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MG Midget and Sprite Technical - Dwell angle
| I hope someone can offer some guideance with a problem on my MG1300 which is fitted with a 25D distributor Despite the CB gap of 15 thou, the dwell angle is 89 deg instead of 61 deg. Being unsure if a high dwell is due to a small or large gap, I then adjusted the gap to 10 and then 25 thou but the dwell remained constant at 89 deg. I then fitted the plate / CB / condensor assembly from my spare unit [which was running fine when replaced by a Distributor Doctor refurbished version] from the midget and that also gave a dwell of 89 deg. I havent changed the rotor or cap / leads but AFAIK they will not effect the dwell. To the best of my knowledge, the distributor [and the car basically] is original as it was taken off the road in 1982 before being recommissioned in 2012/3. |
| D Plumb |
| I believe dwell is the number of degrees the points are closed, so smaller point gap will give larger dwell. Since it didn't change, I would suspect something is assembled wrong or the dwell meter is wrong. Got another car you can try the dwell meter on? Charley |
| C R Huff |
| I thought the dwell cant be changed on the lucas 25D Prop |
| Prop and the Blackhole Midget |
| Prop. As Charley notes, dwell is a dynamic measurement of the number of degrees the points are closed during their recharge the coil cycle. Different points cams, because of their shape and the number of cylinders they service, will have different degrees of ideal dwell. The Lucas 25D4 distributor is designed to provide about 60 degrees of dwell when the points are set to the proper gap using a feeler gauge. But, if the points are gapped to a smaller opening, they stay closed longer and show a higher dwell reading. They show a lower dwell reading when they are opened up to a larger gap. The readings given in the original post would indicate that either the points gap is excessively small, causing them to stay closed longer and barely open to provide the spark, or that the test instrument is incorrectly reading the actual dwell. Cross check the test instrument first, comparing the readings it is giving against another test instrument that is known to be working properly. Remember that older test instruments only had a six and eight cylinder position and four cylinder vehicles are tested on the eight cylinder reading and the indicated reading doubled to give the four cylinder reading. If both the known good and the subject instruments give the same readings, the problem lies in the distributor circuit, assuming that points are being used. If an electronic points replacement system is in use (e.g. and Ignitor) the dwell will be significantly different than the factory setting for points and will be in the 80 degree range. Les |
| Les Bengtson |
| As previously said I would suspect your meter. Dwell is a measurement of how many degrees the points are closed, on a 4 cylinder engine if the points didn't open at all it would effectively have a dwell of 90 degrees. As you must be measuring this with the engine running it suggests your meter is at fault because it would run very badly if at all with the points only opening for 1 degree. The fact that you've replaced the CB assembly with a different unit and still get the same result further points to a fault in your meter. Bob |
| R.A Davis |
| I dont know... id just feeler gap the points on the flat part of the lobe and call it a day The problem is your being to dead on nutz precise, and that aint this car, I seriously doulbt that the 4 corners are that fine tuned and mirror ground and the shaft in its bearings is that perfect to not have any verance, and I doulbt the cap is that looked into place with a super high degree of dead on accuracy... not to mention the plate that the points set on has a lot of built in slop not to mention the slop play in the dissy gear to cam shaft gear With all that and more, id think your dwell would be changing whith each rev. Really, all your wanting is decently somewhat close, if your engine needs that level of precision then go with the 123 tune from peter burguss Ive always looked at lucas dissys like a harley pan head... if you make everything detailed and highly precise its only going to be a matter.of time before it exploded... these things seem to like alot of slop to function and vibrate properly That said... a good double point mallory is the way to go... large cap, small rotor block, and very little slop and it doubles (or maybe its half) the dwell over lucas |
| Prop and the Blackhole Midget |
| Prop, sometimes you need to learn the basics before offering advice. |
| Nick Nakorn |
Is the dwell meter set for 4 cylinders? Most dwell meters can be varied for the number of cylinders they are to measure. If the answer is yes, I feel the dwell meter or at best, its leads are u/s. My multimeter gives a static reading of 89°(89.7 in fact) so does yours actually change when you attach it to the l/t switching terminal and crank the engine? dwell is found by dividing 360º by number of cylinders so 4 cyls =90° dwell 54° ± 5° 5 cyls =72° dwell 43° ± 5° 6 cyls =60° dwell 36° ± 5° + 8 cyls =45° dwell 28 ± 5° I used to use 55° when I was working on four cylinder engines |
| bill l |
| Nick ... your not actually proposing that the lucas 25D is a soild high tolerance slopless dissy with zero play are you?? If your are, that would indicate your an idiot Exactly which part above did I mention would not effect the angle and the amount of the dwell Ill start... A worn out points plate with lots of movement... Nick would that effect the amount and angle of dwell??? Just a simple yes or no, id hate to over tax your feable mind Sorry nick... im a nice guy, but im not a door mat, you want to debate thats cool and welcome but blantly attacking me with no point of referance is unacceptable... Try agian Prop |
| Prop and the Blackhole Midget |
| Prop. I do not know what you are basing your "expertise" on. I have owned a Sun Distributor Test machine and have taken apart quite a number of Lucas 25D series distributors. I also own a lathe, which allowed me to put various parts between centers, then, using precision test indicators, examine what the various parts and subassemblies had to tell me. I built the special tooling that allowed me to hold the distributors safely while driving out the old shaft bushings and inserting new bushings. I built the special tooling that allowed me to hold the distributor body in my lathe and properly ream the new shaft bushing to the proper internal size after installation. I studied and experimented for about five years trying to learn as much as I could about the Lucas distributor, what caused it to wear, how much it wore, and at what point it would be in need of rebuild due to distributor based performance losses. May I ask what brand of distributor test machine you own, how many you have taken apart and rebuilt? Are you trained as a machinist? Do you have other college degrees that give you the basics of quantitative and qualitative research skills to base your opinions on? You seem to come down hard on Nick, but, from my analysis, his comments are correct. If you wish to consider me an "idiot" because, based on years of properly done research, I consider the Lucas 25D4 distributor a generally good distributor, made from some parts machined to high precision, and to be fully the equal of any other distributor design of that era, so be it. Nothing wrong with a Lucas 25D4 distributor that has been properly rebuilt by someone who knows what they are doing. Les |
| Les Bengtson |
| Surrender Prop! Resistance is futile......if you continue to fight with these guys you may actually miss out on learning something! Anyway.... there's a lot of good information being written about here. I noticed on another thread that some one was timing there car at full advance @3000 rpms. I thought you had to leave the vacuum advance disconnected. (I always did that for a car set at idle) If you went for full advance @3000 wouldn't you want the advance connected to get the full effect? Or no? I had never really thought about the function of the diaphragm advance....compared the weights and spring advance. I always checked for function but now thinking about it am getting a little confused. If anyone can enlighten me...I promise to keep an open mind...Ha ha ha ! Not trying to look stupid but just not getting my head around it at the moment. Good discussion here but lets keep it friendly people! |
| Steven Devine |
| Oh ya, one other thing...does anyone no why they use the term dwell? I was always baffled by the reason for that term... I would read it all the time buy definitely have a laymen's understanding of it. so if anyone had the reason I would love to know! :-) |
| Steven Devine |
| Isn't dwell a term of timing? In UK English when one dwells one takes time, one stys for a bit longer, takes longer to decide. One 'dwells' on it ? Like you Steve, I am trying to get my head around the basics. I thought the CB points base plate movement changed the advance and retard of the spark. However from this I think the gap is also an effective variable for timing the spark and so makes a difference to the effective dwell. I always knew the gap was critical for good performance but didn't understand about dwell and that it may affect dwell. ? just asking anyone that would like to confirm or put me right. |
| Dave Squire |
| Les, how long would you say does a well built Lucas dissy remain reasonably reliable at its job and how badly out do you think the average dissy is that's decades old and never been attended to? |
| Nigel Atkins |
| dwell is the period that the points rest closed isn't it? Or is it? All I know is the gap needs to be 55° and the dwell is near enough 0.014" Or is it...? Prop seems to be clumsily berating Joe Lucas for his distributors, oh dear. Prop it is obvious that you never had the pleasure of "dealing" with any of the alternate distributors that found their way into the Austin Rover Mo Co range and others. Let me know how bad Lucas are after you have tried to get sparks from a Ducellier dizzy at midnight with rain cascading round you. Pressed tin and slimey rubber are permanant reminders. Or one of those weird transparent dizzy caps fitted to some French motors of the seventies where you can watch but not stop the tracking of sparks running round in all directions and the points are inbuilt in a replaceable cassete which you CAN eventually get set up right. Honest. No, Lucas mmade very good simple and accurate distributors. |
| bill l |
| Hi Nigel, sounds like a leading question :) I used to reckon I got two years from a new Aldon points distributor before the points seemed to fail too often. Maybe 15,000 miles at the time. I never looked back once I fitted the lumenition chop chop optical system....till I cut through the wires when messing with the timing when running and the chopper killed the wires...soon mended though. Peter |
| Peter Burgess Tuning |
| In mechanical terms the noun dwell means a period of inactivity, so in this instance the period of time that the points are closed and doing nothing. Points gap does have an impact on timing and dwell as if the gap is too large the dwell period will be reduced and the spark will occur earlier and vice versa. Like Bill I have come across other brands far worse than Lucas over the years. Trev |
| Trevor Mason |
| thanks Peter I've read that the Kucas dissys were out within 2-3 years after they left the factory (perhaps it was you that wrote it) so dissys that have been used for decades but not rebuilt are probably well out so Prop does have a point about the accuracy of these units the Distributor Doctor guarantees all his rebuilt units for 3 years and I'd imagined they go on a lot longer but I wonder when their accuracy goes passed the point of needing resetting |
| Nigel Atkins |
| In relatively simple terms, there are a number of factors to take into consideration, the following isn’t intended as a definitive explanation but it may help. To make power in an internal combustion (petrol) engine you need to ignite a compressed charge of fuel and air, this charge takes a finite time to burn. Simple descriptions of the four stoke cycle would suggest the ignition of this charge should take place at TDC (top dead centre) when the piston is at the top of its movement and about to start moving back down. The problem with this is that because the charge takes time to burn, by the time it’s fully burnt the piston has moved down the cylinder and you don’t therefore get the maximum power from burnt charge. The time taken to ignite the charge and get it fully burning is affected by a number of factors but if we take it to be a nominal 2mS (2 thousandths of a second) for this explanation. Please note the following figures are approximations and for illustration purposes only. At 1000RPM (a bit faster than tickover but it makes the figures easier) the crank turns approximately 10 degrees in 2mS, the means we need to ignite the charge 10 degrees before the crank gets to TDC to get maximum power from the charge. Hence why this is a typical advance setting for tickover speed. We can now see that at 2000RPM we still need to ignite the charge 2mS before TDC which equates to 20 degrees before TDC and at 3000RPM it needs to be at 30 degrees before TDC. Other factors take over after this and 30 degrees is about the maximum you’d need. The above shows why we need auto advance and why it’s important it’s working correctly. Now we can look at dwell. The coil that produces the spark takes a finite time to charge, the time it takes depends on a number of factors but I’ll base this on a simple generic coil. The coil is charged when the points are closed and produces a spark when the points open. The time the points are closed is measured in degrees of the distributor rotation. Therefore (using simple figures and assuming a 4 cylinder engine) if the dwell is 60 degrees it means the points (CB) are closed for 60 degrees and open for 30 degrees. It does this 4 time for every rotation of the distributor (2 full rotations of the crank). The dwell in these distributors is fixed, variable dwell with electronic systems is available. Because the dwell is fixed it’s a compromise between the requirements at tickover and maximum speed. If the dwell is to long it can lead to overheating of the coil at low engine speeds. If the dwell is too short, the coil won’t have sufficient time to fully charge at high engine speeds, this leads to a week spark and loss of power. Finally vacuum advance. The reason for vacuum advance is to help with part and light throttle economy. Under these situations the charge entering the cylinder is small compared to full throttle and may be set weak to aid economy. This charge can take significantly longer to fully ignite compared to a full charge. We therefore need to start the ignition process earlier. We determine when this is required by using the level of vacuum in the inlet manifold. We take the vacuum and apply it to the diaphragm in the vacuum advance unit. This is connected to an arm that’s attached to the plate that the points are mounted on. High vacuum (light throttle) causes the diaphragm to move, this pulls the arm twisting the points base plate advancing the timing. I hope this helps Bob |
| R.A Davis |
| Or to illustrate it more simply. I used to set my points using the dwell feature on my meter (which is really only a voltage reading). But really it's not worth being that accurate.  |
| Lawrence Slater |
| "From nigel, how long would you say does a well built Lucas dissy remain reasonably reliable at its job and how badly out do you think the average dissy is that's decades old and never been attended to? From lawerance I used to set my points using the dwell feature on my meter (which is really only a voltage reading). But really it's not worth being that accurate. This is the basis of my orginal coment These where made in mass im guessing at least a million, and they due wear out, to tune these to dwell is not practical because of the amont of slop they devolop over time... the point I make is to mainly take an avg and use the standard tune in using a feeler gauge and find what you can live with or buy an aftermarket dissy that can be tuned to the level of perfection thats required for a special set up As to LES... hes the man, and he can remachine a lucas dissy to perfection and he is also a highly respected gun smith ... so his engineering skills, machining ability and what im guessing must be an amazing work shop of at least $100,000 value for building and tooling ... he clearly can do that... I and probably most of here cant As to nick... it was simply a shove back, a code he himself admitted he lives by, as do I Someone attacks you without merit or not in jest, then you shove back to even the score so that balance and peace can exist |
| Prop and the Blackhole Midget |
| Prop, The big O called, he wants to know if you can dwell on the Gaza strip for a couple of weeks to make some peace! If it doesn't work out he's got a great little hotel pine box he will let you use indefinitely! Dude....just roll with the changes! http://www.youtube.com/watch?v=eGgLPriZUSA |
| Steven Devine |
| Nigel, Les, or Peter I've read that the Kucas dissys were out within 2-3 years after they left the factory (perhaps it was you that wrote it) so dissys that have been used for decades but not rebuilt are probably well out. How long do you think a good rebuilt one now runs before needing a rebuild? oh Nigel I agree with you on this one good comment. |
| Steven Devine |
| I really wouldn't worry or even consider using the dwell angle to set up the distributer on a car of that age...especially as you are not experienced at doing it. As already stated, the components will all be fairly worn anyway.To get a good starting point, turn the dissy to get max revs and then retard slightly. An engine tuner won't even refer to the spec, it's a matter of adjusting everything to get the required type of running and output by purely trial and error.Once you have it running fairly well, try sight adjustments either way until you get the best results. |
| j hall |
| Prop, if any component of a machine is worn out, you can not expect to adjust it correctly; that's a given. As others have pointed out in great detail, setting the dwell does help. If the distributor is in reasonable condition and the contact breaker set is new, and the fixings are good, it's quite possible to set the points correctly with a feeler gauge and then set the dwell more accurately by further adjusting the points. The reason for this is that, while the points might be dressed 'flat' (often they are not)in practice the current passes through the points along the line of least resistance and that's not always the shortest route. So if the material that the points are made of is 'perfect' then the dwell will be the same as predicted by the points gap. In practice this is not always the case. Further more, a set of points can be made to last longer if they are a bit worn out by setting according to dwell rather than gap. The reason I was short with you, for which I apologise, is that you seem to want to rubbish so much of the advice here from experienced people and post opinions that have either already been expressed or have been overturned, or are just wrong. It's not the end of the world or really very important in the scheme of things but in the little, tiny, insignificant world of old cars, some people enjoy getting it right if possible. |
| Nick Nakorn |
| R.A Davis, Essex, United Kingdom Thanks that helps.....So correct me if I'm wrong.... Setting the timing at full advance @3000 Disconnect the vacuum advance. |
| Steven Devine |
| My understanding is disconnect vacuum and seal at carb end before setting the timing. |
| Graeme W |
| And I appricate that nick, I really do, thank you Your opinion, and my opinion...both very debateable and can be done respectfully Prop |
| Prop and the Blackhole Midget |
| Perfectly put, Nick. |
| Steve Clark |
| Steven The figures I used were for illustrative purposes only and shouldn't be considered correct for your engine. For more information try the link below to Paul Hunt’s very useful website. http://www.mgb-stuff.org.uk/ignitiontext.htm#cent Bob |
| R.A Davis |
| I have never thought about it this way, but with contact breaker ignition it isn't really the gap in the points that matters, its the length of time that the points are open for, and closed for, that generates the spark. From that point of view, setting the points gap using a feeler gauge is a "back door" method of getting the dwell right. Easier to accomplish on a basic engine maintained with simple tools readily available. It was probably only with the advent of electronic measuring devices that it became possible for the average person to readily measure dwell - assuming they bought the meter! So, for roadside mechanics, set the points gap to the specified measure, or with a folded fag packet and the necessary dwell angle will be reasonably accurately set. But once the equipment became readily available, then setting dwell directly is actually much more accurate that the one-step removed method of setting the points gap. |
| Guy Weller |
| I agree Guy. Gapping the points is a back door method, but I don't really think it results in less accuracy on our old engines. That's why I stopped using my dwell meter, since it's quicker with feeler gauges. (Btw I should correct my earlier comment. The dwell meter funtion is really a frequency measurement function, but expressed in degrees). On a new distributor and higher spec engine it probably is superior, but with a worn one, with slack in the system, you get close enough with the gap. I compared measurements once, and using the dwell, gave the same results as using the gauges. I'll dig out my meter again, I haven't used it for quite some time. But this is a superior discussion though. In that it dispells the myths that points ignition is somehow less sophisticated, and crude when compared to so called electronic ignition. It's all electronics, and no less interesting because it incorporates a mechanical switch to interupt the flow of electrons. |
| Lawrence Slater |
| Lawrence, Not a frequency measurement function as it shouldn't change with engine speed rather a duty cycle measurement. As mentioned earlier in the thread the dwell angle has to be high enough to give the coil time to charge fully at max engine speed to provide a good spark. |
| David Billington |
| Lawrence, why l said l (slightly) prefer setting by dwell is that it does away with any innacuracy from pitted points. But l wasn't criticising gapping points as a method. If anything l was guilty of confusing the issue of normal wear and tear with how good the system is. Saying that points are rubbish because you have to replace them after 15,000 miles is a bit like saying that pneumatic tyres are hopeless because they wear out. |
| Guy Weller |
| If you use an electronic ignition amplifier the points can last much longer as the current is much reduced reducing points wear. The heel of the points can also be a source of wear and I have had the odd set bed in and reduce the gap to nothing in a matter of months but generally in my experience they don't wear much or didn't with OE points. |
| David Billington |
| Yup. I stand corrected, duty cylce it is. I've got 3/4 sets of "old" points (OE). Left over from the days that I used to change them for the hell of it. Now't wrong with them. So I won't need to buy a new, possibly less well made set, for a long time and maybe never. |
| Lawrence Slater |
| an incredibly informative discussion guys thank you could you give me some advice on the 23d4 that came with the ex race engine Ive rebuilt. spec is: 948 +60 down cammed from a 649 to a standard cam hs2s cooper s 12g295 head Would it be worth changing it out for a dizzy with a vac advance? |
| D Paul |
| For a street engine, yes I would, the vacume advance will give you alot better fuel economy and probably over all a smoother running engine But It may need to be recurved Prop |
| Prop and the Blackhole Midget |
| Thanks to everybody for this thread, particularly Bob Davis for the basics, very well written. I've learned what dwell is - not so daunting a subject as I had thought. Of course I will have forgotten it all within about three weeks, and the next time I need to know, I will ask you (having also forgotten that there's an archive). |
| Nick and Cherry Scoop |
| From the excellent Datsun1200.com website.. figures for a datsun engine of course.. **********+ Set the point-gap or dwell. Use a dwell meter if at all possible as it is much more accurate than the points gap, Set dwell 49 to 55 degrees. If the points are brand new, a feeler gauge can be used to set the gap to 0.020 inch. Incorrect Dwell can cause rough running, as does bad points. For example, it might only run at high idle, and not very smooth at that. NOTE: If the points are used, do not set them via feeler gauge. They pit during normal operation will which render gap measurement incorrect. ********** i gave up with points and put in the Simonbbc unit and never looked back... |
| Andy Phillips (frankenfrog) |
| "" I gave up with points and put in the Simonbbc unit and never looked back..."" I foolishly upgraded to pertronx then gave up on them and went back.to points As to pitting, nothing a finger nail file, or fine sand paped wont fix and only take 30 minutes (minuse the 25 minutes for hot tea and beer breaks ) every 20,000 miles to service and adjust Im a born agian fan of points and condensers Prop |
| Prop and the Blackhole Midget |
| If your points are pitted you need a True Point contact grinder (pic) Alan  |
| Alan Anstead |
| Very cool alan, Ive never seen anything like that before...can you do piston ring gaps with that also Prop |
| Prop and the Blackhole Midget |
| Andy, I agree - I've never had an issue with points but setting them (and thus dwell) is time consuming if done correctly and sufficiently frequently. I've gone the for an after-market electronic set up too for my 1500. I have no idea if it will be reliable but I don't see why not. My last daily driver to have points and condenser was a Landrover with a V8 conversion and I was adjusting them about once every couple of months - no real chore but not enjoyable either, just time consuming. |
| Nick Nakorn |
| If points need adjusting every couple of months or so, then possibly the heel is wearing prematurely and closing the gap, or the condensor is crap, causing the points to burn back. I check mine once a year, and often don't adjust them at all. But I have OE points in mine. Never seen a machine like that before. I doubt they sold many, since when that was made, points were as cheap as chips. You'd replace them rather than dress them. But I have filed mine flat a few times. PS. Not true that feelers can't be used on pitted points. You get a pit, and a mountain. You set the gap between those. Just a smaller surface area for the arc, but it still applies, albeit less efficiently. But on these old tech engines, it's not really that critical. |
| Lawrence Slater |
| I used to grind the top off the "mountain" with a bit of old cylinder bore hone. Then just set the gap by eye as a feeler gauge won't be measuring from the bottom of the pitted point anyway. I have also used tradditional fag packet card as it was good enough for old British bikes. But more often just set them by eye |
| Guy Weller |
| Ditto Guy. |
| Lawrence Slater |
| Alan Great to see old gear like that still about I've never seen one specially for points before - Prop - no they're different I have an old ring gap grinder that probably came out the same museum as Alan's point grinder -- It's different to the points machine as it has a double sided grinding wheel which is very thin and has a couple of angled guides to feed the ends of the ring onto the sides of the wheel I've had it for several years and can't see anything on the market that would do the job any better to be honest I'll get a pic willy |
| William Revit |
This thread was discussed between 06/08/2014 and 18/08/2014
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