All that Glitters is not Gold: A Day on a Rolling Road

Part 1

My 1955 TF1500 is a lovely largely original car (see TTT2 issue 31 front cover) and has won many awards during my 4 years of ownership but, as Shakespeare nearly said, “all that glitters is not gold”. Poor starting and a rough tick-over plagued the XPEG engine, despite a fully rebuilt set of SU carburettors, a simple electronic ignition (Hall effect type), and coil. It was time for a serious investigation. The impetus came from a series of articles written by Paul Ireland, initially in Totally T-Type 2 on-line magazine (issues 40-42) and subsequently re-printed elsewhere. These articles addressed, inter alia, the subject of timing and the impact of modern ethanol petrol on XPAG engines.

I also took note of the comments of John Saunders on Paul’s article in Issue 40.

An opportunity presented itself to address the problems in more detail; namely a day on a rolling road, no, not ‘Drive It Day’ but the rolling road at A B Garage, Hawarden, North Wales. The local MG Owners Club organised a day for 14 cars (mostly MGBs) on the rolling road; the idea was to obtain a basic assessment of brake horse-power at the road wheels and engine torque throughout the normal range of engine revs, from which some possible improvements in performance could be deduced. Some members came away afterwards with that smug air of contentment and some of us were, perhaps secretly, disappointed with our cars’ performance. I was in the second group.

My car returned peak bhp at the wheels of 24.5 at 3,450 rpm [originally 63 bhp at the flywheel or perhaps 54 bhp at the wheels] and peak torque of 170 lb ft at 2,100 rpm after which it dropped to 94 lb ft at 4,000 rpm [my rev limit on this test as I did not know the true condition of the engine]. Adrian of A B Garage suggested the ignition needed another 2 degrees advance but, more importantly, bigger filters than the fancy after-market chrome things on the car fitted by the previous owner.

Back home I advanced the tick-over ignition by 2⁰ to 9⁰ BTDC

and fitted a pair of original type Vokes filters for my SU H4 1.5” carburettors. Feeling a lot more upbeat, I returned the car to Adrian for a full assessment. This is what he found:

1. The connecting spindle between butterfly valves was slightly loose, causing the front SU to run at a lower input than that called for by the accelerator – i.e. loose!

2. The front carburettor was running much richer than the rear, as measured by the position of the jet at the shoulder. Surprising, as all 4 plugs looked the same – honey coloured central ceramic and black round the edges.

After making these corrections, a further emissions test was carried out and Adrian concluded that the jet needle profile was incorrect. It was ok at tick-over and full throttle, but too weak in the mid-range, which was where most driving occurs. New needles with the correct profile were fitted and the subsequent test was to Adrian’s satisfaction.

At this point the car was tested on the rolling road and, oh, what a difference! Power at the wheels was 36 bhp at 3,600 rpm in 3^rd gear, with a steady increase from tick-over at 13 bhp, but a sharp drop from 36 to 22 bhp at 4,000 rpm. Similarly, the torque rose rapidly to 179 lb ft at 2,000 rpm and remained nearly level to 173 lb ft at 3,600 rpm, but also dropped rapidly to only 91 lb ft at 4,000 rpm. Adrian’s conclusion was that at 4,000 rpm [my normal limit equating to about 70 mph with my 5-speed box] there was valve bounce suggesting valve springs needed replacing [see graphs of 2 test runs at the end of these articles]. Driving home, the car ran a lot more smoothly, tick-over was less lumpy, and it seemed to be much more responsive. Time to address the diagnosis.

Part 2

Following the valve bounce diagnosis, I re-read the Paul Ireland articles and came to the view that not only did the valve springs need replacing but that I should pay more attention to the advance curve – did it address the modern ethanol fuelling problem? My distributor is an original A2D4 type and there did seem to be a bit of sloppiness, mainly noticed when trying to set the tick-over advance – this wobbled about somewhat. I also noticed the rotor was skimming one or two cap terminals, leaving fine brass everywhere.

Turning to local MG Octagon Car Club gurus on T- Types, Dave & Ray, it was decided to replace the valve springs by pressurising the cylinders in turn, but during this process a leaking head gasket was suspected as well. Off came the head and sure enough it was leaking, although caught before any damage was done. Whilst the innards were exposed, we noted that the car had been re-bored only once (+20 thou and no liners), suggesting a very low mileage engine, and the bores, pistons, bearings and rings were near perfect.

A quick de-coke (wire brush in an electric drill) and the engine was re-assembled. Everything was fine, cylinder pressures were good at 168-178 lb/in², but still the car was lumpy at any speed, starting was difficult, and on over-run the exhaust popped and banged. I was not a happy bunny and nor was the car! I then did what I had thought about doing for a long time; fitted a fully electronic programmable CSI distributor – expensive, but was it worth it? The old dizzy will be properly rebuilt next winter as a spare and for any future owner to return the engine to original.

So, back to A B Garage again to tweak the carburettors, timing, and anything else needed, and then test on the rolling road. First, the distributor advance curve was set at 10⁰ on tick-over, 26⁰ at 2,000 rpm and 38⁰ at 4,000 rpm or higher (curve 7 on CSI). Back on the rolling road we observed a steady high torque at 178 lb ft, declining to 168 lb ft at 4,500 rpm, and a steadily increasing bhp that was still rising beyond 45 bhp at 4,500 rpm. A good result for 3^rd gear at the wheels, which equates to about 53 bhp at the flywheel. However, tick-over was still not smooth, even though engine performance at all higher speeds was excellent on every measure, including coil, plugs, and HT leads’ performances. Also popping and banging on over-run had been eliminated. RESULT! Was it worth the expense? YES!

As I understand it, Paul Ireland’s research tells us that XPAG and similar older engines are not designed to run on ethanol dosed fuels (bio-fuel), because combustion space design does not allow sufficient turbulence for petrol to vaporise fully during the “suck and bang” cycle, such that several cycles in succession will be faced with varying fuel mixtures. This leads to a condition known as “cyclic variability”, whereby the timing of each cycle’s combustion is slightly different, leading to bouts of smooth & lumpy tick-overs. Exhaust gas analysis during the 3^rd rolling road test also showed high hydrocarbon emissions, strongly suggesting incomplete combustion, but only at tick-over. This causes the slightly lumpy tick-over experienced on my car but does not affect engine performance above tick-over speed. Similar cars will also vary from each other depending on other physical characteristics such as engine condition. Early on-road testing up to 4,500 rpm (75 mph) demonstrated a smooth responsive outcome so I’m quite pleased! I next plan to experiment with the advance curves, I have 16 to choose from, to see if that improves tick-over without compromising overall performance; I’ll report in part 3.

The graphs follow (click to enlarge). Neil Wallace