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MG MGA - Distributor Mechanical Advance
| Does anyone have a DM2 distributor in pieces at the moment so that they can tell me what advance setting is stamped into the cam arm? I am just trying to fine tune (file) mine. Not sure I have got it quite right yet. After dynamically tuning my distributer I am finding that the static timing is still just a little too far advanced. On attempted start up from cold the engine is tending to lock on pre-ignition. I am overcoming this by spinning the engine on the starter for a second before turning on the ignition! For those who do not know the background to my saga, my distributor came back from being refurbished with a 7 degree cam. This would give a mechanical advance of 14 degrees plus the static setting (8 degrees ish - running an 1800). If I dynamically timed the engine to 32 degrees @ 3000rpm this would result in a static of 18 degrees! But if I set the static at 8 degrees I ran out of mechanical advance at 22 degrees at 3000rpm and the engine just ran flat. As I said I am almost there but am curious what the standard cam lobe is. Despite all this the engine is running very nicely on its electronic (Aldon) ignition. It may be my wishful thinking but I get the feeling through the seat of my pants that it is performing better than with CB ignition. Steve |
| Steve Gyles |
| Steve, I think I have a couple in bits, can you tell me exactly where to check? Neil |
| Neil McGurk |
| Neil It is the number on the cam arms. Some examples attached. Thanks Steve  |
| Steve Gyles |
| Steve. If you have a rebuild distributor, the markings on the stop arm of the points cam will only tell you what it started out as, not what it is today. There is at least one tech article on the web which discusses modification of the arm as part of the rebuild process--either filing one to allow more mechanical advance or welding up one to cause the advance to be terminated sooner. Thus, you need to actually test the distributor to find out what its curve is rather than rely on 40+ year old markings. That being said, the example in my collection, manufactured 560 (fifth week of 1960) has the stop arm marked "12", indicating that there were 24 degrees of mechanical advance at the crankshaft. Neil. If you remove the points plate, you will see the points cam which has an arm on it which hits, at full advance, one of the posts having the springs, that control the mechanical advance curve, attached to it. This limits the movement of the points cam to the maximum specified movement, 12 degrees in this case, that the ignition is advanced as the rotational speed of the points cam increases. Distributor advance is one half of number of degrees you see when measuring at the crankshaft. (Because the crankshaft is operating at twice the distributor speed--two revolutions of the crankshaft for one revolution of the distributor points cam.) Les |
| Les Bengtson |
| Les I understand and agree with what you are saying. When I first discovered the problem I was able to make a reasonable guess what my available mechanical advance was through dynamic testing and than checking the resultant static advance. Also, I measured the gap between the post and arm, then drew it up on graph paper to convert it to angular movement. It was the 7 degrees as stamped on the face. I increased it to 11 degrees ( better to take a little off at a time than too much to avoid having to weld metal back on). The result is that I am almost there. One more degree worth of filing this afternoon should sort it. Many thanks Steve |
| Steve Gyles |
| Steve. Looks like you know what you are doing. Please let us know how the experiments turn out. It would also be nice if you could write up your use of the graph to approximate how much metal needs to be taken off. Most of the methods I am familiar with started off as trial and error, keeping records of how much metal was removed to give however many degrees of increase. However, you seem to have a better idea and one which could be used when either metal had to be taken off or when metal had to be added. It would be a good thing to know. Thanks. Les |
| Les Bengtson |
| Steve I have sent you a mail with a lucas booklet that has pictures of various arms and their degrees. Dominic |
| dominic clancy |
| Les I doubt that I have any better idea than you. I just tried to apply logic to the problem I had. I took approximately another 1mm off the face of the arm. Just tested the car and it feels good. I will now leave it to go cold overnight and check that the engine spins ok on start with the ignition on. Looks promising at this stage. I sent the distributor back twice to the reconditioners because I was not happy but they said it met the profile on their test rig. I think they need to recalibrate or they are using the wrong data. Bob West had been using them but was not happy either and changed supplier. How they think a 7 degree cam can provide the required mechanical advance range for our engines beats me. Thanks Dominic. I will have a read. Steve |
| Steve Gyles |
| Steve what beats me is that you accepted a dissy with only 7 degrees or at crank 14 degrees of advance. This would be way too low for most engines, why did you use them in the first place? 12 or 13 (24/26) would be nearer the mark! |
| Bob (robert) Midget Turbo |
| Bob Tell me. But I did not know that was the problem in the first place. I originally sent the reconditioners a very worn out original DM2. It came back beautifully cleaned and rebushed, also fitted with electronic ignition. It did not run correctly and I was also puzzled by the readings I was getting on my strobe gun (later I was told that some guns do not like copper wired plug leads). I even bought a new silencer box because I became convinced that I had blown a baffle. Anyway, I did not take the dizzy apart at that stage and I sent it back under guarantee. It was only after the second time of returning it and being told it was 'in spec' that I took it apart and the reason became obvious. By that stage and, having spoken with Bob west, I wrote the company off as incompetent on the profiling side and reckoned that I could do a better job myself. That's where I am today. The cam is now at or very close to the figures you describe and I have learnt a fair bit about ignition timing, strobe guns, plug leads and profiling. Good fun. Steve |
| Steve Gyles |
| Ok Steve, finally got round to finding and looking at my distributors. This turns out to be pretty interesting. The first one is a NOS item and does not seem to have a cam as such? so we'll forget that one for now. The second one was the original from a 1600 but with a cracked lower casing, this had 12deg. Then I checked my 1622 part that I know was the original factory fitted unit. (known car from new and engine paint overspray). That car was quite quick and very smooth and the cam is 15deg. Finally I checked the unit on my other 1622 (probably original) and that was 13deg. I had no idea they were different! Now it's got me thinking what to use on my rebuild? I didn't check the parts on either of my drivers, but one has an MGB 3 bearing and runs really well, (both fast cam and smooth tickover). Neil |
| Neil McGurk |
| Neil Welcome to the confusion! I have filed mine to 12-13 ish. Just a hint of being too far advanced on static when starting from cold this morning so I am going to back it off a marker on the nurled adjuster and see how that affects the top end. If not satisfactory its out with the file again for another 0.5 to 1mm. Almost there - I keep saying that! Keeps me occupied. Thanks for checking. Steve |
| Steve Gyles |
| Interesting stuff on the sdvance curves. I avoided these problems by fitting a new positive earth electronic distributor from Moss $210 US. I did not give much thought to the advance curve till this thread. Set the static timing at 12% and have approx 30% mech. only. Now I'm not so sure, its a 1500 in good condition and runs well. Does anyone have the specs for these new distributors ,Moss just sell them. Thanks Sean |
| S Sherry |
| Steve, We are waiting to hear the next episode. Have you managed to fettle the Aldon distributor to a point where its working? or have you reverted back to the trusty metro distributor. Regards...John |
| J Bray |
| John In the Middle East at the moment. When I left the car a week ago it was working just about spot on. I will report back later this week. Steve |
| Steve Gyles |
| John Back from my travels in Saudi Arabia. Car started fine today and I guess I have the mechanical advance for my DM2 on the 1800 just about spot on. I will take it all apart at the weekend, do some measuring and translate it all to degrees to see what I ended up with. However, let's not pretend that is all there is to timing the distributor. I may have got the range from static advance to full mechanical advance sorted. Now there is the little matter of a couple of springs that decide what goes on in between........ Need to get my books out and study the website that Dominic has provided the link to above. Steve P.S. Minus 5 in Riyadh the other night. The vegetation out there has taken a real hammering. |
| Steve Gyles |
Steve it will be interesting to see what the static and full advance end up at, and in particular how it compares with the trusty Metro distributor you have used in the past. I tried changing the springs around using some from a Mini Cooper, but ended up with full advance a bit to soon. My understanding is that new springs are no longer available so its a case of choosing the best of those which you already have. Unless of Bob West has found a reliable supply. Keep us in the loop of how its progressing Regards...John |
| J Bray |
| John It's a 'to do' job this weekend. I am going to measure distance between the cam arm and stop post, also the radius of the stop post/cam arm. With this info I can convert it to degrees. I can then make a simple chart so that anyone else who has their distributor apart can take one simple measurement (cam arm to stop post) to check what their mechanical advance is in degrees. I think I need to take a smidge more metal off the arm on my DM2. Static advance still not quite there. Upper end performance very pleasing. Electronic ignition also behaving very nicely. Steve |
| Steve Gyles |
| Completed my weekend 'to do' job this afternoon (Friday). First of all I had a look at the Metro distributor I have been running very successfully in my 1800 engine for the last 8 years. It has a 15º cam arm. I did some linear measurements, drew it all out on graph paper and, amazingly, it came out at 15º. I then took my DM2 out of the engine and performed the same process. It came out at 16º. I have to say that I am a little surprised that the figure was so high. I was expecting 12º - 13º. Anyway, the engine is performing really well with my modified cam arm so I am leaving it be. If anyone else is interested in seeing what their distributor mechanical advance really is see the attached diagram. It is based on my measurements in my distributor, so there could be variations from one unit to another. It all depends on Lucas manufacturing tolerances in the 1950s!! Steve  |
| Steve Gyles |
| Steve, I think it's 8+(2X12)= 32 and does this agree with full advance at 3500+ rpm when measured with the strobe? With your 'seat of the pants' style rolling road would do you now have a better set up all round compared with the Metro or have you gained top end power but lost some of the low down 'grunt' Regards...JB |
| J Bray |
| John I should have known better. And there am I with 3 different maths at 'O' level and one at 'A' level. Brackets are now on the revised diagram! Just checked my notes that Bob West gave me years ago. You are quite correct. At the time I wrote down 32º-34º at 3500-4000RPM with vacuum disconnected. Not sure how I picked up the 3000RPM figure, maybe somewhere on this BBS at some time. On the last point I reserve opinion til tommorrow. I am going to spend some time setting the timing up. I am intrigued by the 40º I should get! I will then give the car a really good long run and let you know the results. Steve  |
| Steve Gyles |
| Steve. The 3,000 rpm figure is a common one for the early MGB engines where the max advance should be 32 deg BTDC at that rpm. Static timing is an interesting concept, a hold over from back in the days when a stroboscopic timing light was a very expensive piece of tuning equipment that only a professional mechanic could afford to own. Now, some forty years later, the price is within reach of anyone who needs one. Hence, static timing, other than to get the engine started after the distributor has been removed and replace, is obsolete. What we are interested in is the actual mechanical advance at various engine rpms. With a timing light, we can measure this and no longer have to "guestimate" by using a static timing figure, followed by a hope that the mechanical advance is correct--we can set it exactly, then test out how well it is working. Les |
| Les Bengtson |
| Les Yes, that is what I am doing. I have a very good light that I can dial the advance into. Cost an arm and a leg but it has been a very good investment over the last few years. Sometimes, though, with modern technology comes problems. Some time ago I had copper ignition leads and they totally confused the timing light. The timing mark was all over the place. At times I had about 6 timing marks showing at once. Changing over to the modern fibre leads sorted that one out. Once I have my maximum advance sorted tomorrow, I am going to plot the advance curve and see if it is remotely similar to anything else published on the recognised sites!! Watch this space. Steve |
| Steve Gyles |
| Steve. I will. Eagerly. Les |
| Les Bengtson |
| Steve, Don't worry about your mathematical skills. Your first attempt was correct. However, the revised version is also correct. Mathematical rules specify that actions are carried out in the following order, brackets, division, multiplication, addition and subtraction. So both of your versions are correct. I remember my primary school teacher, when I was 7 years old, giving me the "ruler across the palm of the hand" if I could not remember the word BODMAS. I was too scared to ask what the O was for. Mick |
| Mick Anderson |
| Mick Order Steve |
| Steve Gyles |
| Steve, I checked my spare distributors today and they both have 10 degree cam's in them, I think the one in the car has 15 degree cams in. I also had a look at some old books on tuning BMC cars and they make very little reference to distributor modifications other than slight changes to the static setting. One book 'BMC 'B' series tuning & modifications by Hansen did have the attatched graph for stage 4 tuning on the MGB. The scan is not very clear but the x axis is rpm and the y axis is marked distributor advance in degrees The top line is for a competition dizzi and the lower one is for a standard one. I suspect the difference is a result of the incresed compression. Regards...John  |
| J Bray |
| Interesting chart John. I believe a couple of things, you will notice that the x axis is dissy revs thus this is half actual RPM. That means maximum advance comes in at 6500 on the competition item. Not much good on a standard dissy where the engine RPM would never reach such dissy Heights So the standard item is all in at about 2500 RPM which in those days was adequate motoring. Remembering also that the competition item was all about ultimate power then as you say they want to keep it on the edge of detonation all the way up the range where as the standard item was all about safe engine performance and good fuel economy. |
| Bob (robert) Midget Turbo |
| Well spotted Bob, you are corect about the x axis relating to dissy revs. What are your thoughts with your racing background about optimising the timing curve? I know you are still using an 'A' engine rather than the 'B' but there seem to be a lot of advantages to getting this right!....John |
| J Bray |
| Bob and John I had spotted the half speed x axis as well. I took some figures on my car today and have drawn them out at engine speed, although this does not alter the shape of the graph. I appear to have got a consistent curve. However, I got the impression when I was tuning the car on the stroboscope that the advance seemed to build up very quickly at low RPM. Could I be right in guessing that this may indicate a weak primary spring? Diagram of my curve attached. Steve  |
| Steve Gyles |
| Steve. Most of the early MGB distributors used two different weight springs with the stronger spring not coming into play until about 1,500 to 2,000 rpms. Thus, a flater curve at higher engine rpms. Les |
| Les Bengtson |
| John in racing we are only interested in the engine at high RPM. So for argument sake we would be happy with 32 degrees at 7000 and 4500, so the engine would always be "on Song". With a road car if it was say fast road and had a hot cam we wouldn't want the maximum advance coming in too early. With these cams we need to up the CR to make them work at high RPM, this leads to possible pinking in the mid range when "effective" CR will be high and thus we want to back of the timing. I must admit to not understanding the limited advance types. We need to be at 32/34 at high revs and no more than 6/8 at start up. Thus we need at least 26 degrees of mechanical advance thus a 13 degree dissy is ideal. how you could use a 7 degree is beyond me although it possibly had something to do with fuel octane ratings which would not require such high advance at peak revs? If someone builds an engine with good high CR and a decent cam then a reprofiled dissy that would not bring in the mech advance too early would be of major benefit. I have just fitted programmable ignition to my midget Turbo so will have hours of fun practicing what I preach!! :-)) |
| Bob (robert) Midget Turbo |
| Bob I presume that was meant for me, not John? I take it that CR is Compression Ratio? Re your comments about the 7 degree cam arm; I totally agree with you, that's what started this whole saga off. Rest assured it is a lot bigger than that now. The engine is currently showing 34 degrees at 3500RPM and feels lively with no knocking. Quite pleased actually. Steve |
| Steve Gyles |
| No Steve I was answering a enquirey to me from John 5 posts up :-) |
| Bob (robert) Midget Turbo |
| Bob, Thank you for your observations. Steve, Apologies for the confusion, I asked Bob'e opinion on optimising the advance curve having forgotten that on a competition engine you don't really need an advance curve as you should alaways be at the point of maximum power (power band) and ignition timing is set to coincide with this. I'm following the thread with interest as I have the same set up as yourself and I feel there should be a little more power available especially at higher rpm. I've tried a number of distributors and although some are better than others I think some sort of matching to the engine is required. I also have high expectations as I want good low down torque to assist the high first gear in my close ratio gearbox, but I would also like the power to still be there at higher rpm's which I know is not really realistic. Having followed this and other threads with regard to your trials with getting the timing optimised, I have high hopes you will get it right and I will then try a smilar set up. However I'm also considering purchase of a modern distributor? which don't have preset cam's instead they appear to have about eight different electronic progammable ignition curves allowing selection of the one most suitable. |
| J Bray |
| John Having set mine up over the weekend to give 34 degrees at 3500RPM, I have since backed it off for the sake of better starting, low RPM pick-up particularly when cold and run-on avoidance. I'll see what I have ended up with later in the week. Sufficient to say that she was a delight today on the way back from our site over the other side of the M6. Happily cruising at 90 indicated at I think it was a tadge under 4000RPM. The only shock I got was the subsequent refuel at a Shell garage to get their 98 octane fuel. £1.20 ($2.34) a litre!! Think I will be mixing with normal unleaded from now on at that price. Steve |
| Steve Gyles |
| Actually Steve on tests I performed on a rolling road to understand advance timing, I found that advance settings between 28 degrees to 34 degrees were very similar in performance at the top end and so with race engines I generally set the advance at the lower end of this band as there was nothing to gain in going higher. |
| Bob (robert) Midget Turbo |
| Steve, not sure what your A has under the bonnet but are you really happy at cruising at 90 if the cars standard your close to its top speed! |
| J H Cole |
| JH The motorway was clear for a change, hardly a car in sight. 1800, 5-speed, 3.9 diff, front discs, not a sign of a vibration anywhere and hardly a movement on the steering wheel. Not a cloud in the sky. Sheepskin jacket, collar turned up, flat hat and flying gloves. That's what driving the MGA is all about. Just one of those rare occasions when everything was right. So the foot went down close to the floorboards and the smile grew from ear to ear. The car would happily have gone faster, just held it back a bit. It did cross my mind to push it to 100. Got to 97 last year when the exit loomed up. Steve |
| Steve Gyles |
This thread was discussed between 26/01/2008 and 13/02/2008
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