QuestionQUESTION: Hi Barrie:
Here is a question I have not found any answer too. It would be great if you recognize something familiar. Here are the details
Some years ago I bought a third hand '80 MGB LE model. I really did not know what I was buying and not long after the purchase I needed the engine to be rebuilt because of serious compression problems. I really do not know if the engine is original because the timing marks are underneath the pulley and the distributor was an OPUS type that had the original (power transistor on the outside of the heat sink) Allison electronic ignition added to it. The mechanic that rebuilt the engine told me that the hot valve stem to rocker clearance needed to be 0.014" (intake) and 0.016" (exhaust) rather than the 0.015" value in the manual. I do not know why. I have a down-draft Weber carburetor.
Now for the strange part. The rebuilt engine timing must be advanced to 30-35 degrees at 800 RPM (carburetor vacuum disconnected) to run well. This is, of course, well beyond the 10-15 degrees in the manual. In fact I find it hard to imagine why an engine would need that much advance at that low an RPM.
If any of this makes sense to you I would be very grateful to hear an explanation.
ANSWER: I don't think the engine timing is wrong, just the timing marks.
The 1800cc engine fitted to the MGB came in various forms. There were 3 different front plates, which all had the timing marks in different places, and there were 3 different front pulleys to match them. It is possible that your engine either has the wrong pulley or the wrong front plate.
I always prefer to set valve clearances when hot, but the valves do expand when they get hot, so they require different settings from the cold ones.
---------- FOLLOW-UP ----------
QUESTION: Hi Barrie:
Thanks for the quick response. I have made a number of measurements in the last week including a static measurement of the piston #1 TDC position to verify that the pulley mark aligns with the TDC mark. I used a wooden rod to determine the top of the stroke and verified it was the compression stroke by noting that the intake and exhaust rocker-arms were loose and the rotor arm was pointing at the distributor segment wired to spark plug # 1. I repeated the measurement several times and each time the pulley mark was within a couple of degrees of the TDC mark. I also used a 12 volt light on the LT side of the coil to statically check that the distributor was "turning-on" approximately 30 degrees BTDC; matching the "advance" measured dynamically.
I interpret these results to mean that there is no mismatch between the front-plate and the pulley. Am I correct about this?
I should note that the distributor segment wired to spark plug #1 was the 8:00 o'clock segment rather than the 2:00 o'clock segment I see in the literature. I do not know of any problem with this set up other than it is not the conventional one.
ANSWER: Your investigations make good sense. They do indeed confirm that you have the earlier cover and the matching earlier pulley.
The distributor has a drive unit underneath it with an offset slot. This can only be inserted in one position. However, the drive shaft underneath has a skew gear with about 12 teeth on it. I guess it has been inserted 180 degrees out (because your rotor arm points to 8 o'clock instead of 2 o'clock). If so, you should extract the drive gear and re-insert it at the correct angle, then swap plug leads 1 with 4, 2 with 3 and turn the body of the dizzy through 180 degrees. The end of the drive gear has a 5/16 UNF internal thread, so it can be pulled out using one of your rocker cover studs. Fiddly, but it can be done.
This still leaves us with the puzzle over excessive advance.
How are you measuring the advance required to `run well'? If you are merely advancing the timing to get the highest rpm on tickover (vacuum disconnected) then I can well imagine that it will be 30 degrees. However, this would produce massive pinking when driving the car.
If the car doesn't pink, then possibly the mechanical advance is not working correctly. If it is stuck, then you would need 30 degrees to get a decent amount of power at 2500 rpm and above.
One thing is for sure, 30 degrees static advance plus another 30 degrees mechanical advance makes 60 degrees total advance and driving it like that would rapidly cause major damage to the engine.
If you want to follow this up further, it would help if I knew the full engine number, including the prefix (18GK or 18V/582/H or whatever). Also, was this originally a UK spec with twin SUs or a USA spec with a single Stromberg?
---------- FOLLOW-UP ----------
QUESTION: Hi Barrie:
I have seen, on the Web, specific instructions for changing the distributor drive unit. Thank you for identifying this problem in the engine rebuild.
I would like to continue to pursue the crazy "advance" measurement that I am getting. You sure know more about this thing than anyone I have access too. So as long as your patience and thoughts hold out I would like to continue this conversation. I am an older fellow that drives neither fast nor aggressively. So I can only report that up to 3000 RPM there is no pinging nor indication that the advance exceeds the Peak-Torque. I have not set the advance to maximize the RPM but really only to get a comfortable sounding 800 RPM idle. I do not know where to set the advance.
Engine description: The engine number plate on the RHS of the engine block has been removed. The vehicle number is GVVDJ 2AG 508323 and stamped 10/79. As described above the distributor was an OPUS type which could have been fitted in '79 or '80. I have recently changed that distributor out with a Pertronix FlameThrower. The "advance" measurements did not change with this installation. I do not know what the original carburetor was. When I bought the car it had a side-draft Weber. This was too much for me and I had it replaced with a down-draft (32/36 DGV) Weber which has a vacuum port.
I am not a mechanic but would try to make any measurements you suggest. You have identified one problem with the engine rebuild. I suspect that there is another somewhere. Any thoughts will be appreciated.
AnswerThe UK distributor for a 1979 car with 2 SUs and no catalyst was set at 10 degrees at 1,000 rpm increasing by another 22 degrees at 5,400, totalling 32 degrees.
The USA spec for a 1979 car with one Stromberg and a catalyst was 10 degrees @ 1,500 increasing by another 35 at 4,500, totalling 45 degrees.
So, depending on which distributor you have, the figures you quote might not be that far out.
Personally, I am very wary of maximum advance totalling more than about 34 degrees, so the USA spec seems excessive.
Vacuum can be forgotten, because we are only concerned with advance under heavy load, when any additional vacuum advance is minimal.
I hope the vacuum is taken from the Weber carburetter, and NOT from a take-off on the manifold.
By the way, I strongly recommend Pertronix Igniter systems.
If you remove the action plate inside the dizzy and look under the advance weights you will find a number. On a UK car it will be 11, indicating half of the advance figure of 22 degrees. On a USA car it could be 18. We can discuss some more once I know this number.
