HONDA CB125T
Ralph Ferrand finishes the twin.
This month I’m on the home straight with the parallel-twin environmental disaster zone from Hamamatsu; hopefully when I’m finished, it will be greener than a wind farm. Last month I had rebuilt the cylinder head and just fitted it. Next up is the cam, which is a bit fiddly to get in, but reasonably straightforward. The left hand piston must be set at TDC (top dead centre), as mentioned last month, and the cam-chain tensioner must be set as loose as possible. The duplex cam sprocket is carefully put into an approximation of the final position, meshed with the chain. While tightly gripping the sprocket, the cam is slid through the sprocket to its working position. The sprocket has a cut-out to facilitate the cam lobes passing through on installation, but I found that not fitting the bearing bushes, until all the way in, gave a valuable bit of extra wriggle room. Once the cam is in, the bearing bushes are refitted either end of the shaft and positioned with the machined cut-out in the end of the left-hand side at the three o’clock position as shown in the second photo. If the cam doesn’t end up in the correct position first time it is easy enough to move the chain back or forward on the sprocket one tooth at a time. In the photo, the dowel in the bearing is not yet in position, but this will not affect the valve timing, and it will be pushed into its locating slot when the rocker assemblies are refitted. Once everything is lined up correctly, check and double check, given the damage that can be done should you get the valve timing wrong. I once got the valve timing wrong on a Hinckley Bonnie and bent all four exhaust valves; a very costly mistake, particularly as I wasn’t charging labour on the job for a friend! Insert the sprocket bolts with a dab of thread lock and seal on their threads. Tighten them to the correct torque setting as specified in the workshop manual. It may be easier to get a second pair of hands to hold a spanner on the crank end while you torque up these bolts; my missus is well used to being asked to hold spanners in this sort of situation.
At this point I put a small bit of tension on the cam-chain tensioner, just enough to take out the bag in the chain, and gently wound the crank over 720° (two turns), repositioning the crank back at exactly TDC to double check that the camshaft ended up in the correct position. Satisfied that the valve timing was correct, I then refitted the rocker assemblies. Before doing this I undid the tappet lock nuts and wound the adjusters out enough, so that (regardless of the cam position) there would be no pressure on the valves. The rocker assemblies are secured by nuts at the top of the main top end studs. As these nuts are also torqueing down the cylinder head, it is critical to follow the supplied tightening sequence and apply the torque in steps. Some manuals prescribe the stages, others don’t: I usually use three steps if there is not a dictate stating the settings. What is critical is that the head is tightened down evenly. I find that once you reach the final torque setting, you need to repeat the torqueing at this setting a number of times until the wrench stops turning the nuts. The reason that the nuts keep turning at the specified setting is that the head and base gaskets are continuing to be compressed. This is why many factory manuals instruct that cylinder heads are re-torqued after a specified time following a rebuild; for example, a Z900 cylinder head is re-torqued 500 miles after surgery. Once the head was properly tightened down, I set the cam-chain tension as specified in the manual. I used my valve tappet adjusting tool for this purpose, as
can be seen in the photo. Though the Haynes manual didn’t mention it, I discovered that it was impossible to adjust the cam-chain tensioner while the top securing bolt was done up. Why this was, I have no idea, but given the tensioner gear did not appear to have any faults, I can only conclude it was an omission from the aftermarket manual. Some service manuals tell you to turn the crankshaft to specific positions to adjust the valve clearances and others just state that the lobe of the cam should be 180° opposite the follower when measuring the clearance. I personally find the latter the easiest. With an engine that uses rockers, like this one, you take the gap measurement between the bottom of the tappet adjusting screw and the valve stem. Once the cam was in the correct position, I fed the correct feeler gauge blade in between the valve stem and the adjuster screw. The side handle of the tool (blue in the photo) is used to
undo and tighten the adjuster’s lock nut, while the graduated knob is used to move the adjusting screw. With the lock nut released a small amount the adjuster was screwed in until it just made contact with the feeler blade, when I retightened the lock nut. When the adjustment is correct, you should feel a very small amount of resistance to the feeler blade being pulled out. Sometimes the clearance for the exhaust and inlet valves is different, so one must always check. I progressively moved the crank with a spanner allowing me to set all the valve clearances correctly. I always give the engine a few revs and then re-check the clearances to ensure that they are all spot on. Next I refitted the rocker cover and carried the lump over to the bike. What a change to be able to carry an engine without spinal damage; there’s a lot to be said for working on a tiddler! The easiest way to fit one of these engines is to sit it on a scissor jack type platform and wind it up into position, to fit the engine mounting plates and bolts. It’s
so hard to be struggling to hold the weight of an engine while simultaneously trying to persuade the bolts through the holes. Even with a second pair of hands, it’s a right royal pain. I tend to take photos with my phone when removing an engine, as my memory is so poor these days, I can never remember which way the bolts go: i.e. are the nuts on the left or right hand side? A quick check back through the pics will save a lot of messing about. It doesn’t hurt to put a bit of copper slip or ACF50 Corrosion Block grease on the bolts and then clean the threads and put a dab of thread lock and seal on them before torqueing them up to the torque dictated by the workshop manual. When refitting the final drive sprocket, it is important to thoroughly degrease the threaded holes in the sprocket and the bolts before applying thread lock and seal to the bolts. It can take the shine off your day when a rear sprocket falls off the final drive when you’re enjoying a weekend ride. With my able (and paying) assistant, we
replaced all the hangers-on like exhausts, carbs etc. The airbox was pretty scabby so we gave that a really good clean. The foam filters were especially manky so we gave them a good degrease and re-oil. The throttle cables didn’t move anything approaching easily, so I used a cable oiler to inject some WD-40 in them which freed them up a treat. Cable oilers cost buttons and are worth their weight in gold. Once the engine is back in the frame it’s time to give it a drink of oil. For an engine that has had a re-bore, it is important to give it a mineral oil because a synthetic, or even a semi, is too slippery and the engine won’t run-in properly. As a rule, for most Japanese bikes I tend to run them on Rock Oil Guardian semi-synthetic, partly because the company is British and partly because they really do specialise in motorcycle oils and have superb technical back-up, which I haven’t found anywhere else. I did re-set the points, but as I don’t seem to have taken any photos of that process, I’ll cover that fun and games in a
later article. The best way to set up points, of course, is to replace them with electronic ignition, unless you’re a serious rivet counter! Even an old hand like my good-self will forget something, and this time it was to set the idle screws. As I had refitted the carburettors, which was a tad tedious, what with getting the rubber ducting to the airbox in place, I was in no mood to take it all off again, when I realised my omission. I dug out my borescope and poked its end through the back of the airbox until I could clearly see the bottom of the throttle slide on the screen. I undid the idle screw a long way and then screwed it back in until it just started to lift the slide again. I then took the borescope around to the left-hand side and repeated the process. I then remove the blanking screws from the cast aluminium part of the carb mounts on both sides and inserted a vacuum gauge adapter in each. I won’t go into the carb balancing in much detail, as I so recently covered the topic on the Z650. The engine sprang into life quite readily given a prod with the kick starter. Once it was warmed up, I balanced the carbs at tickover using the idle screws. As the throttle slides are pulled up by a split cable system, it is important to ensure both throttle slides are also synchronised. To achieve this, the throttle is opened so that both slides are lifted off the idle screws, and the cables are adjusted with the little brass adjusters in the carb tops. Each cable should have a minimal amount of slack at a closed throttle, but you then adjust them so that the vacuum is equal on both pots just above tick over. Once this is completed the engine should sound is smooth as a well-oiled Swiss sewing machine. I then replaced the tank and side panels and took the little bike for its first ride down the road. Even carrying my not inconsiderable load, the little Honda pulled well and was fully cured of its nasty smoking habit and my customer was a very happy budgie! I am told the little Cee Bee went on to pass its MOT and has not had so much as a crafty drag since leaving me. So overall, it’s been a good result allround.