The advantage of this more modern unit is that it has a Vacuum Advance facility which may prove useful with modern fuel. After fitting it I then needed to modify the advance curve to suit the TA engine. To do this I altered the “maximum stop” and “rate of advance”. Previous experience gave me settings that I have used to evaluate the results.
Firstly the car goes much better; the engine feels more responsive. What really indicates to me “improved fuel burning” is the condition of the exhaust valves.
An advantage of the MPJG engine is that having removed the spark plugs and by turning the engine on the handle, the exhaust valves can be seen through the plug holes.
Their condition has improved greatly. Before, they looked like they were struggling against incomplete combustion.
Another indication of the more complete combustion is the exhaust temperature. Sometime ago I installed an exhaust down pipe temperature sensor (see photo).
Using the temperature as a guide, an optimum timing could be arrived at. This time I drove at 2500 rpm, about 43 mph, several times along a test route (level road about 2 miles long) and recorded the temperature with and without the vacuum advance connected. The temperature of the pipe was about 410 degrees C with the additional advance and about 35 degrees C greater without. Lower exhaust temperatures mean lower under bonnet temperatures, cooler carburettors and less vapour lock but most importantly indicates more complete combustion before the exhaust valve opens.
At speeds of 30/40 mph the inlet manifold vacuum is allowing the vacuum advance unit to apply probably 10-20 degrees additional advance (crank). Conversely any engine running 10-20 degrees “late” will be inefficient, run too hot and burn out exhaust valves and seats.
To modify the distributor:-
With the distributor on the bench in a soft vice, first remove the points’ plate by removing the 2 screws and wire grommet. Lift the plate, noting the peg which locates through the vacuum advance rod. When re-assembling it may be better to loosen the vacuum unit’s screws for extra wriggle room (can be quite a fiddle).
Next remove the 2 advance springs. These are quite thick and difficult to pop off.
Lifting out the cam requires a puller, I made one (see photo). Holding the cam in is a small nylon O ring. Centre dot the top of the shaft for the puller point to locate.
The photo also shows the “maximum stop” (arrow) where a small amount of filing will increase the movement. The plate will have a number stamped on it: 8 or 12 for example – this is the distributor factory maximum degrees. File the stop to achieve 16 degrees (32 crank). Use a protractor stuck onto the cam top with blue tack to measure the angle of movement. I replaced the 2 springs with some much lighter which I think came from the Moss spring packet.
Spring dimensions mm
- Wire 0.75 o/s length 17 o/s diameter 5.3 coils 7
- Wire 0.6 o/s length 16.5 o/s diameter 4.94 coils 6
The cam fits over a nylon guide ring which only goes one way around. Check that the cam moves its full 16 degrees before putting the springs on and the O ring.
Checking the advance curve with engine running is helped by the use of an adjustable strobe lamp. Some experimentation with different springs may be necessary, a tedious job. I initially set my idle at 10 degrees/700, vacuum disconnected (connected 28 degrees) since reset to 5 degrees.
The attached graph shows 4 curves:-
A is a typical Metro A+ high compression 9/10cr curve.
B is typical standard T type.
C is John Saunders’ curve. You may have seen John’s article of modifying a type 25 distributor to fit an XPAG in the May Octagon Bulletin. 8.8 cr.
D My curve, ahead of the others to compensate for modern fuel in a low cr MPJG engine 6.6 cr.
I also changed the vacuum unit to help with the slower burning in the low cr TA, from a typical Metro unit 54424167, 3/9/8 to a 54425067, 3/13/12. The last number 12 is the maximum vacuum advance of the unit (x 2 for crank ie 24).
The Metro unit would probably be fine.
At 30 -40mph the vacuum from the inlet manifold is approximately 10-15 inches giving an additional 15-20 degrees advance on top of the 25-30 centrifugal. This is substantially ahead of the standard advance.
My testing has shown me the advantages of vacuum advance. If to protect my engine I have to fit a different distributor…..then I have to!
I run a number 2 carburettor needle. This is a little richer than the AC which I always felt the TA needed.
I use Redex lead replacement additive even though I have hardened exhaust seats.
The tests use standard 95/5% ethanol pump fuel.
As yet there has been no time for fuel consumption tests.
The graphs are known as “wide open throttle” i.e. the correct timing when there is no vacuum available. The vacuum advance is in addition to this and continues to vary as the throttle is changed in response to load. When plotting yours, disconnect the vacuum pipe and plug it on the inlet manifold side.
I am experimenting with idle timing between 5 and 10 degrees (no vacuum) 23-28 with. This moves my graph start point between 5 and 10 degrees. I need to do more testing but with winter here things are on slow.
So far I have limited my testing to about 3000 rpm, my normal maximum.