HONDA CB750 K2
More head (and sump) scratching as Mark finds out if the engine on his K2 works. Project Honda CB750 K2 part 12
Mark Haycock gets to work on the motor.
The importer of the K2 had said that the engine was in ‘some’ working order so maybe it might be possible to get it going properly without giving it a complete overhaul, but first a service. I started by changing the engine oil. The CB750 was unusual for a Japanese bike in having a dry sump circulation system: the bulk of the oil is stored in a tank kept behind a side panel, rather than in an engine sump. The oil pump is in two parts: one (the delivery side) draws oil from the tank and pressurises it to be sent around the engine. The oil then drops into a small sump and it is immediately drawn out via a strainer by the other part of the pump (the scavenge side) which then passes it back to the tank. The scavenge side usually has twice the capacity of the delivery side to ensure that it keeps the little sump clear. To drain the oil on a CB750, two drain plugs are provided: one in the sump plate as normal and the other at the bottom of the oil tank. The only difficulty is that the tank is kept on the right-hand side. You would think that it should be on the left so that the bike could be put on the side-stand rather than centre-stand and the oil would flow well away from the side of the bike rather than all over… The filler is at the top of the tank of course. The oil filter is the old-fashioned separate element type kept in a casing at the front of the engine. My Jardine exhaust system has pipes passing right under the case so there is a certain amount of clearing up needed (Photo 1). About the filter: firstly there should be a spring and seating washer (Photo 2) which are fitted within the case (Photo 3). The case is fixed in place by this special bolt (Photo 4) which incorporates a springloaded pressure relief valve which should be checked by ensuring the spring-loaded plunger can move. The aim of the valve is to prevent damage to the filter element by an excessive pressure difference between the dirty and clean sides. The bolt head looks too small at 12mm AF, but this is deliberate to try to stop it being over-tightened. After-market bolts with 17mm heads are available but not recommended, as they are sometimes slightly too long which results in parts of the internal thread in the crankcase being ejected into the oilway, causing blockages. If you buy the genuine Honda filter as well as the element itself you get the O-rings for both the case and bolt and of course these are always best replaced each time. Valve clearances are easily adjusted as there are only two valves per cylinder and it is a simple mechanism using rockers with screw adjusters. Access is via circular caps and although they are small, visibility is not bad. No special tools are needed but standard adjustment tools (Photo 5) will make the job even easier. You will need to find Top Dead Centre to carry out the clearance checks and marks are provided on the centrifugal ignition auto-advance. This is a good time to adjust the cam-chain tension. The original manual specified that No. 1 (the left-most) piston should be at TDC on the compression stroke when making the adjustment, but later on it was realised that this was not optimal. The perfect position is actually 15° after TDC and rather than using a degree disc, all you have to do is imagine a line drawn from the timing mark down to the centre of the cam and turn the engine beyond TDC until the peg which holds the end of the bobweight spring is just to the
right of that line. Photo 6 shows what I mean. Then all you have to do is to undo the locking bolt on the adjuster (Photo 7) and do it up again. We have looked at points gaps and static ignition timing in the past and it’s a simple procedure on the CB750. One interesting point I found when working on the contact breakers is shown in Photo 8. Just under this area is an engine mounting and I found this abomination: a square mild steel nut with the wrong sized thread, jammed on to hold the bolt in place. I have found things like this on Americanowned bikes and it’s because UNF and UNC sized fasteners are easier to obtain at a local hardware store than metric items. Before attempting to start the engine I thought it best to fit the air-filter. However, a previous owner had obviously had a go at improving engine output by modifying the air-box. In Photo 9 we see that they’ve opened up the air intakes. This was one part that was modified over the years to quieten the intake roar at the expense of performance, so really it would just return things to original specification. More significant though was that the air-box did not fit properly (Photo 10). I could not find a way the get the box in the right orientation because of the vertical strengthening ribs. Assuming that the frame was not seriously bent, the only explanation was that the rubber hoses were incorrectly shaped so I replaced both the air-box to carburettor and carb to cylinder head sets. For the latter, I got this pattern set (Photo 11) from ebay but something wasn’t right about the hoses: see Photo 12. The clips were standard Honda but it was very easy to tighten them right up as far as they could go (meaning the clips couldn’t be tight enough to allow an air-tight seal), which told me that the hose walls were too thin. After returning the set I bought genuine Honda parts and found exactly the same thing! I never did work out what was going on here, but here’s my solution (Photo 13.) I superglued on little strips of viton/cork gasket material to fill up the inset area for the clip location on the hose and this worked okay.
1 Pipes pass right under the oil filter casing.
2 Spring and seating washer...
12 As tight as can be: something’s wrong!
13 Widening ‘gasket’ fix sorts it!
11 Pattern air-box rubbers.
10 Air-box didn’t fit correctly.
9 Air-intakes have been butchered.
6 This does away with a degree disc.
7 The locking bolt on the adjuster.
8 One square-head bodge job!
3 ...fit within the filter case itself.
4 Special bolt keeps case together.
5 Standard adjustment tools help.