Mystery misfire

[COMMANDER]

I am trying to resolve a misfire on my 1960 948cc minor. It occurs when increasing the throttle after starting. It coughs and splutters, revs drop and backfires. Timing is set at 5 dedrees BTDC. Points at 15 thou. Tappets at 12 thou. All valves working as they should. But I have found that the vacuum advance has a leak at the (screw on) connection at the distributer. Removing the vacuum advance mechanism and sucking on the mechanism top shows that it is working, so no damaged diaphram So my question is could this leak at the connection be the cause? The car has not been for a run sinve this occurred.

[myoldjalopy]

Fix the leak and you will find out!

[COMMANDER]

Good thinking Sir.

[IslipMinor]

Things that help the engine to run cleanly after a cold start:

• Timing is correct, and plugs and points gaps are set correctly - STATIC timing at 5° BTDC is fine, as is 0.015" for the points and plugs should be 0.025" with a standard coil. How did you set the timing?

• Both centrifugal and vacuum advance are working properly - definitely no leaks on the vacuum advance!

• Mixture is set correctly and sufficient 'choke' is being used

• Carburettor dashpot is topped up with oil - engine oil is fine

• Tappet clearances set to 0.012" for inlet, but 0.015" for exhaust (unleaded fuel burns slower and hotter, so slight increase on exhaust clearance is now recommended)

If all these are set up correctly, it should run nicely from a cold start.

[COMMANDER]

Thanks Richard.
Most of the above OK. All tappets at 12 thou. Timing by timing mark and test lamp. Believe mechanical advance working (dist. shaft springs back) Plugs at 25 thou. Since all was ok previously just the leaky vacuum possibly. Am devising a method of sealing the connection, then we shall see.

Chris.

[myoldjalopy]

If "all was OK previously" (hopefully not a long time ago) and you have noted a leaky vacuume pipe connection, then that leak is the prime culprit. It may be a nice, easy fix!

[COMMANDER]

That vacuum leak was indeed the culprit. Starts much more easily and revs nicely. Not yet ben out though as the weather is foul. Next trip us to my friendly welder for a little tlc.

[myoldjalopy]

Thanks for coming back and glad you got it sorted. Nice easy fix too - we like them!