Ignition Adjustment – Amazon

Maintenance of your (old-timer) Volvo

In order to have continuous enjoyment of your Volvo, it is important to maintain it thoroughly. There are a few important points of attention like valve clearance and the ignition. These will have to be (re-)adjusted every 10,000 km. When the car runs on LPG and the head does not have hard-seating valves then it is wise to (re-adjust) the valves every 5,000 km.

The Ignition

Combustion time

When the cylinder is filled, an ignition of the mixture by a spark of the spark plug will follow. It will take some to completely combust the mixture.

This combustion time is dependent on the kind of fuel (octane level), the design of the combustion chamber, fuel air ratio, and filling level.
Depending on the degree of filling, the combustion time remains almost equal at any number of revs.

Combustion pressure

During combustion of the mixture, the temperature in the combustion chamber will rise sharply. Consequently the gases will expand, which will put pressure on the piston: the combustion pressure. This combustion pressure is the highest at the end of the combustion and at the smallest space.

The combustion pressure should be at maximum level when the piston has just passed the top dead centre (TDC). The subsequent expansion of the combusted gas will then have the highest impact. In order to achieve this, we will have to (taking the combustion time in mind) start the combustion before the top dead centre (TDC).
This means that sparking of the spark plug will have to commence before the top dead centre (see pic).

During the combustion time, the piston travelled route A and the camshaft passed angle X.

At increased revs, the moving speed of the piston in the cylinder will increase, which means that the combustion time (which will remain the same of course) the piston will take a longer route and the crankshaft will turn a few degrees more.

Now the spark will have to jump earlier in order to maximize the combustion pressure when the piston has just past the top dead centre.

During this same combustion time, but with higher engine revs, the piston travelled route B and the crankshaft went through angle Y.

This means that the ignition, at increasing speed, always has to start earlier. In other words the number of degrees before TDC will increase.

For the B-18 engines the ignition will be at 21-23 degrees before DTC and 1500 rpm, which corresponds with 10-13 degrees before TDC when idling (all without vacuum advance).

Mounting contact points

Mounting of the contact points must be done in such a way that the thread of the contact points only makes contact with the thread that leads to the coil, so make sure that the insulation between breaker housing and mounting bolt remains intact.

Adjust contact points

The contact points should be adjusted with a maximum opening of 0.4-0.5 mm, which, for a B-18 engine, corresponds with a contact angle of 59-65 degrees. The adjustment can be checked both statically (with help from feeler gauges) as well as dynamically (by using a contact angle meter).

By turning the crank, also the distributor drive shaft will turn. The aim is to place a cam in such a way that the contact points open fully. The distance between the cam and the contact points reflects the number of degrees of the ignition. From 30 – 0 is before TDC (so before ignition), from 0 till 10 is after TDC (so after ignition). The adjustment is done correctly when the light turns on the moment the number of degrees pre-ignition is opposite the small cam (contact points open). Do not forget to turn the ignition. If this is not the case, then the breaker housing will have to be unlocked, loosened and turned. Turning to the left is fewer ignitions; turning right means more ignitions.

You will need a stroboscope lamp in order to adjust the ignition dynamically. The stroboscope lamp will flash the moment the spark plug of a running engine sparks. By placing this lamp on the spark plug cable of cylinder one with an induction clamp, it will flash the moment spark plug 1 sparks. By shining the flashing lamp on the fan belt pulley, one can read the number of degrees before ignition. By turning the breaker housing one can now also (re-)adjust the ignition time.

Although the latter is the easiest, there are few things however; one needs to keep in mind: by shining the lamp on the pulley, the lamp will come very close to the running cooling fan. In order to use this method, the vacuum tube of the breaker housing will have to be disconnected.

Do not forget to maintain other areas and parts of your Volvo as well, like:

  • Brakes
  • Checking and changing oil
  • Valve adjustment
  • Changing of tyres
  • Checking and possibly changing the lights

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