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MG MGB Technical Out-of-Spec Ignition Dwell BBS discussion at MG-Cars.info
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MG MGB Technical - Out-of-Spec Ignition Dwell
| My '67 GT had an 18V engine in it when I bought it. The engine has always had plenty of power and looks like it had been rebuilt not too long before I got it. But it always had ignition problems, the main symptom being a misfire under load, especially in cold weather. A year ago, I was gapping a new set of points when I accidentally set them too far apart (.019"). The engine ran a lot better that way, and the gas mileage went from 17 to 20 mpg (city driving). Thinking I measured the gap wrong, I checked the dwell and found it was about 56°. So what would cause an engine to run poorly at the recommended dwell? The carbs are o.k., timing is set at about 17°BTDC, and the timing mark on the crankshaft pulley is in the correct location. The thread about the cam being out of sync with the crankshaft prompted my question, but my symptoms seem milder than those. Thanks. -G. |
| Glenn G |
| Glenn. What distributor are your running. While the Lucas 25D4 has a dwell of "60deg + or - 3", the Lucas 45D4 has a dwell of "51 deg + or - 5". Hence, your 56 degrees is quite close to being in specification for either type of distributor. Also possible that your actual points gap might be a little smaller than the .019" you are measuring depending on how you are taking your measurement. (It is done by feel and can be quite subjective, especially if you have not had someone experienced set up the gap, then let you feel how much drag should be present on the feelers gauge leaf.) Bottom line, if it works better, make a note of it and use that specification on your next tune up. Les |
| Les Bengtson |
| Glen - Since you have a dwell meter, set the gap to give you the correct dwell (point gap and dwell are two different measurements to accomplish the same end). Prior to installing a Pertronics solid state ignition, I always set the gap on the points using a dwell meter. Cheers - Dave |
| Daved DuBois |
| Checking dwell to see if the gap needs adjustment is certainly easier and less invasive than using a feeler gauge, but if the correct dwell reading results in an incorrect gap or vice-versa, there is something wrong somewhere. With old points it will occur if they are spiked, with new points it could be sloppy distributor bearings. Like Les I think your 56 degrees is only marginally out of spec for either distributor type, and wouldn't make that much difference. Running badly at that dwell also indicates something else is wrong. 17 degrees BTDC under what conditions? Static or dynamic? If the latter what revs? What distributor do you have, type and reference number? |
| PaulH Solihull |
| Paul. One thing I encountered, back when I was investigating things ignition related, was that most of the distributors I took apart and checked out had unequal lobes. By this I mean that when the points cam was removed from the distributor, placed between centers on a lathe, and a dial indicator used to determine the exact height of each points cam lobe, the height varied somewhat. In the best cases, the difference was .001" to .002". In most cases, the difference was .003" to .005". In the worst case, there was a difference of .017" in the height of the lobes. I wonder if Glenn has a distributor with unequal points cam heights? Two things support this supposition. First, the fact that the engine ran better with a lower dwell (larger points opening) than standard. This would be the case if there was a "low" lobe on the points cam which was not allowing the points to fully open on one cylinder and not providing a proper charge to the secondary windings of the coil. The second cam related item is ignition timing. When the points are opened at different times, because the cam lobes are of unequal height, the ignition timing may be set correctly for cylinder number one, but will vary on each of the other cylinders. The cam opens the points, this induces a charge in the secondary windings of the coil, the coil voltage builds up until the spark plug is fired. If the points are opening at anything other than the exact same place at each 90 degree segment, the ignition timing for that cylinder is something other than what it is on the number one cylinder where the dynamic or static timing is initially set. Relatively easy to check timing on number four cylinder since the timing marks are the same as for number one cylinder. More difficult to check for variation on cylinders two and three where the timing marks are 180 degrees out. Just a thought. Les |
| Les Bengtson |
| To answer questions, the distributor is a Lucas 25D4 (not sure where to find the reference number). There is no discrepancy between dwell and gap readings; the discrepancy is between published specs and performance. I am surprised that +.003"/-1° has made a noticeable difference, but it has. Maybe I should try an even larger gap and see what happens. Timing is set with a strobe at 1000-1200 rpm. I've never heard pinging, so could probably advance it some more. If I find time, I'll try to check the distributor cam lobes with a dial gauge. Thanks for your thoughts. -G. |
| Glenn G |
| That dwell should not make any difference. Check actual delivered voltage at coil hot terminal, engine running. Check that you actually have a 12V coil. 67 MGB should have Lucas HA 12, which is a high performance coil, normally used on sports models. FRM |
| FR Millmore |
| As FMR said the dwell when set "reasonably close" will not make any difference but by opening the point gap you advanced the ignition timing which did make a difference. Denis |
| Denis4 |
| That's why you must check/reset timing after changing the gap. The reference number is adjacent to where the body is stamped 25D4, typically 4nnnn. In 1967 distributors for the high compression engine were 40897, 10 static, 14 at 600 strobe. The curve data is such that at 1000-1200 rpm you should be seeing 20 to 23 degrees BTDC as the *additional* advance (i.e. additional to static) at 900rpm is 9 degrees i.e. 19 degrees, vacuum disconnected and plugged. However with modern and low octane fuels you may get pinking at that. Low compression used 40916, 8 degrees static, 12 degrees at 600rpm. At 1000-1200 rpm you would be seeing 17/18 degrees. Because of the changes in fuel since the specs were arrived at the best timing for high compression engines is just short of pinking at any combination of throttle opening, load and revs. Low compression engines may stall the starter before they pink. Based on the timing figures you are using, and not having any pinking, if you have an HC engine it looks to me like you could benefit from a bit more advance, higher octane will advance more than lower and you get the benefit of improved performance if you use it, and economy if you don't. |
| PaulH Solihull |
| Without removing the distributor, it is difficult to see the numbers stamped into the raised rectangular boss on the side, but I can read "0897 E" and below that, "68". I guess this would indicate a distributor for a high-compression engine if the 0897 is simply missing the leading 4 (or I can't see it). How do I find out if the engine really is a high-compression build? Compression check? What should the results be for normal versus high compression engines? The spark coil is a stock Lucas unit purchased from Moss. It seems to do the job, but I am considering a higher-voltage coil. The car has always had weak ignition. When I replaced the side-entry distributor cap with a top-entry version and molded spark plug wires, performance and gas mileage improved noticeably. Thanks. -G. |
| Glenn G |
| Lucas coils have Model, part number, date of mfr stamped on the bottom; "Stock" from "Moss" does not help. The fact that you seem to have a problem removing the distributor might tell us that the distributor has not been serviced or examined for correct function since you got it. Also, that you may not know how to set timing from scratch. All 18V engines are low comp in the US, unless modified. Later ones also have retarded cam timing. Is there a full number? Which would tell us what it started out as, but not necessarily what is is now - designations in Moss catalogue. You can look into the cylinder with the piston near TDC, and see how deep the dish in the top is. LC are about 1/2", HC maybe 1/4", and flat tops are definitely aftermarket/performance. Compression is usually around 140 or less for LC, 160 HC, but these are subject to variation either way by enough to confuse the issue, for numerous reasons. 18V also had single row timing chains, subject to significant wear, hence late cam timing. Run like pig, eat gas! FRM |
| FR Millmore |
| Almost certainly 40897 then as the numbers often had a suffix letter. What do you mean by weak ignition? How did you measure it? The spark should be able to jump about 1/4" with a standard coil. Your coil could well be faulty, as could anything else in the system. A higher voltage coil won't increase the HT voltage as that is governed by the spark gap. If replacing cap and wires improved things noticeably, and it ran before, I'd say you were unlikely to get much more of an improvement, but the condenser is another possibility. Unfortunately the offerings from many sources are pretty rubbish these days, and could well makes things worse. Compression figures given in the Workshop Manual are 130psi for LC, 160 for HC. That would be measured hot, all plugs removed, throttle wedged wide open, and no oil in the cylinders. But it's confused by later UK HC engines being quoted as 170-190psi, and American 8.0:1 i.e. LC engines being quoted as 160psi. I'd missed you had an 18V, what is its 18V prefix? There were at least 10 different distributors for North American 18V engines which were all LC as pointed out, most with very different curves to the 40897 which is an 18G HC distributor. |
| PaulH Solihull |
| Almost certainly 40897 then as the numbers often had a suffix letter. What do you mean by weak ignition? How did you measure it? The spark should be able to jump about 1/4" with a standard coil. Your coil could well be faulty, as could anything else in the system. A higher voltage coil won't increase the HT voltage as that is governed by the spark gap. If replacing cap and wires improved things noticeably, and it ran before, I'd say you were unlikely to get much more of an improvement, but the condenser is another possibility. Unfortunately the offerings from many sources are pretty rubbish these days, and could well makes things worse. Compression figures given in the Workshop Manual are 130psi for LC, 160 for HC. That would be measured hot, all plugs removed, throttle wedged wide open, and no oil in the cylinders. But it's confused by later UK HC engines being quoted as 170-190psi, and American 8.0:1 i.e. LC engines being quoted as 160psi. I'd missed you had an 18V, what is its 18V prefix? There were at least 10 different distributors for North American 18V engines which were all LC as pointed out, most with very different curves to the 40897 which is an 18G HC distributor. |
| PaulH Solihull |
| Guys, I appreciate the thoroughness of your replies, but I was really looking for more general ideas to answer my question, which was "What would cause an engine to run poorly at the recommended dwell?" And at this point, unless you have something to add, I believe I have enough to work with. These are the things I'm going to look at: - Engine. Determine compression ratio. - Distributor. Determine what curve should be used for the engine's compression ratio. I have a spare distributor that possibly could be rebuilt to best specs for the engine. - Coil. Upgrade to hotter spark -- why not? Safe to assume that Moss stock coil is not as hot as sport coil. Coils are less efficient in hot weather. It is hot as hell around here. - Spark plugs. Try larger gap with new coil. - Timing. Find point where pinging begins at various loads, adjust for no pinging. And Fletcher, I have no idea why I would need to set the timing from scratch when I have a scratch awl. I guess some people like to do things the hard way. Thanks. -G. |
| Glenn G |
| Glenn- In that case, the answer is: dwell only affects the quality of spark, but in the range questioned will have no effect, so there is another problem. Might be in actual spark, or what you are changing in the process. Hence, we do yammer on! A "hotter" coil might run hotter as well, which means you might not want one under HOT conditions - or mount it on the roof. MGB ran perfectly well under all conditions with HA12 coils, 25D distributors, and .025 plug gaps. If you increase plug gaps beyond .030, you may create problems with caps and rotors - that's why the factory went to 45D when they fitted electronic ignition and wide gaps. Advance curves are not determined solely or even principally by CR, which is why I mentioned cam timing, and the general importance of knowing the configuration/condition of the engine. You can only determine the specific advance curve requirements on a dyno; even then, it is rare indeed for anyone other than the factory to be thorough about this in all possible usage conditions - how many dyno rooms can be cooled to -40F or +140F? And that's awl I'm gonna tell you, so go scratch! FRM (tuned thousands of engines, never owned a dwell meter) |
| FR Millmore |
| Glen, The one thing you will absolutely need to do to return the car to spec is reset the timing. Changing the dwell will change the timing of the spark. If you reset your timing to spec and your old conditions return, I would suggest that you are running with the timing too retarded. Thus advance till you get pinking and retard a wee. Better still, set your timing at approx 3200 for max advance using an advancing timing light. Max advance is way more important than your timing at idle. Alex |
| Alex Waugh |
| Shocking echo in here ... |
| PaulH Solihull |
| Sorry, guys, I have not been able to spend any quality time with the engine lately. Yes, Alex, timing is on my list. I do normally check it after setting the dwell, but haven't tried a bigger advance in the past because of specs. However, we can pretty much throw specs out the window as so much has changed since they were written. Fuel, for instance, not to mention the cobbled-together, rebuilt and relatively unknown status of my particular engine. Which, Paul, is an 18V890AE-L1846, head patent #585394. Adding to the difficulty is the need to drive the car to work every day, making it necessary to limit diagnosis and improvements to fit that schedule. Fletcher, I have surely tuned a few engines a thousand times and only use a dwell meter when something doesn't add up. (I actually bought the thing 30 years ago and found it was inaccurate for the six-cylinder engine I was working on at the time. Almost threw it away, but found it works fine on a four-banger.) How would you recommend checking the cam timing? Can it be done without removing the front cover from the engine? On the coil, I was thinking of using one with an external ballast to mitigate the effect of heat. Thanks. -G. |
| Glenn G |
| You can check the cam timing if its the stock cam by the amount of lift at the inlet valve at TDC. From memory (but I would not swear to it) is .060".open at TDC. Others will know for sure. This wont give you the timing only how accurate its set. Denis |
| Denis4 |
| An 890 L is a low compression, and the original distributor for that would have been a 45DM4 41851 or 41853. They were very similar - Strobe 10 degrees at 1500 rpm, Centrifugal max 16 degrees at 2600 rpm. Vacuum capsule Starts 3 in Hg. (41851) or 5 in Hg. (41853), Ends 11 in Hg., Max 24 degrees. The 40897 was 14 degrees at 600rpm, 20 degrees max at 2200 rpm, vacuum capsule starts 5 in Hg., ends 13 in Hg., max advance 20 degrees. A 6v coil (make sure it is 1.5 ohms) with a 1.5 ohm ballast will halve the heat in the coil - from the current flow at any rate. But I've been doing some tests lately and at an ambient of 10C on the bench simulating running a 12v coil stabilised at 30C and the 6v with ballast at 22C, i.e. about half the rise in the 6v as the 12v. However at a similar ambient driving they both ended up at about 40C, and I know they get hotter than that in summer. But they work perfectly satisfactorily in desert states, so if anyone is having a problem at less than that there is a fault that needs to be found and fixed, not modified. The ignition system you use can make a significant difference. A variable dwell electronic system should result in a significant reduction in coil heating from current, typically one eighth at 2000 rpm, only at max rpm would there be as much current heating. However I have read that fixed dwell electronic systems have more dwell than points, resulting in more current heating effect. The claim is that this results in a better spark at max rpm, and no drop-off in performance at high revs. But Lucas stated that the points system is perfectly adequate up to 8000 rpm in a 6 cylinder engine, and what's more important is that the V8 has half the dwell of the 4-cylinder and has no trouble revving into the red. A 4-cylinder MGB would disintegrate before it ran out of sparks. |
| PaulH Solihull |
This thread was discussed between 02/03/2013 and 12/03/2013
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