While the Honda CB350 engine in stock form is quite potent, it lends itself quite readily to some added horsepower.
If you choose not to build up your engine right away, you still need to disassemble it, check it out and replace any worn components. If you decide to get it blueprinted and balanced (making sure your pistons are equal weight, rods are equal weight, carbs to intake and intake to heads are matched, etc. ) you will find this alone will make the engine much faster and reliable than before.
Since I already had racing experience, I opted for more performance. I chose the 123-20 cam from MEGACYCLE CAMS, San Rafael, California. These cams must be used with racing pistons. I also recommend using high-performance valve springs to prevent valve float at high RPM, which can cause valve and piston damage. Note the difference in the cams. the Megacycle 123-20 cam on the left. Also note the difference in lift and duration.
The reason I chose the 123-20 cam is that it gives me more top-end horsepower without sacrificing bottom-end and midrange performance.
You will need to clean out the entrance for the cam in the cam box so the redesigned cam lobes will fit. You will also need to clean the ridges off the cam box floor and enlarge the opening in the cam sprocket. That's all you need to do fit the new cam.
If your cam rockers are worn you will need to send them to Megacycle Cams to have them reworked or get new ones from Honda.
As far as pistons, I chose the 348cc/12.5:1 piston kit from POWROLL, INC., Redmond, Oregon. Along with the piston kit I also got their high performance valve springs. I chose these pistons for their high compression and light weight compared to the stock units. This high compression piston with its lighter weight helps maintain the low-end torque and gain top-end speed and slightly higher RPM.
If you use these, you will have to check piston-to-head clearance as well as valve-to-piston clearance. You will also have to check piston-to-sparkplug clearance, to make sure that the piston does not hit the electrode. You can go the safe route regarding clearance by following the manufacturer's suggestions, or the closer route used by some racers, but then you will have to watch your tach very closely.
The Powroll, Inc., valve springs are smaller, lighter and stronger than the stock ones and allow you to rev the motor higher with less chance of valve float as they return the valves quicker. The one drawback to installing these springs was in removing the old spring seats which often become heat-welded to the valve guides.
A little trick you may use in removing these is to carefully weld a piece of steel to the seat and use a slide hammer to remove it, being careful not to brake the valve guide. For this little trick I would like to thank Thad, Scott and Coi at CYCLE RIDERS SUZUKI-TRIUMPH of Orlando, Florida. They also did the following head work.
Note the difference in port sizes in the stock and ported heads, the ported head on the left. The intakes were enlarged in the carb area to accept 34mm Mikuni carbs and slightly enlarged all the way through to the valve seat area. The exhaust were only cleaned and matched. Be sure to use new intake manifolds when you do this.
The vlaves were refaced as were the seats to stock specs. You may want to do a multi-angle valve job for better flow.
The rest of the motor was left stock. Disassemble the entire motor and check it for wear and, if you find any, replace the worn part or repair it. A weak point are the cam chain rollers, these are soft rubber and barely last a season of racing, so be sure to replace these along with a heavy duty cam chain such as Tsubaki. Check the crank for play or wear as it will get quite a workout.
The transmission was left stock with the exception of a close-ratio 5th output gear from TODD HENNING, Provincetown, Pennsylvania. This gear brings fourth and fifth gear closer together allowing for less RPMs lost in shifting. This actually lowers fifth gear and you may have to use taller gearing for faster race tracks.
The transmission gears should be checked for wear on the teeth and shift dogs, and replaced, if wear is noticeable. Also check for wear especially in first and second gears as well as the shift drum. Any wear in the tracks of the drum and it should be repalced. In the clutch department check out the steel discs to factory specs and also check them for signs of burn or warp.
Install a set of Honda friction discs and Honda CB500F clutch springs, or shim the stock springs as I did, with spark plug washers.