Peter phoned me this morning where he is having an issue with the car stuttering for 4 seconds when the throttle is depressed.
His mechanic seems to have checked carburation and the distributor.
He says that one carb is running weak and the other rich and says the vacuum advance is working.
Anyhow I am posting here some things I think should be checked.
Use
timing disk or a good strobe gun.
With vacuum advance pipe disconnected, static advance should be zero deg or TDC, whilst at 2000 rpm it should be 22 deg. The advance curve is an approximate parabola created by two linear lines (defined by the strength of the springs) in between those two rev points. The advance should climb smoothly and there should be less than 4 degrees of scatter between cylinders. The exact advance curve depends on the weights and springs but these often wear to create too much advance after starting. Also the advance stop wears to give too great a maximum advance.
With the vacuum advance pipe on, when the throttle is fully and sharply depressed, an additional 10 degrees (no more) should be added for up to 4 seconds whilst the revs climb. Do not let the revs climb to more than 3500 rpm. Make sure the plate inside the distributor moves smoothly when the vacuum is applied and that it moves smoothly back to rest. This can be checked by sucking on a suitable pipe attached to the vacuum inlet with the distributor cap off. The vacuum advance, which is 10 degrees more than the dynamic advance caused by the weights and springs, should be the same across the rev range, even from 750 rpm.
Sometimes the engine may start and idle better with a static advance of up to 3 degrees BEFORE TDC. With modern petrols or tuned engines, an increase in maximum advance may also be possible and give more power before pinking or pre-ignition occurs. The onset of pinking is best detected going up a slight incline at half throttle when a tapping may start in the engine. This can cause serious damage and the ignition should be retarded until it stops. Note that changing the max advance by retarding the engine may make idling and starting worse.
More info in
this topic.
and the dizzie specs
here.
I have discussed the carburation issue at length with the engine rebuilder in Germany. They even fitted an electric petrol pump to perhaps remove one possibility. There are so many possible causes of weak and rich mixture, and with the added dimension of the inherent imbalance in the Jowett inlet and firing order, diagnosis is a challenge. But please feel free to give some assistance to Peter.
Note that Peter has tried three different sets of newly rebuilt carburetors in search of perfection, one from the engine rebuilder and two from JCS.
If the car starts easily without choke it is probable the mixture is too rich, possibly caused by leaking float needle valves or too high a petrol pressure. Low petrol pressure can cause the engine to die when the throttle is depressed. A hole in the float causing a rich mixture is also common and can be checked by shaking them. Washers under the needle valve control the petrol level.
Leaks around the carb gaskets, worn throttle shafts, a leak in the pipe going to the pintel valve on the oil filler or bad pintel valve can cause a weak mixture. Leaks in the float chamber gasket will upset carburation. The carb bodies can be easily distorted, so check fit with engineer's blue.