Seven Fifty cafe-racer. Engine works started.

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.

Even original ‘Oil recommendation’ sticker was left on the crankcase:

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:

But other parts, like cylinder head and camshafts were in good condition: no traces of wearing on surfaces and all measurements are within service limits.

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.

I disassembled it.

And what a relief, there was no traces of any previous treatments.  A have a lot of cleaning to do, but I used to this work.

To be continued.

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9 Comments

  1. Robert Eduard Onut

    Hello, any update on your project?

    I also have a seven fifty and I think about doing a camshaft and cylinder head swap with CBX ones.

    But before doing anything, I was wondering if you finished the project and if you reached 91 hp after the swap?

    Reply
  2. gazzz (Post author)

    Hi Robert,

    Sure, project is finished, here is link for complete story:

    https://www.gazzz-garage.com/category/cb750-seven-fifty-cafe/

    I used CBX750 cylinder head, camshafts and ignition. I also used big bore pistons. After all this modification I had to reinforce clutch.

    Kind regards,
    Nazar

    Reply
  3. Geert

    Hi Nazar,

    Hope you’re doing well
    Maybe I missed it somewhere, which carb set did you use? CB or CBX?

    Best regards,
    Geert

    Reply
    1. gazzz (Post author)

      Hi Geert,

      Carburettors looked pretty similar, but I used rejetted CBX750 carburettors.

      Kind regards,
      Nazar

      Reply
  4. Steve

    Hi, I’ve got a 1995 Honda CB750 seven fifty, when i start it up it sounds OK after a couple of minutes when you rev it there is a rattle, sometimes quite loud, I’ve been told it could be the cam chain tensioner but i don’t know how to adjust it, also when it gets hot it will rev High, very annoying in traffic queue. Could you please advise me

    Reply
    1. gazzz (Post author)

      Hi Steve,

      There are two chains in the Seven Fifty engine: timing chain and alternator chain. They both have automatic tensioners that need no adjustment. Alternator chain tensioner is more known for failing and causing rattling chain sound. Alternator chain tensioner is located inside of the engine between crankcase halves, so it’s quite an issue to replace it. Here you may find photos of that tensioner:

      https://www.gazzz-garage.com/2017/09/01/seven-fifty-cafe-racer-engine-assembling-part-1/

      Check if there is a free play of alternator rotor. If there is free play, it’s likely that the alternator chain tensioner failed. 

      Kind regards,
      Nazar

      Reply
  5. Steve

    Hi, thank you for your advice, is it still OK for me to ride the bike, i don’t go mad, mainly to work and back, many thanks for advice i do appreciate your help.

    Reply
  6. Steve

    Hi, it is me again, would it be worth having it fixed as i like the bike, how long would it take a skilled mechanic to to the job, just so i have a rough idea of what it would cost. Many thanks Steve

    Reply
    1. gazzz (Post author)

      Hi Steve,

      I couldn’t estimate how much it will cost you. Nor could I estimate time for the work as I never have reason to check timing spent on works.  However, to replace the tensioner one has to remove the engine from the frame and separate crankcase halves. It’s quite a lot of work,not counting preparations like cleaning and parts like the tensioner itself, some gasket sand rubbers ( O-rings for oils system for example).

      So in the first place I’d show a motorcycle to a skilled local mechanic to check the issue if it indeed is  alternator chain tensioner. If it is, then the same mechanic could advise about time and pricing.

      Kind regards,
      Nazar

      Reply

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