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

Classic Motorcycle Mechanics - - CONTENTS -

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.

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