❙ YAMAHA TX750
What’s with the TX750’S much-maligned reputation? Does it deserve it? Mark asks the questions…
Mark Haycock asks if this twin was really that bad?
My TX750 is now back in one piece and working well, so I think this is a good time to look at this model and its bad reputation.
It is very interesting, not to say surprising, to look through the collection of ‘Service Notes’ produced by Yamaha USA after the model’s introduction, the reason being that there are so many of them. Many discuss ways to reduce engine noise and oil leaks, which must have formed the basis for a number of customer complaints. There was a series of modifications which could be applied by the dealers but the question is: why were the mods not carried out during the development stage, before the model was released? The answer is no doubt because commercial pressure demanded that it be put out as soon as possible, which is a pity as a couple more months might have prevented the disappointment the customers must have felt.
There was a rather more serious issue though and that was engine failures caused by damaged crankshaft bearings. Recently we looked at plain bearings and found that failure of these can have a number of causes – for instance, in the case of my bike it was the lack of an oil filter allowing contaminated oil to circulate. Back in 1972/3 it was fairly new bikes which were falling by the wayside because the oil was unable to prevent separation of the bearing surfaces, casing rapid wear or seizures. Yamaha were mystified by this as the preproduction bikes had behaved perfectly during tests in Japan.
So what was causing this? There are a number of theories, but one I have seen can be dismissed straight away. This was that the typical American rider would spend their time using the bike for traffic light drag races, which naturally caused the engine oil to be flung to the back of the sump and thus not picked up properly for recirculation. The main objection to this theory is that the TX750 engine does not have a sump!
Let us take a look at what it does have (Photo 1). Here we see the mythical sump, actually called the strainer cover, with its associated strainer. It appears to have an oil capacity of around 250ml which does not seem quite enough for a 750, and of course it is not. The TX750 has a ‘dry sump’ circulation system where the oil is actually stored in a separate tank which holds something over two litres, with the total quantity in circulation being three litres. The trochoidal oil-pump is in two parts, i.e. a normal pressure feed pump from tank to engine internals (via the filter) and a scavenge pump to take the oil from the bottom of the engine back to the tank. The two parts are at the bottom left and top right in Photo 2, which shows all the pump components. The latter part has a pumping capacity 50% greater than the feed and this is to ensure that the ‘sump’ is always kept clear.
This brings us to the second theory about the failures. Looking at Photo 3, we see one of the balance weights which is right at the bottom of the engine (it is upside down here). It actually extends into the strainer cover which has a space to accommodate it. So perhaps the weight could dip into the small amount of oil and whip it up into a froth, which would be less effective as a lubricant. Well maybe, but bear in mind the engine supply oil is actually drawn from the tank, not the ‘sump’. Unfortunately it is not possible to see the oil inside the tank because it has an odd shape (Photo 4).
However, during the pre-production tests in Europe in 1972 a journalist had the bright idea of cutting a hole in the side of the tank and sticking in a piece of clear acrylic to form a window, and the oil was seen to be frothy.
Yamaha addressed this later by designing an extension to move the strainer cover downwards, which is often described as being to increase oil capacity. It doesn’t, of course, because the oil is sent straight back at the same rate to the same-sized tank, although I can see that it might prevent the foaming.
However, I suggest that the oil always will be foamy because the rate at which it is removed is greater than the rate at which it is appearing (as we have seen), so the excess must be made up of blow-by gas in the crankcase.
Let us look at this issue by trying an experiment. In Photo 5 you see I have poured a sample of a special dark foam-forming liquid deliberately carelessly into a transparent test container. After a short while you see that the foamy liquid has separated into two parts with foam at the top and beer (sorry, I mean test liquid) at the bottom. In the bike’s tank, the foamy oil is introduced at the top and it returns to the engine from the bottom. I suggest then, that there should not be a problem, or at least not as significant as it might seem. Whether that is true or not, oil has improved over the years and it is less likely to form foam so this wraps it up I think.
So what did Yamaha have to say? They expressed the failures as... ‘a problem with heat build-up that prevented stable engine performance and insufficient machine durability, both fundamental flaws for the type of high-speed, long-distance riding common in overseas markets but not in Japan’. In other words, they are confirming that the bikes worked okay at home, but the trouble started mostly in the USA.
The TX500 was similarly affected and I must say that it is quite true that my 500 does get extremely hot after quite a short time on a warm day. Yamaha’s solution for the 750 was to specify an oil cooler, but I was quite struck by an American road test report (Motorcyclist, December 1972 issue) which commented that they had... ‘recorded oil temperature readings of 115º C during testing in 46ºc desert heat’. Well what did they expect? Next time I shall look at a little mod to (I hope) reduce the TX750’S engine temperature.