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Old 01-20-2011, 09:48 AM   #1 (permalink)
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DIY Tune-ups for older cars

Something I had to do recently was work on an 85 Subaru Justy. It was running about as well as a pregnant cow on acid... High idle, almost zero engine braking (less than zero under some circumstances...), very little power, shockingly high fuel consumption (12l/100km city), and slow to return (2-4 seconds) to idle. It all pointed to an amateurs efforts to keep the car running. Without specialist tools I was able to rectify most of the problems. This is applicable to most cars with a carburettor and points.

*note: This assumes you have good knowledge of the parts of the carburettor and distributor, and some knowledge of how they work.

1: check static ignition timing. Check your baseline

Manually turn the engine until it is approaching TDC Compression on #1 cylinder (note that on 6 cyl jaguars, this can be the rear-most cylinder) with the ignition off. Disconnect the coil lead from the distributor and run it within 5mm of something metal attached to the engine. Turn the ignition on.

While watching for the spark (or listening) turn the engine VERY slowly. When you hear or see the spark, note the crankshaft position. If the car has not been maintained or adjusted for some time (and is equipped with points) it will invariably be retarded. In my case it was 1* ATDC (yeah, after top dead centre) while the factory spec is 5* BTDC.

2: Investigate why the timing is so far out (no-one in their right mind would deliberately set the timing this far out) and check for correct operation

Turn the ignition off and remove the distributor cap. Can you turn the rotor button in the direction of rotation with little force (but only for a short distance)? Does the rotor button spring back to it's original position? If no to either, the mechanical advance unit is faulty (likely seized). Dis-assemble, clean, lubricate, and re-assemble.

Does sucking on the vacuum advance unit move the points plate? Does it return freely when vacuum is released? If no to either, remove the vacuum advance unit and move the points plate manually. If they do not move freely they are faulty, and should be repaired or replaced (again, Dis-assemble, clean, lubricate, and re-assemble). If they move freely the vacuum advance unit is likely faulty and requires replacement. Also replace the vacuum advance unit if you get any airflow through it (i.e. it cannot hold a vacuum).

3: Check and adjust points gap.

Now turn the engine until the points are open the furthest. There will be a shiny spot on the points cam (shiny because of the constant nylon-metal contact), placing the follower in the dead centre of this shiny spot will result in the widest points gap. The points in the justy were moving so little they appeared to be closed (and actually needed repair). Once you have this point, loosen the screws holding the points in place, and put feeler guages in between the points, to the correct gap (rule of thumb - .015 to .020"). While holding the points against the feeler guages, tighten the screws. Your points gap is now set correctly. Invariably, opening the gap advances the ignition timing. Also, while at this point, it is advised that you clean and lubricate the points cam.

To repair worn points, use fine emery cloth clamped between the points, and rub... Those on the justy were bad enough to warrant dissasemby of the points and use of a bench grinder (should have bought new ones, but with excess time and no money, they were repaired).

4: Now that your distributor is functioning and adjusted correctly internally, it's time to check, and if necessary, adjust the ignition timing.

Repeat step 1. Note the new ignition timing. In the case of the justy, it was some 7* BTDC (merely adjusting the points gap changes the ignition timing a LOT). To set the static timing, move the distributor to its most retarded position (ie the body moves in the direction of rotor button rotation) and move the engine until it shows the correct angle on the crankshaft (5* for the justy). Then remove the coil lead and advance the distributor until the coil fires (ignition on...). Tighten the distributor now.

This is as close to correct as you will get the ignition timing without a timing light

5: Adjust carburettor

Start the engine. Idle speed is likely to have been increased with the correct timing (and it was already high in the case of the justy). The factory issue tachometer is sufficiently accurate for idle speed adjustment. On most carburettors the only idle speed adjustment is the throttle stop screw. Adjust it until the idle speed is correct. Locate the mixture adjusting screw (a screw that typically has a spring on it, that dissapears into the carburettor). Adjust this for the smoothest idle. It is likely that you will have to re-adjust the idle speed screw at this stage.

By this stage, the engine was running a lot smoother, had significantly more power, and had more engine braking. However it still took 2-4 seconds to return to idle.

6: Adjust dash pot

Not all carburettors have this, but Justy's do. When the throttle is released, the last part of its movement is heavily damped. This has two purposes, it reduces emissions (although I'm not sure how, consult an expert on that matter) and it prevents the engine from stalling. This was adjusted to come on way too early (revs would hang around 2800rpm and fall slowly). Release the lock nut and adjust until the end of the dash pot is just contacting the throttle. Wind in a further 1.5mm or so, do up the lock nut. This was sufficient to stop the engine from stalling and provided a smooth enough transition to engine braking while driving.

7: Test drive.

The car was found to be vastly improved in all problem areas (outlined above), however it pinged too much for my liking below 1500rpm at high loads. I'm unsure whether this occurs with a new justy engine, and I don't care. I'm blaming excess blow-by on this 'well run-in' example. I retarded the ignition timing to 4* BTDC. The 1* difference was enough to make a noticable difference to pinging, and the difference in power was not noticed my me.

Also fuel consumption is markedly improved. Unsure how much as we have yet to refill the tank but I'm expecting at least a 20% improvement.

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mnmarcus (01-20-2011)