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MG MGA - clear the air
|A friend who has a 3.4 Jaguar is obsessed with getting a smooth idle with disregard to the advance curve of the Lucas distributor. Driving , nothing happens till 2500 revs then it takes off. Question , Lucas distributors . Not very well made with little reserch put into getting the curve right, plus poor quality springs. ? ? ?|
Question , does Hi octane unleaded call for a modified advance curve ? Moss told me that their new electronic distributors for my A uses the curve from '68 1800 B. If they have just copied the Lucas curve I doubt if it is right. Preformance Ignitions in Melbourne have new springs made to correct the Lucas advance curves , All comments appreciated.. Sean
|Little wifey suggests ..... Lest our British friends see the above as a critisicm of the well enginered products, Jaguar and B series engines. I'm simply questioning if they are let down by the Prince of Darkness. Goggle " Lucas electronic distributors " and open the second site MG+++ Let me know what you think Thanks Sean|
I cannot speak for when they were new; I was too young in the 50s to be interested in that detail on cars.
I can only speak from personal experience about what was being turned out a couple of years back by one of the reconditioners here in the UK. My 1500 distributor came back fitted with a totally inappropriate cam lobe - see the variety below. The MGA should have a cam lobe in the region of 12 - 13 degrees. This would give a maximum dynamic advance in the region of 34 degrees (2 x 13 + 8 degrees static). My one was fitted with a 7 degree cam. This would give only 22 degrees dynamic advance. The result was that it either idled beautifully but ran out of oomph above about 2500 rpm, or it was a sod to start, idle was awful but performed well above about 2500 rpm. And all this was ignoring the quality of the springs that I presume they fitted.
So, when you buy a reconditioned distributor from an unknown source, do not assume it will have the correct advance profile. Have a look inside to see what you have got. It may not always be down to Mr Lucas.
|I think I used the wrong word. I should have said mechanical advance, not dynamic advance.|
|Thanks Steve ,I had no idea there were so many veriations of cams. My A unit is new from Moss and the curve is '68 B. Static is set at 16% at 800 and runs out at 32/34 at 3000. Preforms well by seat of the pants feel, pulling a 3.9 diff, so maybe all is well after all. My mates Jag is a problem being flat up to 2500. He loves the slow steady idle ,probably retarded and running rich, so maybe I'll let that one go through to the wicket keeper as well.|
Don't mention the Ashes ! Sean
No, I won't mention the ashes! - yet.
|I thought I would throw a hand grenade into the advance curve issue.|
I think we would all agree that the advance curve is primarily governed by the weight of the bob weights and the strength of the springs that control them. How many of us would also consider the planned effect of friction when Lucas built and calibrated our distributors? i.e. the friction of the points cam follower on the cam.
Those of us who have converted to electronic ignition will have removed this additional source of friction. The result is that the ignition advances just that little quicker than the original design spec.
I read the above in a very interesting MGB technical section in a magazine from a dozen years back. The writer says he had to fit stronger springs to get the curve back in spec. In other words, when we modify the old dizzies with electronic ignition, it may not be just a case of taking out the old, fitting the new and all will be spot on.
Food for thought.
|What a drag Steve|
|There is no friction on the cam when the points are closed 4 times per distributor revolution, since the points do not touch the cam then, so it takes at most 1/4 turn of the dist to eliminate that bit of useless worrying. And you are supposed to lubricate the cam.|
Yes, but 4 times a revolution there is friction. i.e. during the points opening periods. As they say, 'every little helps (or hinders)'. Whilst I tend to agree with you, this guy's findings were quite interesting. To get the curve back in spec he replaced the small spring (0.4mm wire diameter and 15mm long) with a 0.7mm / 16mm spring; and the main 0.9mm / 17mm with a 1.2mm /17.9mm.
|Steve, the answer is, if your want to be sure your distributor advance is working properly, fit points. If it runs nicely as it is, stop worrying!|
Not worried here. Just sharing info. I just lobbed the grenade. I did not design it nor make it. It's up to you guys to decide on its lethality.
As an aside I put my Ignitor system into one of those new Chinese replica distributors - same one as Bob our midget Man has fitted. My 1800 engine is running brilliantly on it, whatever advance curve I've ended up with, without changing any springs. I am totally open-minded about it all.
PS. The document will end up on the MGCC website technical section in the fullness of time. Happy to email PDF copies if anyone wants a sight in advance.
|Steve, I made the mistake of trading my worn out although perfectly standard DM2 distributor for a 'reconditioned' MGA spec DM2 from the MGOC. The first one I got didn't seem to have any limit to the mechanical advance, the second piled in all the advance in the first 1500 rpm. both these units had been 'butchered'. The knurled advance/retard adjustment nuts had a flat ground on them to stop them from un-winding because the spring clip was absent, and the distributor holding clips were fitted with bits of bent wire. I complained bitterly to the MGOC and they managed to get me one that hadn't been messed about like the other two. This distributor had its faults, but at least they could be rectified (at a price!), so I decided to cut my losses and make it right myself.|
The cam plate was jammed because someone had bodged the earth wire by connecting it to the capacitor fixing screw. This was an interesting fault because everytime I checked the ignition timing, it had changed from the last time I checked it! The cam fitted was a 10 degree instead of a 12 degree, and the vacuum advance unit not only leaked and was thus inoperative, but was the wrong one for an MGA! All this on a so-called reconditioned unit.
So I have sorted out the earth wire, fitted the correct cam and managed to get a brand new, correct spec vaccuum unit from British Vacuum Unit in the USA https://www.britishvacuumunit.com/Home.php
The car now runs better than it has ever done before and in all honesty I don't think it could run any better.
The lesson to be learnt from this story is, don't ever trade your original distributor for a recon, but take it to a reputable distributor repairer or do it yourself!
Like you I kept my original but worn out DM2 rather than swap it when I did my rebuild 13 years ago. At the time I fitted a brand new non-vacuum unit that served me well. Later I sent the DM2 to Aldon to be rebuilt. It came back looking superb, verging on the brand new. However, I could not get it to work properly; the advance curve was all over the place. I sent it back to them as a complaint. It came back as no fault found. They said it matched the MGA advance curve profile exactly on their calibration equipment. I put it in the engine and it was exactly the same - dreadful. I then took the innards apart and found it fitted with a 7 degree cam!!
I reprofiled the cam to 12 degrees and it was much better, but still not that great. Sometime I will play around with it again by trying different springs. In the meantime the Chinese distributor is working great.
What have I learned from this? Don't go to Aldon for rebuilds.
Happy new year
This thread was discussed between 26/12/2010 and 31/12/2010
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