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MG MGB Technical - Distributor Cam Lobe
I normally post on the MGA site. I have an 1800V engine in my MGA. It is currently working fine with a 45D Metro distributor.
I have recently had my DM2 (MGA distributor) totally rebuilt professionally and supposedly reprofiled to the 1800 engine, at the same time an Ignitor (pertronix) electronic ignition was fitted.
I have so far been unable to time the distributor. Setting it to 32 degrees advanced at 300RPM (vacuum disconnected) has left it impossible to start - too far advanced on static. Setting it to about 10-12 degrees at idle has left the engine running flat above 2500RPM.
I took the distributor apart a couple of days ago and found that a 7 degree cam lobe was fitted. It had been filed to increase the advance.
And so to my questions:
1. What cam lobe is normally fitted to the 1800 distributor?
2. Can the lobe's mechanical advance be measured on the workbench? If so, how?
For starters, I measured the radius of the lobe (from the distributor spindle to the point of contact on the mechanical advance stop post. I then measured the gap between the lobe face and the post in the static postion. Drawing this up on paper gave a lobe rotational movement of 8 degrees. Is this the correct methodology?
|Typo Error. That should have read 32 degrees at 3000RPM, not 300!|
|If you set it to 10-12 degrees at idle, what reading do you get at max advance?|
I was getting about 28 degrees. This ties in with what I have measured so far: 8 degrees static plus about 8 to 10 (x2) mechanical from the doctored 'mutilated' cam lobe.
The 8 degrees static is a presumption because I cannot measure it with my electronic ignition.
most 1800V engines will start and run OK with static timing at 10deg. The normal dissy plate will have 10deg advance , which of course doubles to 20deg on the crank, making total advance of 30deg.
|How does filing the cam lobe affect the advance? For a given relationship with the rest of the engine any removal of metal can only retard the ignition, I'd have thought. But then when you retime the engine you would do so to the original specs. But then maybe it is a question of terminology - to me the cam lobe is the thing that opens and closes the points (where fitted). What alters the centrifugal advance is the stop-plate in the base of the distributor to which the springs and weights are attached. If you file that it increase the amount of *maximum* centrifugal advance i.e. at high rpm, but that has no effect on starting. Where do you have vacuum advance connected? Is it functional? If the capsule spring is very weak, and has a large travel, and the vacuum take-off is on the manifold then you could well get a large amount of additional advance during cranking, which can make it difficult to start, and is why carb vacuum is best.|
If you make a test rig capable of driving the distributor and a degree wheel, connect it up to a coil and some plugs, and use a variable speed drill and a standard timing light, then you can measure the maximum centrifugal and vacuum advances.
The 45D for a UK 18V was a 41610 with 7 degrees static, 10 degrees at 1000 strobe, maximum additional centrifugal advance of 24 degrees (hence a 12 degree stop-plate) at 6400 rpm, and vacuum advance starting at 3 in. Hg. and topping out at an additional 24 crankshaft degrees at 11 in. Hg.
|Paul Hunt 2|
We may be talking cross-terminology. To understand where I am coming from see here: http://www.jcna.com/library/tech/tech0015.html
Filing the lobe increases the amount of mechanical advance because the lobe can rotate further before it hits the stop. I have just done some precise measurements on my lobe. It had been filed from the original 7 degrees to 8-9 degrees. I have now removed 1.12mm to increase its rotation to 12 degrees of movement. This will give me 32 degrees at 3000RPM and 8 degrees static.
Before hand, when I dynamically timed the distributor to 32 degrees at 3000RPM, it left me with a static setting of about 16 degrees and I could not then get the starter to turn the engine against the pre-ignition. If I then retimed it to 8 degrees static, the engine would only advance to 24 degrees and ran out of performance above 2500RPM.
On the other part of your question, I disconnect the vacuum advance at the unit. It is brand new and appears to be serviceable. The vacuum pipe from the carbs is fine - runs my other distributor ok.
|Steve. I have covered this on the MGA BBS. What you are referring to is the mechanical advance stop which is a part of the points cam. Filing of the cam would do nothing except change the dwell time. That point is what is confusing Paul. As you note, filing of the stop arm of the mechanical advance will allow the weights to provide more mechanical advance than was programmed into the original distributor. |
I believe that, if one reads the thread on the MGA BBS, they will understand what I am posting about. There are two distinct phases in the distributor: no advance and full advance. When the dizzy is at very low rpms, it should show no advance. When it is at a certain point, in engine rpms, it should show full advance. These are givens. At rest, on advance. At some certain speed, full advance.
It is the time in between which is of concern. And this is known as the "mechanical advance curve" which Paul has listed on his website for various distributors, including the one I have recommended to you as a "basic ideal curve" to use as a basic reference.
Overall mechanical advance is a wonderful thing. One should have one which comes on at the desired time. But, except in racing use, having a mid-range mechanical advance which is not suited to the application can cause engine damage. An advance of 32 deg BTDC is very desirable. If one is turning 3,000 rpm or more. Having that same advance at 2,000 rpm can result in engine damage.
Thus, the curve is fully as important as the overall advance.
|Well, yes, terminology as I said. Whilst the cam controls both the opening and closing of the points and the maximum advance as that article says, the lobes of the cab open and close the points, it is the stop that controls the maximum advance. These parts of the cam aren't labelled in the pictures in that article, which is probably where the confusion arises.|
|Paul Hunt 2|
|Steve, File the timing stop arm until you have about 5/32" between the stop and the end of the arm. That'll give you about 10+ degrees of advance. Couple that with a timing setting of about 10 BTDC at idle and you should have 30-32 degrees advance at full rpm. Whether or not its all-in at 3000 rpms is dependant on the springs that Aldon supplied you with and the weights that are installed. |
This thread was discussed between 07/12/2006 and 14/12/2006
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