Welcome to our resource for MG Car Information.
MG MGA - Wrong advance curve?
|My 1500 had a flat spot or misfire at 3000 rpm. I had fitted it with a new 25D4 distributer with the number 41101A supplied as a replacement distributor for an MGA 1500 by Moss UK. I was not happy that this distributor was right for my engine so I exchanged my worn out DM2 40510 for a reconditioned one of the same type and number. I seem to have a real problem with the advance curve on this reconditioned unit. If I set the static ignition timing to 7 degrees I end up with 50 degrees at 3000rpm (vacuum line disconnected). With the ignition set at 34 degrees at 3000rpm, I end up with 0 degrees at 700rpm. What's going on?|
|Tell us all?|
Why would you want to set the static timing at 7 degrees After top dead centre?
Set at 34 degrees at 3 gran and ending as zero on tick over sounds about right to me? Should run fine
A poor advance curve will NOT cause a misfire IMO but that said the dissy you are trying to fit sounds spot on.
|Robert (Bob) Midget Turbo|
|I didn't say after top dead centre. |
DM2 40510 should have 12 degrees mechanical advance not 17+, correct?
Two things going on:
1) You have once again been taken by the "reconditioners", as many people have.
2) Perhaps you are beginning to learn that flailing about throwing parts and money is not how you fix problems.
At the very beginning of all this I suggested some checks and procedures which would have told you what each distributor was actually doing.
You have now found out that distributors are tricky buggers, and those who sell them more so.
I have seen recon distributors with wrong cams, wrong advance weights, two different advance weights, bent shafts, worn shafts, wrong springs, no springs, stripped screws on the breaker plate, no ground lead. Sometimes ONE distributor has most of these faults!
Thing is, I don't see many recon distributors, much preferring to do them myself, for pretty obvious reasons. Others, like Paul Hunt, Les Bengtson, and now Steve G, report similar experience. It appears that the "Recon" con men throw all parts in a barrel, and tell the blind assemblers to make distributors.
You KNOW you have distributor problems.
You KNOW distributors are misrepresented.
FIND the correct spec curve for your engine.
Plot out the curve yourself, on your engine.
If you can't get a distributor stable enough to read the curve, keep trying.
Otherwise, I don't know the situation in the UK, must be one old fart with a dizzy machine someplace; but, by now you could have shipped it to Jeff Schlemmer/Advanced Distributors and had it back all correct, probably for less money than you've already sunk.
As I understand it, it is impossible for you to get 50 degrees of advance with 7 degrees of advance and vacuum disconnected.
Your 12 degree cam hits the spring post and stops at 24 degrees advance (12 x 2); add on to this the 7 degrees static and your maximum possible is 31 degrees. If you are getting more then you have one of 3 problems:
1. Timing light error.
2. Plug leads incorrect for timing light. e.g. my timing light did not like wire plug leads. Only worked accurately with fibre type.
3. The cam marking may say 12 degrees, but a previous owner has filed the cam to 22 degrees.
|I assumed Lidsey that from you description of the problem that you must be using 7 degrees ADTC otherwise how did you end up with 50 when full mech advance was put in at 3 gran?|
If you only want 24 degrees then you are right this dissy will not work but 34 degrees is not the end of the world and will allow you to use less octane fuel and NOT pink at mid revs as many MGAs running on low octane fuel do.
How does the engine run with the timing set at 34 degrees max advance?
|Robert (Bob) Midget Turbo|
|It's a DM2 Distributor, not a 25D4 that has the positive stop on the cam plate. The DM2 seems to stop when the weights just reach the end of their tether. It appears to me that there is wear on the posts that the weights pivot on and this is allowing the weights and thus the cam to travel further than it should in both directions. I have calculated from what I am seeing on the crankshaft that the advance range on the distributor is 21 degrees instead of 12 degrees. One of the weights is stamped with 12 degrees the other has nothing on it. I have refitted the 25D and with 7 degrees static I am getting 33 degrees at 3000, I wasn't able to achieve this state of affairs when I had the original distributor drive spindle fitted, so maybe that was my trouble all along. As FRM says, I am a victim of a bad reconditioner so I will be taking it back to the MGOC on Monday.|
|I use the Distributor Doctor <firstname.lastname@example.org>. He has beduilt a distributor for my racing Frogeye spot on. The MGA DM2 is sat on the bench ready to go to Martin to be rebuilt - I expect the same excellent service. Peter|
|P B Chappell|
|Sounds like impressive stuff:|
Now I know where to buy a rotor arm if I need one.
|Lindsay. "It's a DM2 Distributor, not a 25D4 that has the positive stop on the cam plate."|
I am not sure what you are speaking of. The MGA Workshop Manual does not identify any distributor part as the "cam plate" on the DM2 distributor. Nor, does the MGB workshop manual note any part of the Lucas 25D4 distributor as being called a "cam plate". Both model distributors have a "cam", which is attached to the upper end of the distributor shaft and activates the points, causing them to open and allowing them to close. On both model distributors, the points cam has a positive stop which limits the movement of the points cam, when activated by the centrifugal force of the rotating weights, and sets the degrees of mechanical advance for the system.
Properly done, the rebuild of a distributor takes time, the correct tooling, and machines. All of which runs up the cost of the rebuild. A properly rebuilt distributor will cost about as much as a newly manufactured distributor and may cost more.
First, the distributor to be rebuilt must be stripped, cleaned, then all of the parts are inspected for visible wear. Then, the run out (out of roundness) of the distributor shaft and points cam have to be checked by mounting it between centers (I use one of my lathes) and checking for run out using a dial indicator (clock gauge). Then, the two parts are checked as an assembly for combined run out. Run out, on either item, means that the points will open at a slightly different time--early on the lobe where the run out is most, late on the lobe which has less run out and somewhere in between on the other two lobes. The opening of the points is what collapses the magnetic field in the coil and allows the voltage to build up until it is sufficient to ground itself--i.e. to fire the spark plug. Thus, points opening directly affects the ignition timing and variance will cause the timing to vary by cylinder.
A secondary effect of run out is the dwell time, the amount of time the points are closed and the coil is charging. The worst distributor ever brought to me had a combined run out of .047". Thus: .000", .017", .047", and .018" deviation on the points cam when zeroed on the lowest lobe. Remember, the points gap is .016". If the points had been set using the zero lobe, they would have never closed on the other three lobes. If the points were set on the either the .017" or .018" lobe, both of those lobes would fire properly, the zero lobe would have significantly different timing and the .047" lobe would not allow the points to close and would not fire it's cylinder.
This is just a single aspect of a job which has a number of vital aspects, all of which must be considered during the rebuild process. Then, the system must be tested, on a distributor test machine, and any minor tweaking done to ensure that the distributor meets the proper specification.
Here in the US, you can buy a "rebuilt" distributor for about $60 or you can purchase a properly rebuilt distributor for whatever it costs to do the job properly. (I paid over $200 for one properly rebuilt distributor and do not regret it at all. This was not for an MG, but does reflect what it may cost to do the job properly.)
You will have to decide what you want to do. But, a specialty rebuilder, rather than a retail outlet, is a good place to start.
|Sorry Les, I am using the wrong words to describe what I mean! The 25D has a plate attached to the base of the cam which is stamped with the degrees of maximum mechanical advance. It is easy to see how this works because the projection on the plate hits the post. I couldn't see what was limiting the maximum travel on the DM2 without dismounting the bob weights, and at this stage I don't want to do that because I am returning it to the shop and I don't want it to look like I have been fiddling with it, but it it didn't feel like there was any kind of positive stop. I shall certainly have some strong words for them, in my dictionary it says "Recondition. To repair, to make as new." !|
This thread was discussed between 10/10/2009 and 11/10/2009
MG MGA index
This thread is from the archive. The Live MG MGA BBS is active now.