The Trials of owning a Sprite or how long it takes to cure a miss-fire.
Those of you who went to Bordertown last year would know that my Sprite continually had a miss-fire and after a quick adjustment to the distributor points gap at Bacchus Marsh the situation became worse. Fortunately the weather also became worse and between Bacchus Marsh and Ballarat it became necessary to turn on the headlights and at this moment the miss-fire became a thing of the past. Well the story started a little earlier than October because the very occasional miss-fire became very noticeable on the way to Leongatha for the OST in August. So in August the distributor points were replaced and, as the carbon button in the distributor cap had a very elongated hole, the distributor cap was also replaced. A test drive along the highway indicated that the miss-fire had not gone away and therefore it was decided that the condenser in the distributor should also be replaced. The night drive home from Leongatha was uneventful and the car ran reasonably well.Perhaps I should tell you that my normal method of replacing the points is to remove the distributor from the car and do the whole job on the bench and then refit the distributor to the car. To assist this process I have a white mark on the distributor body that aligns with a white mark on the distributor clamp. Correction fluid is an easy way to make this mark before you remove the distributor. The distributor itself has no vacuum advance and has 14 degrees of advance on the weights and up until this time had not given any major problem other that that the car was always hard to start for the first time during the day. Lots of cranking was required and a very weak spark at the spark plug seemed to the reason for this problem. Occasionally the car would hesitate under acceleration as it appeared not to fire properly but this seemed to improve when the engine was warm.
So now that we are back home from the OST and before Bordertown I changed the coil and placed the new (second-hand) unit on top of the generator because it is easier to change the unit here. The original position requires two spanners, small fingers and a prayer to remove and replace the clamp bolt for the coil bracket. A quick test drive indicated that the coil was working but the hesitation was still present. But surely the car will get to Bordertown without a problem, a spare coil in the corner of the boot will be added insurance.
Well those that went to Bordertown know haw slow the car was on the hill out of Bacchus Marsh and that only after the lights were turn on did it have any semblance of performance.
The logical conclusion to be drawn is the somewhere we have an earth problem within the ignition circuit and that turning on the lights gives an alternative path for the earthing of the circuit.
Well after Sue Dow’s social drive into the Dandenongs the performance was less than average and even the lights on system seems to be failing so a serious look at the problem is in order. Out with the modified distributor (No. 1) and in with a unit from the garage that used to be in the car but was rejected because of engine changes and the need for a set amount of advance within the distributor. Old distributor (No. 2) in the car and no miss-fire is evident. Remove the cap from the miss-firing distributor and place it on distributor No. 2 and that car still runs perfectly. I am now sure that the problem is within the modified distributor and therefore an inspection is in order. Well two things immediately are evident, one is that the base plate has been brazed to prevent any movement and then sprayed with a silver coloured paint and the second is that there is no earth wire present between the base pate set screw and the base plate itself. (Obviously this earth wire is deemed to be totally unnecessary by those that build modified distributors and only adds extra weight to weigh down a true race-car.) Well the first test is to put a multi-meter in resistance mode between the body of the distributor and the base plate. Guess what, no continuity. OK I’ve got patience so I strip all the paint from the base plate and re-install the base plate. It now has continuity so I can put it back into the car. A refit to the car indicates that it is still misfiring but perhaps the condenser (new) is faulty or perhaps the pigtail wire to the points has an intermittent break. OK well take off some units that we know work and place them into the miss-firing distributor, No. 1, to eliminate any potential problems with the condenser and pigtail wire. Place the miss-firing unit back into the car and run up to about 3,500 r.p.m. Yes you guessed it the miss-fire is still present. I am now certain that we need the earth wire between the distributor base plate retaining screw and the base plate. Out with the distributor and with the aid of the soldering iron and some spare parts from the garage I now place in the missing earth wire. I now have a happy running car that started instantly when the starter was pulled and after a drive at varying speeds refused to miss-fire.
The real mystery is that the car had run for a least three or four years before the miss-fire became a real problem. The only explanation I can offer is that for some reason the distributor clamp had always acted as an earth until last time I removed the distributor to service the points and from there it was a trip downhill all the way.
If you have a modified distributor and still use the good old fashioned points it may be prudent to see if your distributor has the earth wire to the base plate as one day this may just let you down when you least expect it.
Whilst my experience was with a Sprite it could apply to any car with a like type of Lucas distributor.
Anthony Young