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Building a Honda CB 350

Rowbeling road by F Melling Daytona on a Shoestring Daytona, FL,Mar 6, 2001


The Honda CB350 (fig. 1) is a very dependable, versatile yet common motorcycle to come by, since there were more of these motorcycles built than any other single production motorcycle.

My decision to build a CB350 for vintage racing came about for several reasons. I had been racing a 1971 Suzuki T500 as well as a 1971 Honda CB450 for a couple of years with moderate success.
In stock form they were relatively easy-to-ride bikes, but once performance changes were made to them they became ever more difficult to ride and maintain. I spent more time fiddling with the bikes than I did riding them. Always it seemed I was trading one good working bike for the other; one year the CB450 worked well and the next year it was the T500.

The T500 was getting too fast for the stock frame setup and I couldn't use full throttle on the east bank of Daytona until I straightened out coming to the start/finish line. Additionally, it required serious man-handling to get around the infield.
On the other hand the CB450 was giving me fits, in large part, due to my inability to degree the cams; a job that can be rectified with lots of money, money that could be used towards, let's say "a dependable, versatile and common motorcycle."

The whole point of my racing was to have fun and race with my buddies. I was in different classes than they were and tired at watching them have fun, some on virtually stock machines, like Honda CB350s!

I had found the bike I was going to build next.


Upon hearing of my project, my good friend, Kirk Cossairt, showed up at my house with a Honda CL350 rolling chassis, the Scrambler version of the CB350 (fig. 2). As luck would have it, many parts from my CB450 frame would fit.
This frame and also the SL350 frame are AHRMA- and WERA-legal. However, the SL350 frame takes a little more work to clean up and has two front down tubes instead of one.

A word of caution:
It is important to get a clean title or Bill-of-Sale, with address, phone number and drivers' license from the seller. They do at times require the VIN (Vehicle ID Number) on tech sheets and, if your bike's VIN comes up on the stolen list, you could lose all you have worked for.

Once you have a complete motorcycle, you need to disassemble it to the bare frame and swingarm. At this point you need to check for alignment to see if it has been wrecked and make sure there are no cracks.
Strip the frame with a good grade automotive paint stripper or sand blast it, since paint has a way of masking defects to the naked eye. When this step checks out you can start removing all the unwanted metal from the frame and start welding up all the pressed seams.

All areas with arrows (fig. 3) were removed. The centerstand and related brackets, the rider footpeg mounting brackets, the bracket under the steering head, the brake pedal bracket and stoplight switch bracket as well.
All pressed seams were then welded for security and strength at speed. I chose other than stock bodywork so the mounting points for it had to be done at this time also (fig. 4).
The seat mount was made to fit a Yamaha TZ250 tailpiece (by National Fiberglass); the front tank mount was moved forward to fit a 1976 Honda CB400F gas tank, and a rear tank bracket was added also.
The steering stop was moved 1/8-inch higher to clear the Honda CB500F lower triple clamp and a steering dampner bracket was added. (Note: a dampner is required in WERA racing).

Unless you are an accomplished welder, make sure you take it to a professional such as Rick Breckon, owner of ROYAL WELD MANUFACTURING of Longwood, Florida, who did our work. Not only is Rick an accomplished welder but he is also the WERA National Vintage Administrative Director.


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 (fig. 11), 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 (fig. 12). 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 (fig. 13), 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.

While I was gathering up suspension parts the frame was sent to Bill at BIKES ONLY of Orlando, Florida, to get prepped and painted.

While the original suspension is adequate for a stock bike, I chose a different setup.
I used the front-end of my former Honda CB450 vintage racer because it was a bolt-on item (fig. 5). It consists of Honda CB500F triple clamps with tapered steering stem bearings, 1972 Suzuki GT750 fork tubes and a four leading-shoe brake laced to an 18-inch shouldered aluminum rim, 2.15-inches wide.

The rear wheel also off my CB450 vintage racer was a bolt-on item (fig. 6). It consists of a 1971 T500 rear brake hub laced to an 18-inch shouldered aluminum rim, 2.15 inches wide. This rim is known as a WM3 for it's width. While the shouldered rims look nice they are not really necessary. Non-shouldered rims can be used, and are much easier to clean.
Be sure to follow your rule book on rim width, as different racing clubs have different rules on this subject.

The finished rolling chassis, less bodywork, is shown in its final stage of being painted (fig. 7). The rearsets were pirated off a 1994 Honda CBR900RR. They are lightweight and rather short as to allow more ground clearance when cornering. These are mounted in the holes originally used to mount the stock exhaust.
The shifter (fig. 8) had its bottom shift rod bracket cut and welded to the top of the shifter to allow more room for the exhaust to tuck under the frame and foot peg. The front shift shaft lever also faced upwards and was off the CBR900RR as it was longer and allows for extremely smooth shifts. With the rear T500 brake being cable operated, be sure to mount a brake lever stopper, so it won't return all the way up or flop around on you.

The rear shocks are 1/2-inch taller than stock and are dampening adjustable. While assembling your rolling chassis, I recommend replacing all bearings, i.e.: steering stem with tapered roller bearings, wheel bearings with double dust seal type, new swingarm bearings and bushings (bronze bushings if you can find them or have them made). Any play in these areas and you may get into a tank slapper you may not get out of!

In the control department I used a set of clip-ons rather than clubman bars, which work just as well, because of their adjustability up and down the forks. For the clutch side (fig. 9) I used a Suzuki GSXR clutch lever and perch with the ratchet adjuster for clutch cable slack. The reason behind this is if the starts are held up and your clutch gets hot, you can move the adjuster with one hand rather than having to loosen a nut first, keeping your bike from creeping, which will definitely get you black flagged in WERA events.
For the brake side(fig. 10) I used a 1972 Suzuki GT750 brake lever and perch that accepts two brake cables for the Suzuki 4LS front brake. The throttle was off the 1976 Honda GT500 with an on-off switch wired to the Motoplat Ignition from Todd Henning, which we will cover later.


For ignition I went with the battery-less Motoplat from TODD HENNING, Provincetown, Pennsylvania. This ignition gives you a hot spark from first kick to maximum revs, without a battery as it is not a total-loss system. The rotor is smaller and allows you to rev the motor quicker.
This system comes complete with coils and is milled to the stator cover you send, which is then returned to you completely timed. You also receive complete installation and wiring instructions.
While this is the hot setup, points with the total-loss system do work fine (fig. 14), as long as your battery is in a perfect state of charge and the points are set absolutely perfect.

In a conversation with Knobby Clark I asked him about the point system; he told me that was all they had back then. You could rev the Honda 250 6-cylinder to 12,000 rpm.
You may need to double-spring the points to keep them from floating at high RPM, and it is a good idea to check the points after every race. You can also remove the alternator rotor once it is timed to reduce weight on the crank and get a quicker revving motor. Bosch coils with built-in ballast resistors are a good choice for a hotter spark (fig. 15).

The carburetors used were 34mm Mikuni round-slides. I used 34mm for more fuel due to the increased flow from the porting done to the head. Carburetor setup was: 3.0 slides, 6DP17 needles on the 3rd notch, 159-Q5 needle jet, 40 pilot jet, 210 main jet, air jet screw 2-1/2 turns out. The throttle cable came from the 1976 Honda GT500 to match the throttle assembly.

I used gearing from SPROCKET SPECIALISTS, Palermo, Cailfornia, in the following sizes: front: 15-, 16-, 17-, 18-; rear: 30-, 33-, 35-. I chose a 520 chain for its lighter weight and narrowness, which lets you fit a wider rear tire. This gearing allows me to gear for any track! For example: for Roebling Road, Georgia, I use a 17-33, and for Daytona I use an 18-30, because of the long straights and high speeds turned there. At Daytona with shorter gearing the bike would reach peak RPM too quickly and not be able to stay up with the other bikes.

The exhaust system (fig. 16) is comprised of header pipes for a Honda CB450 2-into-1 exhaust, cut and welded to a set of tailpipes from JEMCO PIPES, Houston, Texas. These tailpipes come completely welded and all you have to do is fit the headers and weld on larger brackets. It is wise to rubber-mount the rear hanger brackets as you may crack the pipes from excessive vibration, and get the buzziest feet you've ever had.

For the front suspension I stayed with the stock 1972 Suzuki GT750 components. I use a 1/2 in preload spacer and 10 weight oil to Suzuki specifications. In the rear I use a set of Koni shocks with dampening adjustment set at 2, and have found this to be perfect for most tracks except for Road Atlanta, which has a sharp transition comming out of the "esses" and further up the track going into "gravity cavity".
However, this is an area that is different to each rider.


Eric & bike I wish to thank Luis Hernandez, Publisher of Motorcycle Shopper Magazine for the opportunity to write this article and race under the Motorcycle Shopper Racing colors; Jim Reed for his backing of this project, all the sponsors mentioned throughout this article for their support, as well as Thad Levandoski and Kirk Cossairt for their help and generosity.

I wish all of you lots of luck and enjoyment in your new project and hope to see you out at the track. And when you're there look for the white-and-red Motorcycle Shopper Racing CB350 with #158 on it and stop by and say hi.

Eric Kalamaja