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Golden Oldie – Get the shotgun out!

By autotech-nath on November 1, 2021

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.

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.

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.

First things first…

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 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).

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.

Figure 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.

Figure 2: Battery SOC test.

Air and fuel checks…

I then removed the spark plugs to visually check their condition, they were black with soot, indicating that the mixture was rich. They were in a fairly good condition, so I cleaned them and adjusted the electrode gaps with my feeler gauges to the manufacturer’s specifications.

It was now time to use my trusted 4 Gas Analyser to check the air/fuel ratio and to see how the combustion process was working.

The idle mixture was: CO 9%, HC 1760ppm, CO2 7.5%, O2 0.01%

Cruise 2,000 rpm : CO 9%, HC 1940ppm, CO2 7.0%, O2 0.02%

This was not good; I would need a petrol tanker to follow this vehicle!

Figure 3: Ignition timing marks on front crankshaft pulley.

The most common reason why exhaust gas leaks into the intake air stream through the exhaust valve or cylinder head gasket is improper tuning, resulting in excess fuel being delivered at high rpms.

This causes incomplete combustion inside cylinders with
large amounts of unburned oxygen left over after each power stroke. When too much fuel enters these cylinders, some of that air/fuel mix leaks out the exhaust valve as exhaust gas and explodes in the exhaust system.

I removed the air filters and hunted for my carburettor balancing gauges and then got to work on balancing the carburettors at idle and cruise speeds. I then adjusted the mixtures the best I could – you need plenty of patience here! It took me a while to get them near perfect, as the carburettors were worn, and the linkages also needed renewing.

Figure 4: Cruise speed 2500 rpm emissions.

New carburettor needle jets and linkages should have been fitted to get the vehicle running better, or ideally a set of new carburettors, but like most customers they want to spend as little as possible on their vehicles. We don’t do miracles here!

However, after balancing the carburettors, I was very pleased with the resulting 4gas readings, see Figure 4.

Finally, I added fuel treatment to the fuel tank to help clean the fuel system and eliminate the carbon inside the combustion chambers and around the valves. I took the car on a road test and the vehicle ran well, resulting in a happy customer. The car was now fixed!

Using your experience, knowledge, manufacturer data, the correct tools and equipment, and understanding the basics of systems, will help you to diagnose faults, and fix the vehicle correctly.



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