Motor-mania!
Mark’s on the finishing straight with his TX750, but a little engine work now needs to be done to get it sorted. This is the first part of a two engine re-build. articles.
Last time I made the decision that the safest course of action would be to take the engine to bits to inspect what was in there, if not to completely overhaul it. It might be helpful to know what to expect so let us take a look at what we know.
After I had fixed the starter mechanism, I wanted to see what sort of state the engine might be in so I did a compression check (Photo 1). Looks pretty good doesn’t it: 190psi. The only thing is that the maximum reading is meant to be 145psi, so how can it be more than that? This was not a representative test as the engine was cold and the carbs were not fitted, but probably the most significant point was that I had poured oil through the plug holes to prevent corrosion some weeks before. Or, of course, the gauge might be inaccurate. A later test done when in running order showed the pressures to be 170psi which was at least more normal. The main point was that the left and right pressures were the same so it was likely that the bores, piston rings and valves were okay.
I had set the throttle valves to be exactly synchronised when cleaning the carbs out and the vacuum readings were surprisingly close when the engine was running – another good sign. The same applied to the ignition dwell angles.
But on with the work. The Yamaha manual shows what to do to get the engine out and as you might expect it is simply to undo everything that attaches the engine to the rest of the bike.
One thing I had previously not been very happy with was the mounting of the exhaust balance pipe (Photo 2) which did not seem to fit. I had tried removing the bolts to re-seat everything but had been unable to move two of them. That was a pity because the pipe needed to come off for clearance to get the engine out. It was a bit of a job and eventually I had to force the lower two as there was no chance of getting heat or penetrating oil where it was needed (Photo 3). You can see that the exhaust joint had been leaking.
You need to drain the oil from both oil-tank and sump and because I had changed the oil just 15 miles earlier, the oil I had seen on the tank dipstick and the engine connections was clear and new looking (Photo 4) as you would expect. However, the surprisingly small amount of oil I drained (bearing in mind that it was supposed to amount to three litres) was completely black. How that could happen I cannot explain, but no doubt the reason
will emerge later. The book does not mention that the coils must come off
(Photo 5) as there is no clearance for the engine to lift. Then it sank in that my coils were not standard items but generic replacements. This was a popular mod as the original coils are said to be a bit unreliable. I found it also easier to take off the oil-tank to give more room.
Just under the tank was what appeared to be a mounting lug on the frame which was stopping the engine from coming out. A closer inspection under the dirt showed that it was in fact a bolted-on plate which, when removed, turned the impossible into the possible, I am glad to say.
Yamaha tell you to lift the engine using the kick-start and the left-hand exhaust port, but as I could only get two fingers in there, that was not much help. A much easier technique was to lift the engine with a little trolley jack, guide it out with the kick-start and right-hand cylinder and allow a controlled descent on to a soft surface (Photo 6).
The engine is quite heavy to lift but a little while later I had it on the bench and ready for action. The first operation is to remove the valve cover (Photo 7) and here I want to point out a couple of things.
A few purists will be horrified to see that I have not cleaned the whole engine before starting work. My stance on this is that everything will be dismantled, right down to individual components, and thus it will all be comprehensively cleaned later on; there is no point in doing the job twice. The other point to note is that the engine is facing me so that I can easily reach what I need to access, so the left-hand side as I look at it now is actually its right-hand side. This is important when it comes to labelling parts such as valves and pistons so you need to have a rule to follow. Mine is that Right means the Right side as I sit on the bike.
You can see that the valve gear is very simple and conventional and the ‘special tool’ you need is simply an M6 screw to pull out the rocker shafts (Photo 8). One point here was that one of the inlet rockers fitted so tightly in its mountings that I needed a pry bar to get it out
(Photo 9). Evidently these engines were assembled with the aid of a hammer. I always put the valve components together in separate labelled plastic bags (Photo
10) and, of course, RE here means Right (or left as I am looking at it) exhaust!
The cam-chain is not endless but has a staked link and as I did not have a suitable splitter for this size, a small grinder did the job instead (Photo 11).
Don’t worry about the grinding dust or swarf, like I said earlier. Similarly, I don’t need to bother stuffing rags in to stop bits falling down.
By the way, the reason why the head will not budge at this point is that you have forgotten to undo the two M8 bolts at the middle front and rear (Photo 12).
The combustion chambers look okay I suppose, but what is the significance of the rust on the exhaust valves (Photo 13)?
It is perhaps a result of running the engine for very short periods (not by me), when condensation has formed and been left there. The pistons looked okay – maybe a bit oily and I shall take a closer look later on. Removal of the cylinders and pistons is very straightforward.
The clutch is quite well specified with both a roller bearing (Photo 14) and a needle roller thrust bearing. We can see here an interesting feature of the primary gears. If you look at the outer clutch gear at the four o’clock position you can make out a hand-inscribed number: 48. The drive pinion (Photo 15) bears the number 53. The gears were measured in the factory and the backlash between them is determined by these numbers. They need to add up to 139 or 140, so 48 + 53 is: 101. What has gone wrong here? Eventually I realised that ‘48’ is actually 86 when you look at it the other way round, with the centre at the bottom like reading the pinion number. So 86 + 53 is 139 and thus the gears are completely silent with no backlash – in theory, at least.
Now I am able to turn the engine upside down and take off the bottom plate to see what lies behind it (Photo 16). What we are looking at here is the balancer mechanism, which consists of two rotating eccentric weights driven by quite a substantial chain. And now I see that this is the source of the rattling and banging noises because the chain is completely worn out: I can move the balance weights and see the chain moving up and down without turning the crankshaft. Maybe adjusting it a bit tighter would have helped? We shall look at that later. The crankshaft and gearbox shafts seemed okay (Photo 17) but after removing the crank, I could inspect the plain main bearing shells which turned out to be a problem which I shall discuss separately.
Now I have the engine in bits, all I need to do is to clean everything thoroughly, assess what I have and hence what I need to buy. See next time.