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MG MGB Technical - Dizzy advance on modded engine

I'm trying to get the timing sorted on my '74 GT and have trawled the archives to learn that maximum advance should be about 35 crank degrees? I'm recording about 45 degrees maximum at about 4500rpm and 12 degrees at 800rpm. Is this too much? Also, I'm using standard unleaded. Would higher octane fuel be a wiser investment?

Unfortunately the engine is too noisy for me ever to hear any pinking so I would appreciate a scientific approach or a good guess. The distributor is modified (don't have the specs)and non-vacuum as the carb is a 45DCOE, the block is 1995cc, the compression ratio 9.5:1, and the cam has a duration of 284 (37-67) degreees inlet and 294 (72-42)degrees exhaust.

Many thanks
Steve Postins

Steve

45 degs sounds far too much to me. generally , all the gurus recommend no more than 34 degs total advance at top speeds . the exception to this is with vac adv part throttle when very much more can be used. I should try static of about 2 degrees and see how it feels at full throttle

i should warn you that with a DCOE it is almost impossible to obtain proper response at both low and high rpm - full throttle response wiol probably be ok, but part throttle almost always results in a flat spot unless the low end is impossibly advanced, and if that is done, the high end becomes far too advanced with resultant loss of power, harshness and detonation

If u wish to discuss e mail me at cstoreyqc@aol.com

Chris
chris

Steve,
After having my car rolling road tuned, I was surprised at the amount of advance that was required.
I'm note sure what it is because it goes off the scale.
Perhaps 35 degrees is the play it safe figure.

Charlie.
Charles Goozee

I just bought a 67 and while doing a comprehensive tuneup I discovered that the vacuum advance was not working among other problems. I visited a respected MG restorer and racer and he built a dizzy for me from a few used ones. He explained that Bs came with many different advance plates and that some, probably like yours, used an 18* plate which he said was too much mechanical advance. He said that the best plate to use was the 12* plate which means that the dizzy provides 24* crank advance. Including the 12* initial timing and the mechanical advance there is a total of 36* advance. He told me to plug the vacuum unit also as it doesn't help much except at partial throttle but running on the highway at about 3500 rpms I'm running the full advance as we installed lighter springs to bring in total advance much sooner. Maybe you can go to a breaker and see what dizzies they have to modify yours. The 12* initial advance helps the bottom end and you don't get too much at the top end. Just some ideas for you to ponder.
M Landskroner

Thanks,

I happen to have a 45d with a 12 degree plate so I might try a few variations. I've also spoken to Oselli who confirm that 30-35 degrees is the very most advance the engine should get, although the dizzy is designed to give more advance than usual at low revs to help the running of the larger bore and weber combination. 12 degrees at idle is therefore about right, but this is always going to put me off the scale at the top end with this dizzy with another 30+ degrees of advance adding on.

If anything works I'll let you know!

Best regards,
Steve
Steve Postins

Since vacuum advance can add anything from zero to 24 degrees and centrifugal anything up to 40 degrees then theoretically one could get up to 64 degrees, and 50 degrees is not uncommon with the dizzies used in MGBs. But with a WOT at 4500 one would get no vacuum advance, and with a light throttle giving max vac advance the rpms and hence centrifugal advance would soon drop. However off-load I could imagine that one would still be getting significant vacuum advance at quite high rpms, which could explain the 45 degrees. But that is unrepresentative of real life which is why these things have to be done on a rolling road and many combinations of engine speed and throttle opening tried to obtain the best centrifugal and vacuum curves. With a racing engine the vacuum capsule is usually done away with which reduces things from a 3D to a 2D situation. But for road use the vacuum capsule makes a big difference to cruising economy. Even on my bog-standard roadster pinking is a lot worse on Unleaded than on Super, even using an octane booster additive. On the V8 it seems to make absolutely no difference - but then neither did a punctured vacuum capsule and a siezed centrifugal advance.
Paul Hunt

Just to update, I've played about with the springs and plates. Thanks to the Weber, there is no vacuum to worry about so fortunately it falls in the simpler 2D category. Having mapped the advance at 500rpm intervals I could see that the timing was going stratospheric over 5000rpm. Stripping the dizzy, the first thing I could see is that the secondary spring looks long, possibly stretched, and with the 15* cam only comes under tension at pretty much full rotation. Substituting a secondary out of a spare 45D4 seemed to give too little advance in the midrange but took the top off the maximum. Using the original springs but a 12* cam as suggested has restrained the maximum advance to 36* but the secondary spring may as well not be in there as it never comes under tension. Seems wrong, but I guess I shouldn't worry as the car drives well and I feel happier with the maximun capped to something sensible.

Thanks all for the advice,

Steve
Steve Postins

This thread was discussed between 22/04/2003 and 27/04/2003

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