Wednesday 16 September 2020

The AJcette Shakedown

Anyone that has rebuilt a vintage motorcycle will know that, when you think it is finished and can just fire it up and start riding, the fun really starts! There is always something that needs adjusting or fixing and the AJcette was no exception.

Although the engine felt tight for the first couple of miles, it was encouraging that the bike would go very well when run in BUT I got back from my standard ~3 mile first-run-on-the-road course covered in oil. It was “breathing” more than a bit and if I had ridden another ½ mile the oil tank would have been empty. The problem was that far too much oil was going into the cambox and was simply pouring out and covering the back of the bike. I had already plumbed a small tap into the cambox oil feed and calibrated this to what I thought would be an appropriately small amount of oil but this was obviously far too much. The entire plumbing can be seen below with the tap for the cambox oil feed on the RHS.


The first obvious step, therefore, was to reduce the amount oil reaching the cambox by closing the tap further. Unfortunately, this was starting to move into dangerous territory as the tap was already nearly closed – it was about ½ a turn open and the flow rate at this tap setting is very sensitive to the slightest change. Another simple flow rig was built to test the tap setting and the tap closed down appropriately. Same problem as before - too much oil. Next try ….and now with the tap nearly closed (and lock-wired) and a very low flow rate on the rig. This resulted in ZERO oil flow rate on the engine and 2 cooked rocker skids; luckily the cam was OK. The picture below shows the 2 destroyed rocker skids together with a new one.


It became clear at this point that using a model steam engine tap to control the oil flow rate would not work and a new design would be needed. The difficulty in knowing how much oil is required is that it is pretty well impossible to measure the oil flow rate entering the cambox in a cammy Velocette engine although it would be possible to measure the flow rate of the oil being returned to the engine ...but I didn't have one handy at the time. Oil is forced upwards into the upper bevel drive from “down below” and then passes through slots in the shaft and bush that overlap once per revolution. Any leakage through the shaft and bush, which depends on their clearance, needs to be added to this.

One way to control flow rate is to use an orifice, the simplest type of orifice being a hole in a plate. The downside of using an orifice is that if the hole gets blocked there would again be zero oil reaching the cambox. This can be mitigated by using an appropriate sized filter upstream of the orifice and my revised solution therefore consisted of a self-contained flow control device that consisted of an orifice and a filter.

The first incarnation of this is shown below.



This was set up initially with a 1mm diameter hole as this gave a rig flow rate equivalent to the last (reasonably) successful flow rate, ie a flow rate that did not destroy bits of the engine!

In spite of reducing the orifice diameter to 0.4mm (testing each time on the flow rig) and introducing a much finer 100 mesh filter (this should, theoretically, stop any particles that would block a 0.4mm orifice) there was still too much oil “breathing” out of the cambox.

As I did not want to use a hole less than 0.4mm, attention was now turned to the oil drains in the cambox and how to ensure that oil was not prevented from coming out due to the downstream pressure. On the Velocette engine, the oil from the various gullies is collected and fed back into the crankcase and into the inlet valve guide. This was obviously not entirely successful as Velocette introduced a scavenge pump on the end of the camshaft on the KTTs to feed excess oil back to the oil tank.

On the AJcette I had the option of either feeding the oil into the bottom of the timing case or about halfway down the timing case where there is a hole to check operation of the chain tensioner.  In both cases, there are pressure pulsations that result from the crankcase pressure and there is therefore no guarantee that oil will flow, under gravity, into either.

Another flow rig, this time on the engine itself, was therefore constructed to check independently the oil flow rate out of the cambox by collecting it directly in a container and also to check that oil would flow into the engine and, in the case of the latter, by inserting a one-way check valve into the oil line. The check valve is another item from the model steam engine world and uses an extremely low inertia shuttle. The hope was that, even if the check valve was not perfect with the high frequency oscillating pressure, it would allow oil to flow in only one direction and might even provide some suction.


Testing of this arrangement showed that it worked and the final solution was to incorporate the check valve and a small catch tank into the oil line. It also provided confirmation that oil was reaching the cambox …and approximately how much.


The cambox lubrication was the only major problem that needed solving and the bike has subsequently been ridden a few hundred miles, including a trip to the 2019 Manx GP, without further issue.

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