This section is designed to provide helpful "HOW TO" technical tips on building vintage road racer's. We are always looking for more information, so if you have some helpful advice.....please feel free to email us and we will post it. Some information shown within this Tech Tip's has been gathered from other website's(EUROSPARES.COM, ETC.). Our links page has several good site's with tons of valuable informaion. Most of the "Tech Tip's" listed below are related to building a racer from a Honda CB350 twin. The individuals sharing their knowledge below come from various backgrounds as racer's, mechanic's and all around CB350 expert's.
Thank you gentlemen for sharing your knowledge.
Coversion Charts for Fork Tube Sizes & Steering Stem Bearings
Chris Marshall, Frame modifications: No major ones on my bike. The backbone has been MIG welded solid, and all unnecessary brackets removed. I also welded a tube in front of the tank to mount the steering damper to. and reliability.
Hoyt McKagen, Close up open stampings, shave off the spot-weld flanges and weld the resulting butt seams. Rear motor mount especially needs to be boxed in. Could be lighter or stronger with seat loop replaced with tubing, needs bigger downtube. Or, frame needs beams running from Steering Head to Swing Arm pivot; I'm building one like this now using a 550 as basis. Bike would probably benefit from steeper fork rake, can be achieved at least partially with suspension height settings.
Chris Ford, I didn't make very many modifications to the chassis of my bike, other than to lose as much weight as possible, and install a tapered steering stem bearing. There are a lot of gimcracky little brackets on a CB, and you can cut, chip, or file almost all of them off. That will save you about 3 1/2 pounds, and make the bike look cleaner. I stripped my bike down to the last thing I could unbolt (swing arm), and then started rebuilding, piece by piece.
Buff Harsh, Since you will be using a stickier tire on your race bike(and you'd better!) the frame is going to be loaded beyond of what was intended. The metal is soft (and heavy) and wants to flex. The first modificcation I made was to attach a post under the seat to "tie" the top shock mounts together.(especially if the rear loop of the frame has been cut off) In order to save weight all stock tabs should be removed as well. Footpeg,kickstand, helmet lock, anything that is not useful. The other frame mod I have made is the commonly seen bracing from the main backbone to the up tubes of the frame just below where the stock battery box would be.
Seg Niebuhr, Pro-Flo sells steering head bearings of the tapered variety for the CB/CL 350 although they do not specifically list the CL 350 in their online catalog. The appropriate part number is SSH-750. When the bearings are installed, the top bearing will stick out of the top of the steering head approximately 1/8" - 1/4". That's O.K. Steering damper - the NHK damper available from Pro-Flo is just fine.
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.
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.
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.
EXHAUST TECH TIP'S
Chris Marshall, Exhaust modifications: I use an exhaust I built myself. It has 1.5 headers with a 22 primary length. Mufflers are tapered meggas with no reverse cone, 30 long expanding to 3 diameter. To quite them down they have an external absorption type muffler. Never tighten the header nuts too tight, leads to cracking. Rubber mount mufflers too.
Hoyt McKagen, Use formula: Length = 204,000/.85 X redline, and thank Jim Schneider for this every time you fire it up. Length is from ex valve seat, so actual pipes are shorter than indicated. Pipes should start exactly at port size and increase about 5/16-3/8" in diameter in whole length, preferably proportioned as an exponential curve. These pipes make more power over a broader range than can be imagined and the exact taper isn't really critcal. If a meggaphone is used it needs to be 85% of primary len and have a 45 deg reverse cone stepping down to maybe .75 of megga exit dia; actually I think two shorter reverse cones spaced a couple inches apart and with diverging diameters would be even better. If silencer is used, the pipe must enter for some distance and the silencer contour there must be bigger and distinct from the end of pipe. Anti-backflow headers a la Jim Feuling give a definite advantage. Steeper tapers on pipe and megga give peakier motor.
Chris Ford, The only exhaust modification I made was to go to a two into one collector pipe. Better ground clearance, less weight. That was pre-WERA Belly Pan rule (Year 2000) so make sure any mod in this area can also accomodate a full motor length/width belly pan.
Buff Harsh,ALL exhaust systems for the 350 are custom made and all are different in some ways. There is no right or wrong setup here... I have seen 2 into 1 work great as well as 2 into 2. The Jemco megaphones seem to be the standard route but the welders out there have their own ideas. I will say that shedding the weight of the stock headers is a good idea if you can make something else work.
Chris Marshall, Ignition modifications use a Dyna ignition I built myself, similar to the Barker Sparker. It is mounted on a plate where the alternator mechanism used to be. Its uses a fixed advance of 40 BTDC. I use Dyna coils (3 ohm) and recommend Nology Spark Plug wires. They are expensive but produce a beautiful spark.
Hoyt McKagen, Cam mounted ignition is immune to crank whip which can seriously affect timing and even make good tuners cry. The stock points system is OK to 12,000 RPM. The traditional K-Mart cheapo universal coil makes sparks like you wouldn't believe. A special rig is in development for the crank-mounteds to eliminate whip effects, uses its own bearings and a slop-free driver. Ball-bearing camshaft conversion frees up ~.5 Hp.
Chris Ford, I used stock Honda coils with a "constant loss" system. This is a system that removes all of the bikes electrical paraphenalia, ie starter motor, wiring, etc (saves about 17 pounds), and relies on a fully charged wet cell battery to run the ignition while you race. The down side is that you have to recharge the battery a lot. Still, it's the simplest system around, and it works. Ask Hoyt.
Buff Harsh, The Dyna S seems to have been a good choice, but it's life is limited in the hot crancase oil. The Dyna III is also used by some with success, but I like the simplicity of the S. It runs off the crank and hooks easily up the coils.
SUSPENSION TECH TIP'S
Chris Marshall, Suspension modifications: I use modified Marzocchi 35mm dirt bike forks, revalved for increased damping both ways. I use Progressive Suspension CB750 shocks in the rear. They are adjustable for preload and rebound damping and are high quality. Tapered roller head bearings are a must, as are new swingarm bushings. I had mine built by Hoyt McKagen and they are very good. I also use 78 CB750 triple trees, which have less offset, and therefore more trail, making the bike more stable in fast corners. Other good forks are CB500T forks with straight rate springs from a CB400F in them.
Hoyt McKagen, Rear dampers are not up to racing. Progressive springs are winners on both ends. Swing Arm has good cast iron bushings and they're fine if not loose. But inner sleeves are wear-prone and should be replaced with chromed items. Fork needs better damping, stock unit with some machine work would be OK but really too small for the performance and weight, should be 35 mm min and better at 38. Billet clamps and stiff well-fitted stem make more difference than can be imagined. Good fork is the XL250/350 unit: 35 mm, more overlap, axle fits CB wheel, four-bolt caps, stem fits CB Steering Head and bearings, slightly longer travel which you set to pref with damping rod spacers or springs. Or the legs can be used with any 35 mm tubes. Plenty of them on hand, just ask me. Good solid axle mounting probably helps with chatter and brakes work better, etc. CPSO fork/damper oil makes a real difference in striction. Two words: fork brace.
Chris Ford, Mike Gill, the Service manager at Fredericksburg Motorspots here in Fredericksburg, rebuilt my front forks to stock specs, but changed the oil viscosity to a slightly thicker mixture. At the back, I bought two five way adjustable shocks with greater valve diameter than stock, and stiffer progressive wound springs. I would tell you where I got them, but they are no longer available. The best advice I could give on this issue, would be to see Hoyt's comments.
Buff Harsh, unload the stock rear shocks and get anything else... it does not have to be top of the line at first, I used Progressives for two years. Also a part of the suspension is the swingarm. You must make sure the bushings have no slop. The way you do this is buy new ones and learn how to put them in! The front gets much more complicated. First of all invest in the best steering dampner you can afford, it will save your butt somewhere down the line. Next, install some tapered steering head bearings and check the steering head nut torque frequently.( I just learned this one) The stock front end will work fine until you you start pushing the bike harder. Eventually 35mm forks can be fitted. CB500T, 550, or 750 forks all work and can be had at any salvage yard. A new aftermarket spring would be a good idea at this point as well. It is worth taking these forks apart completely to clean them and replace the seals. Vive' la difference.
BRAKE TECH TIP'S
Chris Marshall, Braking modifications: I use stock rear and a 4LS Water Buffalo front. Nothing out brakes this, and it never fades. It is heavy and does tend to increase chatter in the front end. I use stock compound brake shoes only. Other brakes I would consider are T500 or XS650. Cb450 are also pretty good and cheap.
Hoyt McKagen, Use Mike 'Mercury' Morse's (Vintage Brake) shoe compound. Any time you use a bigger brake it's better. Mike told us his favorate brake choices a while back and picked the all-time top as being the CB450 DLS, and you should all be guided by that. A rarer but still better unit may be the Suzuki '72 GT750 4LS. Carefully set the linkage so it gives synchronised actuation. Make sure cable free-runs while under bump condition, is slick and slack-free. Use ultra-minimal amounts of hi-temp lube on anchor pins and linkage, cam flats/shafts, the cable barrel in lever eye, and the lever pivot, and clean/re-lube both brakes often. Shoes do glaze up some in use and light sanding to clean up often helps. Place levers close to bars by using a master link to space them, or use dogleg levers. Brakes should be 'set' by actuating them hard as the axle is tightened; this makes a surprising diff in performance and maybe more if you spin wheel forward as you activate.
Chris Ford, As far as racing is concerned, braking is the biggest short coming of the CB 350's. The front brake, a double leading shoe with an impressive swept area as far as street use is concerned, will definitely fade away on lap seven of an eight lap event. Anyone considering serious racing on a CB will have to contemplate serious brake mods. The most common is to look for a Suzuki Titan 500 front brake, and lace it up to the Honda rim. This gives the Honda 32% more braking capacity. Race compound brakes shoes for stock CB's are another consideration (see Buff's comments), and drilling large holes (1" or greater) in the backing plate for cooling also have some positive effect on braking with stock components. If you are just going to motor around the track, then stock CB brakes will work out OK, but if you are really going to race, you are going to need to spend on the brakes.
Buff Harsh, Just like the suspension, the stock brake will work ok at first until you start pushing them. there are three directions that people go usually. The sought after '72 Suzuki GT750 4LS, the Yamaha XS 650 2LS and the Suzuki T500 2 LS. I use the T500 brake because it is very effient for a 2LS and also has a weight advantage on the GT750 unit. Vintage Brake linings sure do help as well.
BODY WORK TECH TIP'S
"HOW TO BUILD AN ALUMINUM(ALLOY) HONDA CR750 GAS TANK"
Chris Marshall, I use Ducati 250 Vic Camp style fairing from Clubman Accessories. They are well priced and fit the Honda well. I use early tanks since they are the slimmest with an aftermarket seat. Aftermarket rear sets suck and are weak. Learn to shift GP or upside down style and mount a shifter (any Honda one will fit) backwards. Passenger pegs can be used as rear sets, but solid, non-folding are best, especially when you crash. Use good clip-ons with removable tubes, Like Pro-Flo ones or similar. WERA requires a belly pan now, I used a heavy tin baking pan and banged it until it fit and then mounted with safety wire. It actually does work.
WHEEL/TIRE TECH TIP'S
Chris Marshall, Tires: I currently use a Dunlop KR825 80/90-18 front with an Avon 110/80-18 rear. I used to use the 90/90-18 Avon front but switched to the Dunlop because it is lighter and just as stick. The Dunlops wear out very fast. For just starting the Avons are absolutely the best. Race compound only of course. Rim wise, I use 2.15" rims front and rear. My rims came from the junkyard, the rear off an early XS650. They are flanged alloy rims like Akronts but much cheaper. I used stainless steel spokes and nipples from Buchannan's to lace them up.
Hoyt McKagen, Avon, ask Lyn Garfield for specifics.
Chris Ford, Strangely enough, tire choice proved to be the most difficult of any decision I had to make concerning bringing my bike up to race specs. The bottom line is this: go to your local race track, find out what tires everyone else is using that season , and buy those tires!
Buff Harsh, Avon racing tires or Dunlop TT 100 and KR 124's. The Avons are the benchmark and a fine race tyre used at your local track and all over the Vintage racing world. Same for the TT 100 Dunlops and KR124's. DO NOT USE 591's or you'll be picking yourself out of the tire fence!
Seg Niebuhr, Please note that the end of the racing season right before GNF is NOT a good time to try to find racing tires. They were pretty much all gone by the beginning of November; so, I went with the Avons that are considered the "next model down" from the all-out race tires. I was scraping pegs long before the tires gave way; so, going with "B" tires at first is probably O.K. As a separate note, even though I was getting passed by the modern bike riders in the school, I managed to match their corner speeds with the Avons.