Ok, time to bring a bit of light on this project engine section.
It is well known fact that engine of Honda CB 750 Seven Fifty is based on CBX750 one. In fact, on crankcases of the first years Seven Fifty engines you may find serial number starting with ‘RC17’ instead of ‘RC42’, which is the index attributed for Seven Fifty model. RC17 is CBX750 index and that may as well mean that Honda just used stock of CBX parts when decided to produce Seven Fifty in the in the early 90s of the 20th century.
But there are some differences between these engines. Thus, CBX750 has gearbox with six gears instead of five in CB Seven Fifty one. Another difference is a clutch unit, where CBX has hydraulic drive and Seven fifty – mechanical and these facts cause quite significant difference between both clutches build. The CBX engine uses part of motorcycle frame as an oil reservoir, as a result rudimental part in Seven Fifty engine (with all its oil stored in a crankcase pan) is available. As Seven Fifty owners may notice there is a plug on left engine side near the gear shift pedal with “DO NOT OPEN” warning on it. The hole in which this plug screwed is an oil filler in CBX750 engine.
But the main difference between CBX750 and Seven Fifty engines is their performance characteristics. According to specification CBX750 has 91hp/9500rpm and 70Nm/8500rpm while Seven Fifty has 73hp/8500rpm and 61.8Nm/7500rpm. Almost 20hp and 10Nm, even shifted up on 1000rpm look quite attractive to find and bring them back, am I right?
But where we may find all those horse powers and Newtons per meter? The answer is simple: camshafts, cylinder head and ignition. So I began a little hunt for CBX750 parts. As a result I found and bought one lot of CBX parts which consisted of disassembled engine, carburettors, wiring, handlebar switches and relay rectifier.
The condition of parts varied from unit to unit. The clutch plates and crankshaft were in condition “ready to be thrown out “. Here is a striking unfortunate example of “folk art” I found in this engine clutch:
So now I have to make some combination of CB750 Seven Fifty and CBX750 engines. I am not sure about which of two gearboxes (5 gear Seven Fifty or 6 gear CBX) or clutch I’ll use, but I suppose I’ll find the best solution. And anyhow I began to work from cylinder head to be sure that it is in really good condition.