Kawasaki Bighorn 350 Street Fighter / Road Racer Build-Up
Overview The Kawasaki 350 Bighorn is a rotary-valve 350cc 2-stroke single Enduro (aka dual purpose) bike built from 1970 to 1975. In the day, a few brave souls raced the 350 Bighorn in motocross and desert racing with occasional success, but in generalit was a horrible off road motorcycle. That said, the Bighorn was a very successful platform in dirt oval racing and road racing (where long travel suspension was not needed). Today, the 350 Bighorn is easily one of the most under-rated and overlooked high-performance vintage machines around. The rotary-valve engine design gave the Bighorn a huge horsepower advantage over other two-strokes of the day, and Bighorns often ran just as fast as 4stroke machines twice their displacement. From the riders standpoint the Bighorn is a unique vintage candidate because it is uncommonly well suited for taller and heavier riders. The chassis offers excellent high speed handling characteristics, and the motor has the power to satisfy any vintage sport riders needs.
The large production run of the Bighorns from 1970-1975 has resulted in a plentiful supply of affordable restoration candidates. In addition, there is an abundance of affordable spare parts available on Ebay and other sites. The Bighorn easily offers more high-performance bang for the buck than any other lightweight vintage two-stroke platform.
Our History- In the early 70s, we road raced both production class and GP class 350 Bighorns with great success in the So-Cal club circuit. With this background experience, we decided to develop and build a 21stcentury 350 Bighorn resto-mod dual purpose machine . With a twist.Our350 Bighorns would be built to excel at high performance street ridingandVintage Class road racing. We spent almost a year testing and developing the prototype, and then built two identical machines for street and racing use. Our goal was to have a fast, affordable, low maintenance, machine that offers secure handling at all speeds. While these machines would need 110 octane race fuel for the sustained high rpm use of road racing, we developed them to run reliably on 91-octane during street use.
We had been away from road racing for 35 years when we went to our first AHRMA race. All our competitors were younger (a lot), lighter (a lot), and better skilled than us. Despite that, our Bighorns were never once beaten (in 2 years of racing) by any other machine in the class. We never experienced one mechanical failure, and even won both days of racing after a practice day crash event.
In 2011 we took the Bighorns to the 4300ft. altitude Bonneville Salt Flats and set the AHRMA classification records for Lightweight Production (97mph), and Open Sportsman MX (109mph). While we were not road racing the Bighorns, we regularly rode them on weekends and at classic bike events. In our eyes, all of that meets our definition of mission accomplished.
AHRMA Miller Motorsports Park 2010 : 1st & 2nd both days
Bonneville 2011, prepared for 97mph pass for the AHRMA Lightweight Production Class record.
It is the purpose of this document to outline the buildup of these 21stcentury 350 Bighorns. We decided to develop four different levels of modification that can suit the full gambit of owners budgets .. and the varying needs for speed. All these versions can be reliable street bikes, but they do have different racing abilities. Our terms for these different level machines are as follows:
Stage 1 – Casual Recreation This machine is a stock appearing Bighorn that offers exceptionally easy starting, smooth idling, and much better than stock overall power. It runs reliably on 91 octane unleaded fuel, and is well suited for commuting as well as general sport riding.
Stage 2 – Sport Cafe- This is a reliable high-performance sport-bike that runs the stock pipe and carb, yet yields the speeds and acceleration that can be competitive in AHRMA Production class racing. While race fuel is needed for road racing use, it can easily run on 91 octane for day to day riding.
Stage 3 – Elite Production racer- This version is the very best that a stock-pipe, stock-carb Bighorn can be. It offers strong high range power to speeds just over 100mph, as well as stable and secure high speed handling characteristics. Like the sport-cafe, it runs high-octane for racing, and 91-octane for street use.
Stage 4 – GP Class- This version is a 120mph+ road weapon that offers handling that matches its speed. The GP version is also suitable as a very competitive platform for WERA 500GP vintage class racing. While it can be run on 91-octane fuel for casual street outings, it does require higher octane fuels for more aggressive street-riding and road racing situations.
About engine Setups- As mentioned above, we do our best to offer setups that suit every budget and speed preference. With respect to the engines, here are the general tech parameters.
A Note about Pipes and Carbs- We have been building racing 2stroke for 40 years, and we are well aware of the power that can come from expansion chamber exhausts and larger carbs. We are also well aware of the shortened engine life caused by high-rpm chambers, and the heavy fuel consumption of bigger carbs. If we were building these Bighorn setups to win national championship pro races, they would be built very differently. However, our goals for these Bighorn setups is to make them only as fast as they need to be to kick the butts of other vintage bikes. That is a much easier standard to meet. And one that often does not require expansion chambers nor big carbs.
Our Elite Production Bighorn can run 104mph at sea-level with a 220lb. (fully dressed) rider. With a 150lb rider it could likely run 110mph. If you plan to spend lots of time riding your Bighorn at speeds over 110mph . You will need a chamber and a bigger carb and lots or race gas. But if you dont plan to regularly run those speeds, you can easily hand the vintage boys their asses with a reliable stock-pipe stock-carb Bighorn . We did and were old fat guys.
While we originally configured our Bighorns for AHRMA Lightweight Production class racing, its worth mentioning that the Bighorns outlined in this document are an excellent fit into the vintage classes of numerous other racing associations all over North America. Below is a list of the most prominent:
Candidate and Donor bikes- The various vintage class rulebooks in North America require varying machine cutoff dates from 1971-1978. Like most other Assn rule books, the AHRMA rule book allows machines that are built and sold before Dec 31st1972 . OR LIKE MODELS. In the case of the Bighorn, the like models are:
Buying Your Bighorn- It bears noting that no matter what machine you buy to build up your 350 Bighorn, you should expect to completely teardown the entire chassis and engine for a race quality assembly. Given this, its not really important to find a perfect donor bike . But it is important to find one that has endured the least amount of hard use and molestation. Its everyones dream to find a perfectly preserved low mileage barn-find bike but there are few of those left (at a reasonable price). If you can find a low-mileage barn-find.buy it.
The more common (and practical) approach is to buy two or more very rough machines so that you can pick the best parts of the lot to build your Bighorn. We purchased 4 non-running Ebay bikes to build our two race bikes. After building up our bikes from the best parts, we still had plenty of useful spare parts that we could take along to the races with us. The rest of the hardware that was of no use to us.we just sold on Ebay
In this genre of motorcycle shopping, its very common to find not running/ no title bikes for sale.
Such bikes are worth $400-500 so long as they pass the basic priority tests. The items youNEED NOT be concerned aboutare engine compression, lighting electrics, fork seals sealing, tires & tubes, front wheel condition, ignition components, seat padding or seat cover condition, air cleaner box, instruments, instrument cables/drives, oil injection pump.
The priority itemsYOU DO look forwhen purchasing your Bighorn are:
Frame- no serious flat spots on underneath cradle or down tubes no rewelded frame joints.no frame tube fractures or fractured paint on frame welds or gussets.
Forks Hatta 3 axle position type forks are preferred…reasonably smooth up and down fork movementno rust on fork leg area covered by fork seals.no big dents in fork sliders even steering stops in both directions. Also spin the wheels looking for rim flat spots and notable sideways bends.
Engine Spin thru engine with kick-starter to confirm starter gear operation and a free engine (good compression is nice, but not really important). Lift the rear wheel from the ground to confirm shifting into all 5 gears and neutral. No visible fractures in the aluminum clutch cover, ignition cover, or sprocket covers no broken cylinder or head fins, no big dents or fractures in exhaust pipe (the longer behind-shock exhaust exit pipe is preferable to the forward of shock exit pipe). Confirm that the shift shaft is not bent (badly).
If a non-running no-title bike cannot pass these tests, it is not a not abike it isjust parts, and should be priced accordingly.
Assumed Replacements- Unless you found a bike that had a recent full-frame-off restoration, there are a number of items that you SHOULD EXPECT TO REPLACE.
Those items are divided into two categories A) Chassis, and B) Engine
Replace plastic swing-arm bushings with needle bearings
Replace steering head ball bearings with tapered roller bearings
Replace front and rear wheel bearings, tires and tubes
Replace Handlebars (with lower & narrower bars)
All engine seals gaskets and O rings
Connecting Rod Kit (with Yamaha rod kit)
Entire ignition system (with light flywheel ignition)
Crankcase and side case cover Phillips bolts replaced with Allen bolts
Starting The teardown- If your donor bike is badly caked with grease, mud, dirt, etc.Its worth your while to try to get as much of the grime as possible off at a high pressure car wash before your teardown. With that done, its time to just tear the entire bike down to the last nut and bolt. Its wise to assemble as many loose bolts and parts as possible onto subassemblies, i.e. put rear axle, spacers and chain adjusters on rear hub, swing arm bolt and bushings back in swing-arm, etc.. the fewer loose bolts and parts you have, the better. For the bolts and parts you cannot put on sub-assemblies, its wise to have a box of zip-lock bags on hand so you can bag bolt groups together (fender bolts, engine case cover bolts, etc).
The initial teardown is never a pretty thing. Do your best to keep things together in one place, and quickly separate away all non-candidate parts that will not be part of the final build.
Engine Format Choices- There are two big technical choices each owner needs to make that will forecast the direction of their 350 Bighorn build. Gearbox gearing, and ignition system.
Gearbox gearing- All F5, F8, and F9 engines come with the same lower end, clutch, and gearbox. This stock gearbox has a very wide ratio gap between 4thand 5thgear. This stock gearbox will require an engine (and cylinder porting) that offers an exceptionally wide powerband to allow for strong acceleration to pull the 4thto 5thgear shift. The Klemm Vintage Sport Caf spec porting offers competitive peak ground speeds along with the powerband needed to pull this 4thto 5thgap. Our Elite Porting also has the ability to pull this 4thto 5thgap for riders under 160lb. However 200+lb riders wanting to run 100mph speeds will struggle to cover that gap. These riders may want to consider an elite gearbox.
Elite Gearboxes- In 1971, Kawasaki sold the F81M 250 that was a special off-road model built specifically for racing. The two items that made the F81m truly special where the lightweight-flywheel points ignition, and a close-ratio gearbox both of which could fit into any 350 Bighorn motor. In truth, only the 2ndthrough 5thgears of the F81m gearbox were different. The 1st gear, transmission shafts, shift forks, and shift drum were all identical to the F5, F8, & F9 models. A further truth was that the F81M second gear set was inherently weak (prone to breakage) so most racers of the day used only the 3rd, 4th, & 5thF81M gear sets. The closer ratios of these F81M gears allowed for a slightly narrower powerband (ie. higher port timings) that yielded better peak speeds. Sadly, it is tough to find an F81M gearbox. Or the 3rd 5thgear-sets . But they are still out there.
Another option is to install a Kaw H1 500 5thgear set into the Bighorn Transmission (it fits right in). The only down side is that the H1 5thgear countershaft will not mesh with the F5 gear next to it. The solution is to custom make a replacement for this F5 gear that meshes with the H1 5thgear. While it is a bit of work, this results in 4thand 5thgear ratios that are close enough together to allow for the more aggressively ported Elite/GP cylinder. As of this writing, Klemm Vintage does not yet offer this special made gear . But it is in the works. Either way, an Elite or GP Bighorn would have to use the F81m 3rd-5thgear-sets, or the H1 5thgear conversion.
Ignition Choices The stock cdi ignition electrics of the F5/F9 350 Bighorns were often problematic and prone to failure when pressed into sustained high-rpm usage. In addition, the stock Bighorn ignition flywheel was incredibly heavy (a whopping 4 lb). For a high performance 350 Bighorn, a more modern, lightweight flywheel (under 2lb) ignition needs to be fitted for the purposes of sheer reliability, and improved acceleration.
The most affordable option would be to find a genuine F81M points-operated ignition with its light ignition rotor (20oz). The F81m ignition bolts right on and works nicely as long as the points and condenser are kept in good repair. The F81M ignition would be a great choice for a sport-cafe machine, however it lacks the advance/retard curve that would be desired for an elite or GP build. (read about advance-retard ignitions here).
The next best ignition option would be to install a Penton Racing ignition. Penton offers two different ignitions for the Bighorn. The MZB model that has a very light flywheel and a charging coil, but it is a straight-firing ignition (no advance/retard curve). This straight firing model would be suitable for a recreational or sport build, but not an elite or GP build. Penton also manufactures the PVL model ignition that has a very light flywheel and an advance /retard curve. But no charging coil.This is an excellent choice for an elite or GP build. While these Penton ignitions are a bit expensive on the outset, they are much more reliable that stock F5 electrics, and still cheaper than trying to buy soon to fail NOS 1975 cdi boxes and stators for the stock Bighorn ignition.
The final ignition option is to custom fit a 2000+ KX 250 ignition. These very reliable ignitions have a light flywheel (18oz), a nice advance/retard curve, and are often very inexpensive on Ebay. The only down side is that they require custom machining of the crankshaft taper and a backing plate adaptor to fit on the Bighorn cases. Klemm Vintage will do the custom machining to fit your KX250 ignition to your Bighorn motor, but this custom machining is a bit expensive. (see price list below below)
Bottom left is the 18oz. flywheel from the 2000 KX 250 ignition. Besides the very light flywheel, this ignition also has a very desirable advance-retard ignition curve feature.
Right is the stock Bighorn flywheel that weight in at 4lb (64oz). The weight of this flywheel yields slow engine acceleration, as well as rapid left side crank bearing wear.
A Note About Ignitions and Starting- While the heavy stock ignition flywheel is horrible for high-performance and competition use, it does make for very easy starting of the big single. The inertia of the stock 4 pound flywheel helps the rider to spin the engine through a kick-start, and greatly reduces the occurrence of any kind of kickback. Converting to a light flywheel ignition reduces this flywheel assistance, and so requires the rider to deliver a stronger and swifter kick for starting. This is not a difficulty on the Recreation, and GP versions because their compression ratios and ignition timings are relatively conservative. However the stock-pipe Sport and Elite versions have much stronger compression, as well as more advanced ignition timings (in race spec settings). These Sport and Elite racing setups can exhibit some mild kickback in race spec, but very little kickback with the 4 retarded timing used for street spec riding.
The natural answer for most riders would be to simply push start the bike …. but not with a road-going Bighorn. The combination of tall gearing and high compression of all these setups insure that only an 200+lb Olympic sprinter, or the steep down-hills of San Francisco, could bump start these bikes. Fortunately, AHRMA rules (and most others) permit the use of kick starters so long as there is a retaining device to hold the start lever folded in (we use a rubber strap attached to the frame). The only other starting option is a battery driven track starter. We never used such a starter, because you cant take one on a road ride with you. We felt it was stupid to develop a road bike that you cant easily start on the road.
Engine Build Objectives- Our build objectives for every level engine package are A) Long Term Reliability B) Affordability C) Competitive racing speeds. At the same time, we do our best to attend to the Bighorn engines known weakness areas. The 350 Bighorn engine had a long reputation (from back in the day) as an engine that experiences frequent piston seizures and this reputation has some footing in truth. However resolving the causes of these past seizures is not difficult at all. With all of our Bighorn builds, our first order of business is to attend to all the weakness areas that resulted in the poor reliability reputation that 350 Bighorn had back in the day.
Air-leaks- By far the number one cause of Bighorn engine failures is/was air-leaks. There has long been an assumption among careless mechanics that a freshly rebuilt engine will be free of air-leaks. And nothing could be farther from the truth. At Klemm Vintage, we pressure test every engine we rebuild, and find that about 80% of them still have very big (and lethal) air-leaks in odd places that no one would never suspect. The oddity of these air leaks is not important. What is important is that ANY air leak can (and will) result in poor carburetion at low speeds, and a scored piston at high speeds. There is no such thing as an acceptable air-leak . Any leak must be fixed. For more detailed info on how to test for air-leaks click here.
Head Dome Design- The design of a 2stroke cylinder-head dome has a big effect on the engines operating temperatures, and detonation risks. The dome of the stock 350 Bighorn is easily one of the most detonation prone designs that we have ever seen (who knows why they made it that way). The good news is that this poorly designed dome can be modified, and/or filled in with weld, and the head-dome can be re-shaped to have a very detonation resistant and cool operating design. This head dome modification is a service that Klemm Vintage offers (see price list below). For more detailed info on detonation risksclick here.
About Spark Plugs- The stock Bighorn head accommodates a short reach spark plug, and Kawasaki recommends an NGK B10 heat range plug (which is a very cold plug that fouls easily) For our high rpm Elite and GP setups, we welded additional aluminum material onto the head so we could cut a more deto resistant dome design, as well as use a long reach (3/4) spark plug. We use the long reach plug because it has more thread surface contact area with the head, which helps it to exchange its heat away to the cylinder head. This improved heat exchange allows for the use of a hotter heat range spark plug that is more resistant to fouling. To further improve spark plug heat transfer, we also removed the stock spark plug washers and ran only solid copper spark plug washers (to get better heat transfer from the plug seat to the cylinder head (every little bit helps). With this long reach design and the copper washer, we could safely run a hotter NGK B9EV heat range plug during sustained high rpm road racing, and a B8ES/B7ES during our street riding. With these hotter heat range spark plugs, the bike started easier, had better throttle response, and never once fouled a spark plug during racing or street riding.
This is the stock F5/F9 Bighorn head dome. Its conical shape and absence of a squish band made it very easily prone to detonation even on the 98 octane premium fuel of the day …. and worse yet on todays 91-octane premium.
This is our Recreational Head modification that offers a reshaped dome with a squish band. Even though the squish band is narrow, it helps greatly in reducing detonation. This head mod also includes a slight increase in compression ratio over the stock head.
This is the Elite head mod that features a center mounted long reach spark plug, and a wider squish band. This design offers the excellent detonation resistance and improved spark plug cooling that is needed for sustained high-rpm applications
Oil Injection- While the oil injection can be a great convenience to recreational riders on stock machines, it holds a very big reliability risk for any engine that will turn sustained high rpms, or higher than stock (6000) rpms. Removing the oil injection and converting to premixed fuel will yield a big reduction of failure risk, as well as a big reduction in air-leak risk.
Connecting Rod and Crankpin- The stock 350 Bighorn uses a stepped crankpin with a very large diameter lower rod bearing. This large bearing works fine in low-rpm Bighorn motors, but it is a big failure risk in a high rpm competition engine. The standard fix for this problem is to replace the rod kit with a particular Yamaha rod & bearing set that is far more reliable than the stock Bighorn Parts (see price list below). This Yamaha rod has exactly the same center to center length as the stock rod, but it has different diameters at each end. Even so, it fits perfectly with the modified forged pistons we use in our Bighorn engines.
Small End Rod Bearing- The small end rod bearing in a 8000+rpm Bighorn engine has a very hard life. It helps that our Yamaha rod conversion replaces the stock 18mm bearing with a 20mm bearing. However, not all 20mm bearings are created equal. We opted to use a special high load bear that has more needles, and longer needles than any conventional 20mm bearing. We consider the high-load bearing to be mandatory for Elite and GP Bighorn engines.
Piston- The stock Bighorn piston is a very heavy part that uses 3 piston rings. For a low-rpm stock Bighorn, this stock piston offers great torque and good long term wear. However in a higher rpm engine, the three rings of this cast piston sap off acceleration ability as well as high rpm ability. Back in the day, Kawasaki offered a lightweight forged 2-ring piston for the 350 Bighorns but the supply of those pistons vaporized many years ago.
More recently, we have found another 2-ring forged piston from a current model two-stroke that fits perfectly into the Bighorn engine (fitted with the Yamaha rod), and offers excellent long term service. We strongly recommend to upgrade to this more modern forged 2-ring piston that is lighter, easier to find, and available in numerous sizes. Converting to this piston does mandate cylinder-head machining. But the effort and expense are well worth it.
Attending to these pivotal areas of the Bighorn engine, results in a motor that has excellent long term reliability, even under the most extreme high-rpm racing usages.
Left- Stock F9 3-ring Piston Right:- Modified forged two ring piston
Note : weights are piston, rings ,and wrist pin
Left: Stock Kawasaki F9 Rod Right: Yamaha Rod
Preparing a Reliable HighRPM 350 Bighorn motor
About Crankcase Preparation- The crankcases of the 350 Bighorn engine have a few areas that need to be scrutinized and attended to. Most important of these, is the press fit of the crankshaft bearings in to the cases, and the flatness of the base gasket surfaces.
Crank Bearing Fit Both of the crank bearings of the Bighorn should be a gentle press fit into the crank cases. Unfortunately, over long term abusive operation, the ignition side bearing can become loose in the case as a result of loads from the excessively heavy ignition flywheel. At the same time, the drive side bearing sometimes becomes loose from the sheer loads of delivering power to the primary gears. This bearing fit problem is compounded by the somewhat soft steel bearing holders that are cast into the crankcases. If these loose fit bearings are not tightened, the result will be very significant vibration at higher rpms.
The fit between the loose cases and these bearings can easily be restored by re-sleeving the cases (for more info click here). We strongly recommend that any loose crank bearings in the cases be attended to by this resleeving process.
Base Gasket Surface-In the 1970s, it was very common for manufactures to make crankcase halves as individual parts, not as matched pairs. Given this, there were often significant variations in base gasket surface height between the left and right case halves. This difference in heights results in the cylinder not sitting square on the cases, not to mention significant airleak risks at the case center joint. To resolve these problems, we assemble the two empty case halves together, and mount them in a mill to cut the base gasket surface perfectly flat, and square to the crankshaft. Of the four running machines we bought for our project, all four has significantly mis-matched base gasket surfaces. This service included as part of our Engine Blueprint Assembly, or can be done as a separate machining operation.
Above is a bored crankcase & new bearing sleeve ready to be pressed in.
These cases (from a running engine) had a replaced left case whose base gasket surface mismatched .008 and air-leaked badly. They are now even and square.
About Crankshaft Preparation A stock, low rpm, recreational Bighorn can easily offer good long term reliability with the stock Bighorn crank and connecting rod. However (as mentioned above), we strongly recommend the installation of the Yamaha rod kit in any high performance 350 Bighorn platform. Beyond the installation of the Yamaha rod, there is another modification we offer for the Elite and GP level Bighorn engines . a Top Located Rod Modification here is why its better.
A stock Bighorn crank has a thrush washer on each side of the lower end of the connecting rod. These copper thrust washers locate the rod on the center of the crankpin. While this Bottom Located Rod setup works great on a low rpm recreational engine, it does have a performance down-side. As the engine spins up to higher rpms, there is significant amount of rotational friction resistance between the connecting rod and these thrust washers. As well as between the thrust washers and the crank flywheel halves. To address this power loss, we cut down the thrust washers (and crank flywheel halves) so that the rod cannot make contact with the thrust washers or flywheels. We then install some custom fabricated stainless steel centering washers to fit between the piston and the top end of the rod. Since the rotational surface resistance at the top of the rod is far less than the bottom of the rod, this top Located rod setup offers noticeably quicker acceleration and throttle response, as well as much better big-end rod oiling. The only down side to the top shimming setup is that the top end rod bearing runs considerably warmer. To attend to this, we replaced the top end rod bearing every 40 operating hours (we have a digital tachometer/hour meter mounted on our Bighorns).
About the First-Year F5 and F8 Cranks- There are some minor differences in the engines of the first-year 1970 f5 / 1971 F8, and all the other later models of the 250 and 350. By far the most meaningful difference is in the crankpin and crankshaft flywheels. All F5/F8/F9 engines use a stepped crankpin whose diameter is larger in the center where the big end rod bearing rides. However the first-year cranks used a crankpin that had a center bearing surface that is 2mm narrower than all the later cranks. We can only presume that KMC engineers realized that the first-year crankpins did not offer enough bearing surface area for the torquey 350 engine, and so updated to a crankpin with a wider bearing surface area. To accommodate the updated wider crankpin, KMC engineers made the later crank flywheels 1mm narrower than the first-year cranks. For a very short amount of time KMC sold rod rebuild kits for the first-year cranks. However after 1972, when these first-year engines began to fail rods, Kaw shops were instructed to install the later crankshaft with the wider crankpin. The later cranks were cheap and plentiful, and often the crank upgrade was covered under warranty.
Fast forward to today, and (surprisingly) there are still many low miles first-year engines out there that still have the original first-year crank. Sadly, the few rod kits that were mad