in mind that whenever you change gap/dwell it changes the timing as well, so that must be corrected afterwards. Whilst the most 'aggressive' MGB distributor does have a 3-11-24 characteristic, it would have 3-11-12 on the vacuum capsule. As Les says 10-11-24 on the capsule would give 48 degrees of crankshaft advance, which is a huge amount. As '12' seems to be the maximum quoted on the above site I suspect you are giving crankshaft degrees and not distributor.
PaulH Solihull

Thanks for the input Les and Paul. I am embarrased to say that age and eyesight caused the dwell problem, ( I was reading the 6cy scale) as to the dist itself I have it know working fine. The vac numbers I quoted are correct and have a call in to the purveyor for more info.
Thanks, Greg
g e granstrom

This thread was discussed between 20/07/2011 and 21/07/2011

MG MGB Technical index

This thread is from the archives. Join this live forum now