Honda CB350 Road Racer FAQ, Engine Modifications Page

CB/CL 350 RACE BIKE FAQ ENGINE MODIFICATIONS PAGE

Chris Marshall, Motor modifications: If you are just starting, no major mods are required. You must absolutely replace your cam chain with a Tsubaki cam chain (same as CB750) and the tensioner should be replaced. You can use stock tensioners, but must replace them at least once a season. I recommend the KA Performance slipper cam chain tensioner, which I use in my bike. It has no moving parts to fail and is good to the chains. For fullbore racing: Todd Henning pistons are best. Makes the bike a 362cc with 12:1 compression. Either the megacycle 12340 cam, or if you want larger valves, the “X5” cam from Megacycle. I use stock valves and sizes, with Bronze valve guides from Precision Machine/Black Diamond. No valve stems seals. If you do not replace the guides, the valves will weld themselves to the guides above 11,000 rpm. I use R/D valve springs and titanium caps. Replace the springs at least once per season. Carry a spare set, just in case. I use Bore Tech carbide coating in all my cylinders. Better heat transfer and longer life. 32mm Mikunis are good. An SL350 throttle cable with some outer housing removed will make a good cable that uses the stock throttle. I use CB360 intake manifolds. Tranny is stock except for Todd Henning close ratio fifth gear. Barnett clutch plates and springs. Amsoil 2000 full synthetic or similar only.
Motor Notes:
200” pounds torque w/oil on head bolts
.005” valve clearance both intake and exhaust (megacycle cam)
.035-.040” squish clearance (.005” extra if using solder check)
.040” intake valve clearance to piston
.060-.070” exhaust valve clearance to piston
Plugs should be B9 for racing.
Carb notes:
32mm Mikuni
3.0 slide with 6DHP needle in middle position. Built motors will use anything from 180 to 220 main jets. Get the full set.
Hoyt McKagen, Careful assembly with good dimensionally accurate parts will pay dividends. Copper head and base gaskets make the top end cooler but must be used with sealer or they always weep. Stock cam and springs are robust combo and will make real good power under the right carbs and pipes. Racing springs usually accelerate cam wear and don't give advantages at higher revs unless used with radical cams. Higher compression is a must and if obtained partly by milling the head or block this also retards camshaft, for more peak revs. A vernier sprocket is helpful in timing cam. Set ring gap about .003" max, obtained if needed by filing oversize ring sets. If gaps grow over .008-.01" you'll lose some bottom end. Piston clearance should be about .0012" with cast or .002" with forged slugs, and cast last longer because of higher surface hardness. Slightly closer fitting followed by longer breakin is probably better. Valves need be flat all the way across seats, so if sealing surface is recessed the first step is perfectly concentric finish-grind. Valves must seat perfectly, by lapping with fine grade grinding goop and final polishing with oil alone. Should be checked by pushing onto seat by hand and spraying starter fluid or whatever into port. If done right they will show no seepage even hands-off. Valves should have anti-backflow cuts. Ports should not be enlarged more than needed for bigger carbs and this should be tapered into the stock profile just past the guide boss, or to a profile matching bigger intake valve if used. Porting should be to the top of the port even if it means offsetting the carb holders, but be careful not to break into the cambox, undercut the guide boss, or denude the guide. Gas velocity must stay high so stock port sizes with a good touch-up are not at disadvantage with reasonable displacement increases. Even with much bigger displacements the ex port may not need be enlarged; if the motor is more efficient due to higher CR, it passes cooler denser ex gas hence can do more of it thru same time/opening. On the whole the space is better used upsizing the intake valve. In some heads the intake port has a fairly obvious corner a short distance upstream from valve and the area just beyond it, under the guide boss, is a great place to build up with metal-filled epoxy, to round off the corner and prevent flow separation. You can run with one primary gear half-pinion removed (and spacer added) for a bit less power-train loss and it won't wear much faster, and you have the other side for spare. Clutches are usually adequate with stock plates and if necessary shimmed springs. Basket wear causes missed shifts and is hard on the tranny and shift mechanism, so be alert to it. Motors built with lots of poke may show signs of case-halves shifting in vicinity of Rear motor mounts, so the long fasteners rear of bearings should be made 8 mm. Head studs seem to be breaking in some powerful motors, so they should be replaced with a set of 10 mm from Yam XS 650; especially with copper gaskets these would be fine at 15-18 ft/lb. A word on cranks: The stock crank is as heavy as the one in XS 650, and the vehicle acceleration will be improved by taking off some of that. The lighter cranks may lose a touch on top end because the average crank energy is a bit lower when wound out. But races are mostly won in the slow where chassis counts more than motor anyway, and even fast tracks show the advantages of having extra acceleration from corners. So I would advise cranks be gone over in the lathe and magneto rotors be left off. The one caution in that is the only major CB350 crank failure I ever heard of came in a racing bike campaigned by G Jennings. It broke somewhere probably the middle and the individual halves did their thing with the valve train until it locked up. Caused a wonderful T-W-O crash too. It may have been a lightened crank and possibly resonance in higher modes than standard is what cracked it. So I think getting too savage with weight removal isn't good idea.
Chris Ford, For the first season, the only motor modifications I made were to the cam chain tensioner, rollers, and slipper. That should be the very first modification any would be CB racer should make to his or her bike, unless it is already race prepped, and you should ask the seller if that has been done. If you are buying a yard sale bike with the hope of going racing, remember, these bikes are over thirty years old. Age takes a toll. Ask me. I can predict the rain by my left knee. The second season, I got a two into one collector, bigger jets, and velocity stacks for the carbs, because they look neat. If you are going to make various motor modifications, then do them one at a time, and analyze the result, before pushing on to the next mod. There are a lot of things you can do to the CB 350 motors that increase horsepower, and there are a lot of things you can do that will decrease engine life. Make sure you know which is which.
Buff Harsh, 350 mills are strong for their size. A pipe and carbs is the first and easiest mods because the motor stays in the frame! Porting of the cylinder head is another good idea, and it is worth the money to let someone do it who knows what removing metal from a port really means. There are many stock parts in the motor that are fine, and some people will say the tesioner rubber wheel is one of them... don't believe them. Chris Marshall has a slipper style now and I have had good luck with Stanley Liperts extinct metal rollers. Obviously, the next step is big bore pistons, R/D springs and Megacycle cam. Todd Hennings pistons work very well and You are wasting your time trying to find maximum power from something else. Todd has really developed the Hondas and has designed pistons specifically for these bikes- use them.

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