Sunbeam Recurring Themes

A few issues that occur again and again

I have repaired, restored and recommissioned a lot of Sunbeams and each one has been different in one way or another. But there are a few recurring themes that run through several of the scenarios that I have dealt with. One recurring theme involves a bike that has been recently acquired by its current owner and was restored by its previous owner shortly before he sold it. Often the bike comes with a pile of receipts. The new owner seldom reads through them and is often inclined to accept the previous owner's assertion that the bike has been “fully restored” and the engine “completely rebuilt”. Sometimes it is true. Mostly it is only sort of true.

I have dismantled and repaired a lot of recently rebuilt engines. In the process, I have become accustomed to the realisation that the word “rebuilt” covers a very wide spectrum. Its meaning ranges from the thing that we would all like to think it means through to a partial attempt at rejuvenation by an amateur with only a rudimentary mechanical knowledge.

Another familiar scenario is a bike that needs a fix for an apparently simple fault. It could be a leaking or blown head gasket or it could be difficult starting. The owners often ask me to quote a price for fixing it. I always give the same answer – it depends on what caused it. Everything has a cause. When a cylinder head gasket blows, there is always a reason.

The bike that is the subject of this article is a nice looking S7 Deluxe that was acquired by its current owner some time ago, since when he has not ridden it very far and never on a public road. The previous owner had restored it himself and had “rebuilt” the engine. It looked nice but it never really ran well. He tried a number of things to try to improve it but without success, so he sold it.

The current owner, Bond Ranwell, rode it up and down a short section of private road a few times but it never seemed as if it performed well enough for the open road. On the last of these very short excursions, its performance deteriorated sharply and it started spitting hot gas from a hole in the cylinder head gasket. That was when I became involved.

Bond called me to ask if I could fix the head gasket and get the engine running as it should. He delivered the bike to my workshop in mid-July 2017 and I started work on it a few days later. Lifting the cylinder head revealed a huge gap in the head gasket by the front cylinder, between the front-centre and left-front studs. This the most common location for a gasket failure.

There were two other features of this engine that caught my interest. One was that the brace plate that is normally fitted under the front engine mount had been modified to make it easier to install, by cutting a slot into one of the holes. In effect, that renders the plate useless and, since it is there for an essential purpose, it is not a modification that can be recommended. Even so, it is surprisingly common.

The other interesting feature was that the front cylinder liner had been replaced but the rear one was original. They had both been bored to +0.030”. However, the front liner was not a new one, it had previously been used as a rear cylinder liner in another engine, evidenced by the fact that the original small recess for the locating pin was in the wrong place. A new recess had been crudely filed by hand. Neither had the block been skimmed after the liner was fitted, indicating that the rebuild was done under the constraint of a very tight budget. But why was only one liner replaced? Was it a catastrophic mechanical failure or did the liner crack? Was the failure related to the butchered bracing plate? That will remain a mystery.

There was no indication that the gasket failure had been caused by serious distortion in either the cylinder block or the cylinder head. During removal of the head, I had noticed that the tightness of the studs varied greatly, with none of them being what I would call tight but the two large centre studs at the front and rear of the head were particularly loose.

Sometimes that can happen as a result of a poorly executed tightening procedure but it can also be the result of failure of the threads in the alloy castings. So, whenever I come across a loose stud during disassembly, I tighten it first to test the strength of the threads. Coincidentally, the front and rear central studs are the ones that fail most often. But, in this case, the threads were fine. So, in went a new, freshly annealed solid copper gasket (I almost never use composite gaskets) and the assembly proceeded. Until........

I reached the stage where the valve timing is set by aligning the positions of the crank-shaft and the camshaft. This is normally straightforward but, in this case, it proved impossible to get it even within a few degrees of the correct setup. There is only one possible explanation for this. The timing gears must have been installed incorrectly. The bike would never run properly like that and the valves are very likely to collide with the pistons, particularly if your engine is fitted with the flat-topped variety. This engine wasn't, which was fortunate.

Sadly, it is impossible to resolve the problem without removing the engine from the frame, dismantling the timing gears and reassembling them properly. So, out came the engine, the gearbox separated off, the clutch dismantled, the flywheel pulled and the timing chest opened up. Sure enough, the timing marks were perfectly clear but they were nowhere near aligned. They appeared to have been ignored during assembly. A two-minute job to rectify when the gears are accessible but it involved several hours of work to get to that point.

Observations of numerous small points in the engine led me to the conclusion that the engine had been assembled by an amateur mechanic. Not least the fact that the large flywheel nut had been released and then re-tightened, at least once, with a chisel! Not the mark of a professional. That being so, I checked for a few other common mistakes, one of them being the amount of end-float in the crank-shaft assembly. There was virtually none, which can often lead to over-heating and swelling of the white metal on the thrust face of the bearing. That results in the crank-shaft becoming stiff to turn when the engine is cold, leading to difficulty in starting.

Having spotted the problem, it would be daft not to fix it while the engine was apart. The rear main bearing carrier was removed so that the end-float could be adjusted. At which point it became obvious that the bearing thrust face had been filed by hand. The face was crooked and the file marks were still clearly visible, together with a nasty high-spot caused by a side impact on the bush. I mounted the carrier on the milling machine to clean-up the damage and to remove sufficient metal provide the correct clearance.

While removing the carrier from the engine casting, I noted that one of the studs that hold it in place was loose. Following my usual practice, I tightened it to test the thread, which immediately failed. That thread must have failed during assembly but was just left like that. Did the mechanic think that it would heal by itself? I executed a repair of the thread before the carrier was fastened in place to test the end-float again. This time it was within spec.

Following reassembly of the engine, the valve timing was set accurately, with no problem. The engine/gearbox was lifted back into the frame and final reassembly went without incident. The engine started easily and was thoroughly warmed through, so that the head nuts could be tightened again once it was completely cold.

Next came the test ride, which went very well for the first few hundred yards. But as soon as the bike was running at around 30mph it started to misfire, which became worse as speed increased.

Incidentally, the red covering that can be seen on the frame rails and the black tape on the rear mudguard and horn are protective materials to prevent damage while lifting the engine/gearbox in and out of the frame.

Checking the coil, the condenser and the ignition points revealed no faults but a second test ride confirmed that the misfire was still there. Re-checking the ignition timing and the operation of the auto-advance mechanism with a strobe light revealed only that the timing was spot-on and the auto-advance was working as it should.

The carburettor was the next thing upon which my suspicion fell, notwithstanding the fact that it was a brand new and very shiny 624 Concentric. Stripping it revealed a throttle slide with the wrong cutaway ( a number 3 instead of 2.5), a ball bearing sitting loose in the float bowl, a curious hole drilled into the idle circuit and a partially blocked main jet. I replaced the slide with a new one of the correct type, discarded the superfluous ball bearing, cleared the main jet and tried again. The persistent misfire was still there and the main jet was partially blocked again.

I removed the fuel tank, drained it, washed it, flushed it, dried it and stripped and cleaned both fuel taps. I gave the carburettor a good session in the ultrasonic cleaning tank, to make sure there was nothing still lurking in its crevices. Yet another test ride proved only that the misfire was still there. I then swapped the carburettor for a brand new one out of the box, which I had bought to use on another Sunbeam. This time there was no misfire and the bike performed faultlessly.

So, it was definitely the carburettor that had been the source of the problem. My attention then drifted to the curious hole in the idle circuit, the purpose of which is still a mystery. I searched the ether to see if it may be an obscure modification that has been recommended by an “expert” to improve performance. Most “improvements” of that kind tend to make things worse but I could find no reference to it anyway. The effect of the hole would be to feed unmetered fuel from the float bowl into the air side of the idle circuit, the purpose of which is hard to imagine. I made a small brass plug, which I pressed into the hole to block it permanently.

During the next test ride there was no misfire, the bike performed faultlessly and I could relax and enjoy the fact that it also handled as well as any S7 that I have ridden. I have ridden an awful lot of them.