Yamaha TX750, the missing link

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Category: Stories from the Netherlands
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Jurjen de Jong from the Netherlands calls his Yamaha TX750 "the missing link"! It is a truly amazing story of his relationship with this exciting motorcycle.....!

Yamaha TX750, "the missing link"

I became interested in the TX750 because of a colleague at work. He told me about the design with the balancer shafts and how rare the TX was (certainly a road worthy machine) because of all the design troubles it encountered during its short lived career (1973-1974). I think that the TX750 looks the way a motorcycle should look. On close examination it is a history lesson on wheels. Maybe more important than we like to think. I do mechanical design work for a living and when working on the bike, I can almost follow the drawing pen of the Yamaha engineers and see what’s going on (or going wrong) in their minds. They had good ideas and bad ones. To me it is the Missing Link between classic and modern motorcycle design. It is the difference between keeping up with the competition and getting ahead of the competition with new technology.

I was on my third Triumph when I learned about the existence of the TX . The first one was a 1981 750cc T140 Bonneville (I really wanted a 1969 T120). The second one was the iconic 1969 650cc T120 Bonneville(what a surprise!) and the third one a 1996 Speed Triple that represented the re-birth of Triumph as a motorcycle manufacturer. I swapped it for the T140. One classic machine and one modern motorcycle.

But something was still nagging me. At the age of fifteen, I bought my first Yamaha (FS1 DX moped) with my own hard earned money. I loved this little moped like nothing else. I cleaned and polished it every week. It may even have helped me to impress the girls. I must have looked the part in my army boots, leather jacket and black crash helmet with skulls. You can ask my wife (she was one of the “impressed” girls back then but she will never admit that, of course). At that age I was reading the leading motorcycle magazine MOTO73 in the library every week. I read the comparative test of the Yamaha SR500 and the BSA Goldstar.

 

I think the conclusion was that the Yamaha was probably the best single cylinder bike ever made in the tradition of British motorcycles. I liked British bikes and I liked Yamaha, because they seemed to have engineers that liked the British bikes that were no longer made. I liked the XS650 too, because it was a (pretty good) copy of the Triumph Bonneville. But I didn’t like it completely: the XS1 had frame trouble. The Triumph didn’t. This problem was later solved by Percy Tait who used to be a test driver for Triumph and later redesigned the frame for Yamaha. That proved to me that Yamaha was somehow managing the Triumph assets and continuing in style.

Nowadays the SR500 is also in my stable, it being my son’s ride. I have been riding a lot of bikes, but the SR500 is still giving me the boyish grin and does make me feel sixteen again. I bought that one in Germany and it was inspected by the Dutch RDW before it was allowed on the Dutch roads. The inspector had probably been testing a lot of almost new Harley’s and some rice rockets because the moment he saw this one, he got up and said “Ah, this reminds me of the old days” , jumped on the SR and kicked into life. Didn’t see him for at least 15 minutes: he went for a ride! When he got back he didn’t inspect anything and just filled in the paperwork.

Back to the seventies: Yamaha needed a superbike like the Honda CB750 and the Kawa 900Z1. Yamaha knew, because of exhaust emissions, that the 2-stroke engine was on its way out. The XS was a good learning experience for the 4-stroke engines, but engine vibration was a limit to the maximum number of horses that could be squeezed out. But the two cylinder was still an attractive concept because it wasn’t as wide as the Honda and Kawa and the road handling of both the XS650 and Bonneville was much better than both four cylinder bikes. Someone must have said: what about balancer shafts? We will have low end torque, the high rev feel of a four cylinder and road handling like a two cylinder: a sure winner . The prototype was to become the TX750, all design foibles included.

That’s what I wanted for a winter project, I wanted something difficult, an engineer’s nightmare, the Missing Link. I found my TX750 in the south of Holland (in September 2006) and it was a complete wreck.

 

 

Yamaha TX750
Yamaha TX750

The Yamaha TX750 is picked-up with 2 large boxes of bits and pieces.

 

 

Yamaha TX750
Yamaha TX750

The Yamaha TX750 brotherly together with a Triumph.....!

 

 

Yamaha TX750
Yamaha TX750

15 Years of storage was enough for some mice to build a comfortable nest under the seat!

 

More than complete that is: it came with a spare engine in parts. It was partly dismantled, so I started assembling it again and met some very friendly and enthusiastic TX750 people on my way. By the time it was December I had it running again. I had collected missing parts, done the front and rear brakes (both rusted tight).

 

Yamaha TX750
Yamaha TX750

It needs real effort to undo the stuck calipers!

 

I repaired leaking petrol taps and lines, ultra sounded the carbs, re-wired it completely with a second hand wiring loom, adjusted the rebuild ignition, bought a new battery, cut new gaskets for the engine covers, put some fresh oil in and cleaned everything. It still looked like a wreck, but it was complete and should be functional. It started right up … on one cylinder an after a minute the second one came to life. Warmed up it ran pretty well, but it was spewing out oil near the rear wheel (nice!).

 

 

Yamaha TX750
Yamaha TX750

By December 2006 the Yamaha TX750 was ready for his first outing.

December is not the ideal time of year for test runs, so I removed the colored parts and had them spray painted in gold flake. During the Christmas holidays I rebuilt the wheels. I took the wheels apart and sanded and polished the original high shoulder DID rims by hand (10 hrs each), found new spokes on the internet and re-spoked them myself.

 

In the spring of 2007, I put everything back together again and the bike was looking the part. Starting it was troublesome. The proud sign “ELECTRIC” on the side covers should have been written between two other words: “NO” and “START”.

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Electric start? You are joking!

The kick starter would only function when the bike was resting on the side stand (like an old man on a stick). With the bike in upright position it wouldn’t “grab”. Correction was easy though. Just remove the engine cover and remove the hairpin spring from the starter gear. Bend the circular part together a bit so that it holds on to the starter gear. Put everything together, e presto, as the Germans say.

 

It was still starting on one cylinder and when using the kickstarter it felt like it had no compression on one cylinder. I adjusted the valves but that didn’t help. I did a compression test with a warm engine and found out that compression was low on one side. The alloy of the head must have been of an experimental aluminum mix: it gets very soft when warm and I immediately stripped the spark plug thread. Also the oil was still seeping out and I lost interest: the riding season had begun and some Brit iron was begging for blue skies too.

The best thing would be to dismantle the top end to see what was going on there. So in the autumn of 2007 I looked through my parts bin and started collecting parts (headgasket etc) and courage to remove the massive engine.

I had found out that the engine needed to be removed from the frame to remove the head and cylinders, although the workshop manual was stating: “Note: The cylinder head cover can be removed with the engine in the frame” . Maybe it was intended, but I know from experience it is impossible. Other motorcycles from that era can have the top end removed without removing the engine, basically because they all had normal cast iron cylinders or steel liners that needed a re-bore every 50.000 km. The TX had steel liners as well, so…

To get the engine out I removed the oil strainer cover and the spacer that made the sump deeper, this is a factory upgrade that prevents the low mounted balancer shafts making whipped cream of the oil.

To get enough room to lift out the engine, I also removed the oil lines and the oil cooler . Mine has the Chausson cooler that was supplied by the French Yamaha importer Sonauto. Others had a Lockhart oil cooler. The Chausson cooler reminded me of the cooler in my 2CV Citroen. By now I had a distinct feeling I was working on a prototype engine.

 

I had a small hand operated winch hanging from the ceiling of my garage and the engine came out easily. I winched it up and moved it a little sideways and lowered it on my faithful workmate (B&D WM747). I hadn’t lifted a metric gram myself.

 

Yamaha TX750
Yamaha TX750

The Yamaha TX750 is dismantled after its first test run.

 

Now I could remove the cylinder head cover to access the cylinder head bolts, or so I thought. The construction was a little bit awkward: the rocker shafts need to be pulled and taken apart before you can reach the bolts for the camshaft bearing brackets. You basically have to dismantle the camshaft and bracket part by part before you can remove the cylinder head. With the camshaft removed (after splitting the camshaft chain), the head could be removed easily.
I found out that it already had the first oversize pistons and the cylinder in my parts bin was original bore. So I found some NOS 1st oversize rings, and had my cylinder bored to the first OS pistons.

I also found out why I had no compression on one cylinder, I had a bent valve with an impression on top of the piston. I didn’t have a clue about how that had happened. Or is it the workshop manual, that has a wrong instruction for installing the cam chain? (The instruction is corrected by a Yamaha Service bulletin that can be found on http://www.tobyfolwick.com/tx750 ).

I used a cylinder head from my parts bin and got some used exhaust valves from Yvon Wolters (the Dutch TX750 guru). The good news here is that the valves, guides and seals are the same as used on the XS650. So I took the valves and lapped them by hand to fit the seats. I already noticed that the valve stem play was much less than on the original cylinder, but I didn’t give it much notice because my TX had already 52.000 km on the odometer and many of the basket cases hadn’t even reached 10.000 km! I used new gaskets, new stem seals and even a new timing chain and was careful to install it in the correct manner (thanks to the service bulletins). Everything went together well and I had loads of compression. The first test drive was a disappointment, the bike was still throwing out oil before the rear wheel. It looked like it came out of the crankcase ventilation pipe. But compression was perfect?! I dismantled the oil tank and oil cooler lines and found a small leak in one of the hoses. Easy repair no. 26, I grinded of the crimped fitting and got a length of universal oil resistant hose and used two hose clamps to clamp it to the oil pipe. Starting it was still not super and I installed new points, coils and condenser, starting it was still not super and I installed new needles and jets and because starting was still not super I checked and corrected the float height and starting was still not super….what can I say?

I thought that maybe electric starting would be better. So I reinstalled the cable from the starter solenoid to the starter and with the ignition key “on” I pushed the starter button: nothing happened. I then bridged the starter solenoid, and it made some crunching metallic sounds. I took the solenoid apart (you will have to do some soldering) and found burned contacts, I corrected that with a file and put it back together again. Now the starter motor was running, but not starting the engine. Yvon made it clear to me that a new starter sprocket was required and I found one on the Evil bay. Had a puller made for the alternator rotor and installed the part without much trouble. The starter was rotating, but only catching maybe one out of ten times.

 

At least I had a running bike and was ready for the yearly “TX750 Treffen” in the Eastern part of Holland.

 

Yamaha TX750
Yamaha TX750

In May 2008 the Yamaha TX750 went to its first real TX750 Club run.

 

This was the first long trip for the TX with constant speeds of 100 kmh. It performed well, but something strange happened in the cool down phase of the engine, one of the valves was stuck open. I always give it two slow kicks to get the oil going and some fresh mixture in the cylinders. The slow kick movement was stopped by the valve that was stuck open. I pushed it a little harder and heard a metallic “cloink” like the valve jumped back on the seat again. The engine was free again and could be started again. This only happened after 20 minutes of cool down and could be repeated on other occasions. Those valve guides must have been really tight on the valve stems. I talked to Leen van Dorp (known as Ledo on the www) during the Treffen and he said that he always put a small amount of two-stroke oil in the tank. I thought nothing of it, but was desperate to correct the sticky valve problem, so I put a mixture of 1:150 two stroke oil in the gas tank. I couldn’t believe the result: no more sticky valves, so it must have been an inlet valve? Not to worry, problem solved.

Sometimes it made a grinding noise when you pulled the clutch and finding neutral was hard. I told Yvon Wolters (who is the initiator of the TX Treffen) and he said that not many people have that problem because not many TX ‘s have done 50.000 km. He told me to pull the clutch apart and take a look at the springs between the clutch basket and the primary gear.

I had to get back home first, and I almost managed. 30 km before my home it cut out completely. No more electric power. I didn’t have many tools with me, but wrench no. 10 was present so I removed the battery cover to find a blown fuse (it has only one). It was a 10 Amp fuse and maybe it was caused by a short, so I held the wire directly to the battery positive on the starter solenoid and the power came back on. Without a fuse I got back home and put and 20 Amp fuse in and never had that problem again.

 

The season was not lost yet, so I removed the engine’s right cover once more and dismantled the clutch to find three of the six springs broken and one (out of three) rivets broken with the head from the rivet rotating between the clutch plate and clutch basket. This is normal, even on the XS650 this is normal. A repair kit will set you back about €35 (www.xs650shop.de). I drilled the two other rivets out, split the clutch, got the broken springs out, put three spare ones in, put the rivets in backwards and then had them welded up from the outside. Problem solved, the clutch feels much better and neutral can be found again.

Starting was still not super, but not too bad either. Acceleration was sometimes troublesome and I found out that I had a sticky timing advance governor. The timing advance governor will develop only two problems: The shaft is chromed and the chrome will wear off, the advance mechanism will not rotate (and thus advance ignition) very well. I tried oil, that didn’t work, the shaft will get warm and the oil will evaporate, same for normal bearing grease. Then I used a old fashioned chain grease with molybdenum disulfide. That worked wonders: the advance mechanism was free now. Because the governor had weak springs as well, I was rewarded with full advance during kick starting, resulting in backfires that have pushed the carburetors out of the rubber inlets more than once. New springs for the governor are still available from several vintage Yamaha parts sellers. At that time I did not understand the backfires during starting and thought that it was all part of the TX experience. Only later when I installed electronic ignition, it came to my mind that it must have been the advance governor that was the cause of the backfires that made the carburetors jump from the bike.

 

I drove the TX the whole summer season of 2008 without many problems, but my last trip in September ended with a spluttering bike that stalled in front of my home and I pushed it inside and tried to forget about it. I was asked to bring the TX to a show in December (it being more rare than a Vincent Rapide and all) and I intended to go on the bike. It still wouldn’t start, I screwed two new spark plugs in and it started right up. But again it wouldn’t start when I wanted to drive it to the show. By now I got pretty mad and promised myself to get the bottom of the starting problem and on top of that to try and solve the electric starting problem too.

I already found out that the starting fuel mixture was too rich, even with the choke (enrichment) off. The spark plugs were wet and when I dried them on the stove and screwed them back in, it would start. I could start it with the throttle opened for ¼ to ½ but risked sending the carburetors back to Japan. I already had new jets and needle in the Solex/Mikunis and checked the float level for the third time.

The original ignition is known for its weak spark, even with new coils. I decided to go fully electronic, that means ignition timing incl. advance by the electronic ignition unit (so I didn’t need the advance governor any more). I found an unit from Volker Sachse (http://www.elektronik-sachse.de/index_en.htm ). I bought the ZDG3 in one channel version, so I could hook up a decent ignition coil with dual outputs as well: the Dyna 3 Ohm DC1-1 (http://www.z1enterprises.com/detail.aspx?ID=2092) this baby will give you a 30.000 Volt spark instead of the lousy 12.000 Volt of the original. The ignition came complete with all hardware required and was easy to install. It required no cutting into the original cable loom which is a good thing for originality suckers like me. This ignition gives a spark on both cylinders at the same time (so one spark is wasted, this is common practice on most ignitions now) and can be used with the original coils (two channel ZDG3) or with one coil with dual output (one channel ZDG3). It has 9 selectable advance curves and I choose the one with 8 degrees at idle and 39 degrees at full advance. I can assure you that this combination can ignite water. That means the TX750 could be started the normal way.

 

I couldn’t use the choke: the mixture was still rich. The choke is really an enrichment jet and as soon as it was operated the engine would die. One evening I sat with a carburetor in one hand and the workshop manual in the other and by chance I compared the spring pressure of a new float needle with a used one . The new one was much stiffer! That meant that with the old float needles the floats would raise higher than with new needles making the mixture (too) rich. I installed new float needles in the Mikunis , pressed the choke (excuse me, enrichment), turned on ignition and gave it a kick. It started at first spark, and was burbling happily along. After half a minute I cancelled the enrichment and took it for a spin. It was performing like it should.

Electric starting still didn’t work. The clutch wouldn’t catch. I had been doing some reading on the German www.tx750.de website and found that the rollers, caps and springs in the starter clutch needed to be replaced as well. This set has been used in the V-max until 2004 and can be found on Ebay Germany and UK (the same set as the early XS400 and XV535). I ordered a set and removed the three left engine covers (or should I say peeled the onion?) once more. I installed the rollers etc . and tried to do an electric start. Still not catching. Once more I looked through my parts bin and found the only starter clutch part that I had not replaced: the outer ring. I noticed that the curve for the clutch rollers could wear out as well. I found the clutch outer ring on Ebay USA. So for a third time I started peeling the onion and installed the clutch outer ring. It was completely new now: sprocket, clutch outer and rollers, caps and springs. Believe it or not: I now (Feb. 2008) have electric start.

Other strange design features that make you wonder.

The balancing pipe for the exhaust is a beautiful cast aluminum part that is fixed to the front of the cylinder head. I wonder if the chief engineer was away or ill for a week or so and the design was done by an apprentice that didn’t know what he was doing. Exactly on the spot where an air-cooled engine needs the most cooling a heat source was introduced. Only one word comes to mind: stupid. I am convinced that removing this piece (of cr@p) will solve the thermal problems of the engine.

 

Another thing is the position of the points ignition right below the Solex/Mikunis. After removing the left engine covers (onion, what onion?) several times, the part of the gasket covering the breaker points will break off. Since your side stand is left, leaking gas will run straight into the ignition and set your bike on fire at first kick. A Kind of bike hara-kiri. In hindsight the chief designer at Yamaha must have liked this particular feature.

Is that all that is wrong with the ignition, you ask yourself? No, there is more to come. If petrol doesn’t dribble in, oil will. The timing advance governor is on the same shaft as the oil pump and seals will start leaking on the points that then will be burnt away and … well, the service bulletin can be found here: http://www.tobyfolwick.com/tx750/textual/sb305_1.php . Real salvation is within reach for those who believe in electronic ignition.

 

I sympathize with the guy who designed the original oil filter inside the crank case. He must have been as happy as a bunny with seven tails, because he found an unused spot big enough to house the filter . Even the chief designer grumbled something in approval. The crank case went into production and the service people started working on the manual. Then they found out that the chain had to be removed to service the oil filter. The oil filter had to be serviced every 5.000 km and the chain? 10 to 15.000 km? The service man talked to the head of international sales. The head of sales talked to the managing director of the Yamaha Motor Co. The director made a short phone call to the chief designer. He redesigned it so that it could be serviced easily. That’s how the funniest oil filter in the automotive market came about…or so I think it happened.

 

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Jurjen's Yamaha TX750 came with a nice Asaha fuel gauge in the tankcap.

 

Another mysterious system is the innovative reserve lighting unit. There is a difference between the Euro gauge clusters and the US ones. The US has an additional white lamp to indicate the status of the headlamp. The system is simple, when the low beam is faulty the high beam takes over and vice versa. Since the Euro models don’t have a white lamp I don’t know how that is made clear to the man in the saddle.

Same for the brake/ tail light. When the tail light is faulty the brake light takes over and it will have a resistor of 3.9 Ohms in series to emulate a normal tail light. A nasty habit of the indicator on the gauge cluster is that the red light comes on every time you operate the brakes. Since most of us are trained to recognize a red light as danger/malfunction/stop sign this is plain stupid. That it is next to the red light that indicates low oil pressure doesn’t make things easier. I even thought my reserve light unit was faulty and tried two others with the same result.

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A Yamaha TX750 remains the real "Joe Bar"bike.

So why don’t I sell the darned thing? I kind of like it, I like it very much. Riding it reveals the ideas that the engineers had. It does have low end torque, and keeps pulling at higher revs. The balancer shafts do the job because it has practically no vibration. Although it is a heavy bastard to push around, as soon as it is rolling it feels nimble and easy to control. Roundabouts and curvy roads will have the footrest touching the tarmac every time. And the sound of the parallel twin is unmistakable and will always sound like music to my ears. The design of this Missing Link seems to be timeless, it does not look like an classic bike and people are always amazed when you tell them that the bike is 35 years old. Basically I love it just like I loved my little moped, and it’s always immediately cleaned and polished too.
Jurjen de Jong
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