My 750F2 project.

[Cb750r]

I know I’m not alone when it feels like a bike you’ve owned for a long time is an ever evolving, never ending project. Even if your end goal is closer to keeping a bike in stock trim unless you have it in a temperature controlled museum and never ride it, there’s constant repair or refurbishment.

I’m on a new major phase with my CB. I’ve owned it since roughly 2004, it was a poorly researched purchase and I got scammed. The seller slapped together a bike/motor with a hodgepodge of parts and the head gasket blew a day or so after I purchased it, then the engine itself blew up after I replaced it. Long story long there was high domed F2 pistons in a K5 head that had been also milled down below specs because the idiot had wire wheeled the head and damaged the surface. (Could still see wire wheel marks in the old head under close examination.)

I couldn’t afford a new motorcycle, let alone a high horsepower build, I rebuilt the motor with used head, cylinders, eBay, used con rod, new stock size pistons etc. I also discovered that the engine cases had previously lost a fight with the drive chain and was welded back up. Glad to report my collection of new and used parts did turn into a well functioning engine for many years and lots throttle cable stretching action. However I have had to deal with oil weeping from the case halves where the damaged case was repaired due to unperfect mating surfaces. I made various attempts of repair over the years with RTV but obviously it never lasted. Last winter I pulled the motor out with the goal of repairing and rebuilding the engine.

I originally attempted to repair the cases I had, and used some specialized aluminum epoxy to build up the case as well as re drill and heli coil a damaged thread near the area where I had over tightened it to try and reduce the oil leak. The repair is better than it was but I could still see light between the gap when I mated the upper and lower cases. So project sat… until I found a Cb750f2 "parts bike" for sale randomly on marketplace! I started on tearing down and inspecting the "new" motor that came out of the back of a motorcycle shop where it sat for many many years after it had rear ended someone and broke its fork stem. Carbs and exhaust robbed but otherwise just left for dead. The motor tuned over so was stored "properly" but the oil left in the pistons had left the chat in subsequent decades and this engine top end needed a rebuild, however the bottom end and the cases look great.

So I contacted Mike Rieck in the USA, and outlined my goal of wanting to build up just about the strongest street motor I could with stock F2 rods… Even though I had a good 410 core, due to shipping costs Mike sold me a good core and then went to work doing his magic. A full stage 3 port new valves and seats, and spec’d a mega cycle cam that he’s used on several bikes to much success. I ordered, a set of high domed 836cc JE pistons designed to retain compression ratio on the F2 heads….  And for insurance a set of Carillo rods! So much for the Stock rods as my self limiting measure!

I have my crank and and rotating assembly as well as my cylinders and pistons at the local hot rod shop for balance polish cylinder boring and ring gap filing.

I’m painting case halves and just rebuilt the oil pump, and have welded in a set of the frame adapters to give access to the top end with engine in frame.  Hope to keep a log of the progress here to share my progress

[Cb750r]

Some expensive pictures: [ Guests cannot view attachments ] [ Guests cannot view attachments ] [ Guests cannot view attachments ]

[Cb750r]

I swapped out the wheels, front forks and front and rear brakes, back in 2005. I used mid 90s Yamaha fzr600r. I weighed the old wheels with tires and rotors and combined the new wheels are 50lbs lighter than the old comstar setup. I had them powder coated just before I decided to pull the motor…

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[K2-K6]

Very interested to see your build progress, I always liked the F2 and a friend still has his originally bought one.

Head work looks good, any more pictures of it from combustion side too ?

[Laverda Dave]

Good to see an alternative sled being built, makes a nice change. I agree with Nigel, looks a very nice porting job.
What's all the white powdery stuff under the front wheel, snow already 😱

[SteveD CB500K0]

He’s in Canada Dave.

And it’s the end of November.


Sent from my iPhone using Tapatalk

[Cb750r]

Snow indeed but that’s from last year. It’s still bare in the valleys so far here.

Here’s a couple more shots of the head detail

JMR heads are quite well known on the US SOHC forum. MRieck is a long term member over there and does great custom work.

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[Bryanj]

I will second that, i have a 500 head done by him

[Laverda Dave]

Is that the original shape of the combustion chamber or gas he built up a squish band and blended it in? Dave Degans of Triton Dresda fame did similar work.

[K2-K6]

Originally like that Dave, bigger valves than std for the F2 as well....which pushed them back into the head such that they don't touch in the centre. Needs higher dome piston to maintain compression ratio to go with it.

[Laverda Dave]

Is that the reason why the F2 suffered with premature valve guide wear Nigel? I assume if the valve was higher in the head this would have an effect on the cam/rocker to valve stem geometry compared the earlier models unless of course Honda accounted for this in the design of the F2 head?

[K2-K6]

Is that the reason why the F2 suffered with premature valve guide wear Nigel? I assume if the valve was higher in the head this would have an effect on the cam/rocker to valve stem geometry compared the earlier models unless of course Honda accounted for this in the design of the F2 head?

Yes, the geometry didn't change as far as I understand it. The rocker arc is compromised in its contact with the valve to give slightly more sideways loading through it's initial travel, coupled with raised spring rate for higher rpm too. Tgey deleted some of the oil drainage holes from head too, this to keep more oil around the valve gear prior to going back down into sump.

Honda realised this with development of endurance race 16 valve head onto CB 750 bottom end with the first "RCB" motor in 1976, I think. Then development of the road going twin cam motor.

[Cb750r]

I think the 77/78 bikes used a less durable guide material, heavier valves, more lift in the cam, and poor designed retainers. Also factory tuned leaner due to smog era.  All of those reasons have given the big valve SOHCs a bad reputation. However they still had bigger valves so it’s a great base for a high flowing head when you replace everything that was a disadvantage!

Unfortunately for my pocket book it also means I have a head and cam that will allow more airflow than my stock carbs will allow for so it seems I’m also in the market for a larger bank of carbs.I will cross that bridge a little later but I’m leaning towards a set of Mikini RS34. I have the case haves a coat of paint over the weekend.

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[Cb750r]

Installed my freshly painted bronze top clamp and also found a nice little fly screen and installed it. [ Guests cannot view attachments ]

[andy120t]

Looking good . I like your headlight/screen brackets - did you make/mill them?