YAMAHA TX750
Mark Haycock delves deeper...
After sorting the contact breaker bits, I did need to dig a little deeper with the Yamaha TX750.
After removing the rather inaccessible and tiny auto-advance drive pin with forceps (Photo 1), I could take off the large circlip and that odd-shaped plate, then using a couple of 6mm screws, pull out the component underneath (Photo 2). The shaft for the contact breakers is actually an extension of the oil-pump drive and this component (called the rotor filter cover) stops oil from reaching the points – or is meant to – but it has been the subject of a Service News item. The old cover, which had a single oil seal, was replaced by one with two seals (like mine), one behind the other, which sounds strange, but I bought and fitted the new replacements anyway, along with the correct outer sealing O-rings.
Oddly, the shaft has a sudden change in diameter with a very sharp edge (Photo 3) custom-made to cut the seal lip when putting the cover back. There is an unobtainable cone-shaped Yamaha special tool for this, but I thought maybe I could use a bit of thin sheet metal or tape instead. It didn’t work, of course, so I had to just shove it in and hope for the best (that has always worked ok for me). Before we move on, have a look again at Photo 3: what is behind the rotor cover? Yes, nothing but empty space, so where is the rotor filter that is being covered?
We shall see in a minute, but for now I am going to look a bit further back at this odd feature: two flattened metal pipes (Photo 4). I was surprised by this, and to see why let us look at the spare engine (Photo 5). That strange-looking thing is the oil filter, so what has happened with the ‘working’ engine?
I think I need to explain the history of oil filtration on the TX750. When first released, the engine had no less than
three separate oil filters – well, maybe two-and-a-half! There was a centrifugal filter (like we saw on my Honda CL350) incorporating a magnet to pick up ferrous contaminants, which occupied the mysterious space behind the contact breakers. There was also a curious looking filter comprising a set of disks. What form the disks took I cannot say as I have only seen a rather fuzzy photo of the assembly. It sounds quite comprehensive, but evidently it did not work as after a short while the collection of filters was replaced by a single canister filter.
As you can see, Yamaha found that there was no room for a screw-on conventional filter so they devised this weird flat device fed by two snaking metal pipes, which wrapped round the top run of the rear chain.
Looking at the back of the semi-circular plate, we can see that what has happened is that someone has sawn through the metal pipes and blanked them off with grey filler (Photo 6).
After removing the component upon which the filter is mounted, which is called the oil pump cover (strange name, as it does not cover the oil pump), we can compare this with the same part from the other engine (Photo 7). Someone has driven out the ball bearing which is meant to block the hole. I think this is actually the filter bypass valve, which opens if the filter gets blocked.
What is going on? Evidently, someone decided to take off the filter and run the engine without it. Why would they do that? One reason is that this is a part which is unique to this short-lived model and as a result it is obsolete. Strangely though, Yamaha still list its retail price: £147, which must make it the most expensive motorcycle oil filter in the world.
Ok, I needed to think of an alternative, but I was very disappointed to see that the engine had been used without an oil filter, which was bad news. I did think I might fit a remotely mounted conventional canister filter, but then I found that the TX750 club in Germany has developed a solution. They have produced a design which uses a filter from a BMW R65 or R80, which is mounted in the space occupied by the ‘oil pump cover’. To do this, it is necessary to replace the oil pump cover with a specially made component (there is a dimensioned engineering drawing) and I have knocked a couple up (Photo 8), which was very tricky without a lathe. Ok, not quite true, an acquaintance did it for me on one of his CNC machines.
This is the rather strange-looking filter (Photo 9), which is in two halves. We only need one half and the rubber end replaces that on the cover (Photo 10). So this is the new assembly (Photo 11) complete with O-rings, and this is what it looks like when fitted (Photo 12) – a very neat job. One thing is that the new filter is quite small so it will need frequent replacement.
After re-assembling the contact breakers I needed to adjust the gaps and set the static ignition timing, just to get the engine working, and attend to other service items. First was changing the oil and this is a wet sump system and the reason for that was given by Yamaha as being the same as on the CB750: to reduce engine height by using a smaller sump. The oil did look a bit black and the magnetic oil tank drain plug had quite a bit of gunk on it so again, not good signs.
The valve clearance adjustment is very conventional, but you do, or should, need a 27mm spanner to undo the very large access caps (Photo 13) as there is no room for a socket. Adjusting the camchain tension was perplexing: this was one area which had been revised and was covered in the supplementary manual. Firstly, no crankshaft position was given, but turning it over showed that it did not seem to make much difference. The odd thing was that the revised manual shows diagrams saying that the tension is correct when the operating rod is flush with the adjustor. Then it goes on to say that you adjust it by starting with it flush, then screwing the adjustor in by one-and-a-half to two turns, but then it looks just like the ‘needs adjustment’ diagram! (Photo 14). So I just… left it like that.