Golden oldie – Get the shotgun out! By Des Davies, Top Gear Motor Services Vehicle: MGB Roadster 1.8L 1972 Mileage unknown as it had gone around the clock The customer said he was too frightened to drive the vehicle because of the backfiring – it was difficult to start and running poorly. This was the worst case of backfiring I had experienced in my whole career, producing the loudest banging noise that I have ever heard. It was difficult to start and in too bad a state to drive – it would have probably ripped open the exhaust system or exploded the vehicles behind me. Therefore, I had to recover the vehicle into my workshop. This car took me back a few years to when I was tuning these vehicle systems in the late 70's and 80's. I used to enjoy it so much and I gained a wealth of knowledge, skills and experience from these diagnostic methods.
connected to the battery positive cable near the starter motor. The battery was located at the rear underneath the seat, so this was a quick and easy, non-intrusive testing method (work smarter not harder).
Luckily, I still had my Crypton Engine Analyser CMT 2000, a brilliant bit of kit in its day. What a machine, but no good without the operator! It was gathering dust in the corner of my workshop and after the year 2000 you could not update them, which was a shame. The training I had up in Nuneaton with Crypton in their training centre was excellent, they had first class instructors and I learnt so much.
This test could indicate if the valves were sticking because of excess carbon on the valve seat or stem, stopping them seat properly and then exploding in the exhaust system, but all looked good, see Figure 1.
But back to the present, this vehicle is owned by one of my first customers when I started up my garage business in 1987 and it is his pride and joy. So, I set about getting my brain in gear and get it running again.
CASE STUDY
First things first
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There were no fault codes or live data to help me here with this 49-year-old classic car, so I needed to use that computer between my ears! I began with a visual check of the engine and ignition system to see if anyone had previously worked on it and fitted the plug leads back in the wrong locations. I looked for traces of carbon tracking on the leads, a problem with the distributor cap or a cracked rotor arm – igniting the incorrect cylinder at the incorrect time – but all looked initially good. I have come across a few vehicles with exploded and damaged exhaust silencers and melted exhaust pipes in my time, due to stuck valves caused by carbon build up, and flames coming out of the air filters! Because the vehicle had not been used for a while, I decided next to check the relative compressions using an amp clamp
Fig 1 Relative compression test was good
I like to do a battery SOC test first to make sure that the battery can perform to the manufacturer’s requirement before commencing the relative compression test and this proved to be good, see Figure 2. Checking the ignition timing was the next step, to see if it was igniting the spark at the correct time, see Figure 3. It was time to use my redundant strobe light. This was also within spec, and this vehicle had electronic ignition fitted to the system, which was also good.
