Thread: Ignition timing for 2l spread bore

15 degrees compared to 10 degrees - is that retarded or advanced?

Hi Scallops

An old dizzy can be worn in the shaft and any wobble there, can upset the timing by a huge amount by the varying the points gap, check that, then at the same time look at the balance weights, the vacuum linkage and the plate that it operates, making sure that their movement is free and not sticking any where.

There is a flexible wire that earths the moveable plate inside to the body of the distributor, if this has gone open, the earth path is then down through the bearing and the arcing that it can cause, puts the spark anywhere in the timing.

Back fire can also be caused by a leak of air through the manifold gasket.

To test, carefully spray a little volatile gas such as aerostart around the intake manifold gasket joint, do this while the motor is running at a fast idle and listen for any change in the rhythm of the idle.

Needless to say do this test outside well away from other objects, do make sure that your insurances are up to date and the wife is well off.
Oh yeah, but don't tell her! otherwise she may be hiding behind the car with a lighter!

Cheers Arthur

Originally Posted by Scallops

15 degrees compared to 10 degrees - is that retarded or advanced?

Hi Scallops

The spark should occur before top dead centre, as a rule, the spark should be advanced more for poorer quality fuel.

Cheers Arthur

Thanks Arthur - but I think I know what the problem is now...especially as it was virtually fine 24 hours earlier....I now know that 10 degrees is retarded from where it was...and retarded timing can cause backfiring....first thing to do is advance the timing again before I take the carby back off.


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Originally Posted by Scallops

Thanks Arthur - but I think I know what the problem is now...especially as it was virtually fine 24 hours earlier....I now know that 10 degrees is retarded from where it was...and retarded timing can cause backfiring....first thing to do is advance the timing again before I take the carby back off.


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Hi Scallops

It isn't popping from the exhaust is it?
That cause is by a burnt exhaust valve and/or seat, that is a quiet different thing to backfire though the carby, a burnt valve could be a highly probable cause of your troubles

Backfire is the incoming manifold mixture being ignited,by the too far advanced ignition, this is caused by the inlet valve not being fully closed for the cylinder under compression.

The ignition being too far advanced, also causes the mixture charge in the cylinder to burn more fully before top dead centre, this causes pinging or rattle when the motor is pulling hard up hill or accelerating.

On a motor that has trouble cold starting, retarding the spark stops kickback.
This is a lesson learnt the hard way if you try crank start a motor with too lean a mixture and with the timing advanced.
They are not joking when they say, that you keep your thumb out of the way when using a crank handle, it is for this reason!

the inlet valve not being fully closed

There is a whole host of reasons for this to occur;

Timing issues, as we have just discussed.

Burnt inlet valve seat, not impossible but unlikely.

Sticking valve stems from a build up of crud from a life time's history, a top overhaul and decoke fixes this.

Bent valve stems, unlikely, unless the motor has been drowned.

Timing chain problems, allowing the camshaft to lose it's timing position in respect to the crankshaft, can happen if the motor is very worn and sick, so unlikely in your case.

Cheers Arthur

Also, I now realise the vac advance knob was adjusted instead of something inside the distributor.....so I'm confident the problem is not with the carby or any other mechanical parts....because the vehicle drove fine the day before, so I think we'll simply start by undoing what we did last night.


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Well, after playing with the vac advance knob and the mix crew, I've got it to rev without backfiring! Stay tuned (pun intended)...

Hi, there are 2 Philips screws that hold the plate that the points etc sit on, you will see them when the cap is removed. They come lose and give all manner of problems.
Measure your point gap pulling the shaft towards the points as wear in the shaft bush will give problems.
"Remember 90% of all fuel problems will be ignition."
Dennis