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We take a systems approach to tuning, that is, viewing the bike as a whole with an eye toward selecting parts that work together. First, one should determine what they want the bike to do. A racing cam and huge carburetor will be liabilities on an otherwise stock engine; likewise, it makes no sense to choke off a race motor with a small carb, stock muffler & cam. Component selection is best covered on a one-to-one basis. Most of what is referred to as tuning involves carburetion and gearing.


Carburetor tuning: there are four basic areas of carburetor adjustment: idle speed adjustment, pilot jet and air bleed adjustment, jet needle, main jet.

  • Idle adjustment - the idle speed adjusting screw is usually located on the left side of the carburetor, near the bottom of the slide. This limits how far the throttle can close. By turning the screw clockwise, the throttle slide is opened further, increasing idle speed. Counterclockwise rotation decreases idle speed. The idle should be adjusted with the engine fully warmed-up to operating temperature.
  • Pilot jet & air bleed adjustment - from idle to 1/4 throttle, the amount of fuel delivered to the engine is determined by the pilot jet and how much air enters the pilot fuel circuit. The pilot airscrew is located on the LH side of the carburetor, near the carburetor inlet. Turning the screw clockwise decreases the amount of air that enters the pilot circuit, richening the mixture. Turning it counterclockwise increases the amount of, leaning out the mixture. The screw should be adjusted to get a smooth idle and  a clean off-idle throttle transition. If the screw must be turned in to less than 1/2 turn from fully seated, then the pilot jet needs to be larger. If it must be turned out more than 2-1/2 turns from seated, then the pilot jet is too large.
  • Jet needle - works in combination with the main jet. The thinner the needle, the more fuel that can pass through the main jet, richening the mixture. This covers the  mainly 1/4-3/4 throttle range. By moving the needle retaining clip up, the mixture will be leaner at any given throttle opening. Moving the clip down, to a lower slot, the mixture will be richer.
  • Main jet - larger jets flow more fuel, richening the mixture. The main jet comes fully into play at WOT. Generally speaking it is advisable to select the largest main jet that will allow the engine to rev-out cleanly. Overly lean mixtures can generate high combustion chamber temperatures and cause engine damage. The larger the jet number, the larger the jet and the richer the mixture.

If the air/fuel mixture is off by a large amount, the engine will not run right at a given speed. A lean idle mixture will result in difficult starting when cold, weak idle and off-idle stumble. Rich idle mixtures can allow a cold engine to start without using the choke and may result in rough idle. Lean mixtures in the 1/4-3/4 throttle range will usually result in flat spots in throttle response or outright carburetor bogging. Rich mixtures in this range usually result in misfiring or dead spots on deceleration. A main jet that is too small (lean condition) can affect performance from 1/4 throttle to WOT. It can result in weak throttle response, decreased power at WOT, inability to rev-out and even backfiring. Too large of a main jet (overrich) will cause the engine to misfire at WOT, prevent it from revving-out and will result in black smoke from the exhaust.

Once the mixture is fairly close, the only way to verify that it is spot-on is through exhaust gas analysis or reading the sparkplug. The engine needs to be run at the specific speed and throttle opening being tested for a plug reading to be accurate. It is usually done at low speed/idle, or WOT. For WOT/full power readings, a plug chop is done. The bike is run to redline at WOT, then shut down. The plug is then removed and inspected. A black plug indicates a rich mixture, lily-white indicates lean. Brown-to-tan indicates correct mixture. The plug color is valid for other speeds as well, however, once the WOT and idle mixtures are correct, the rest of the fuel curve is usually correct as well, unless the carburetor is oversized for the engine..


In order to get the full top speed from your bike, you need to know how fast your bike can go with the amount of power the engine makes, the rpm at which that power is developed, tire diameter, internal transmission gearing including the primary ratio and the sprocket combination. For a racing application where top speed is limited, you will need the same data as above, except that actual top speed will be used, since it will be less than max potential. In either scenario, the gearing should allow the engine to be at it's peak horsepower when the bike is at its top speed. In non-limited riding, the bike will reach its full speed potential. In speed-limited riding, the bike will have maximum acceleration.

The formula for determining speed vs rpm is as follows:

.006 x engine rpm x wheel radius (in inches)/final gear ratio (overall gearing) = mph

For example: a bike running a 10" wheel, with a tire diameter of 17.2", 3.888 top gear ratio (.958 trans x 4.059 primary),  with 17/28 sprocket combo = .006 x 10,000rpm x 8.6/6.4 =  516/6.4= 80.625mph


Winter Storage

Those of us living in areas that see winter weather typically put our bikes away until spring. Improper storage is where most of the problems begin. Yet, it doesn't have to be that way.  All it takes is an hour or two of simple prep and your bike will be the way you remember it come spring, instead of greeting the new season with nasty surprises. There are three main issues to address: keeping the fuel system clean, preventing moisture damage to the engine, preventing rust .


  • FUEL SYSTEM...Gasoline, especially the formulations used in recent years, tends to break down rapidly once exposed to air. By the time it has evaporated from the carburetor, it will have left a thick, chalky, residue and varnish that clogs tiny precision fuel passages. Worse yet, it becomes acidic over time and can actually dissolve enough metal to create holes in the float bowl. Rubber parts such as the petcock seal don't fare well either. The end result is a bike that runs poorly (if at all), leaks fuel or both. The solution is surprisingly easy. First, drain the tank and fuel lines. Next, remove the float bowl from the carburetor and make sure that all remaining gasoline is gone from inside the carb and petcock. If you have access to compressed air, it's a good idea to blow out the fuel lines and tank. Replace the float bowl on the carb. Make sure that the tank has no traces of gasoline remaining. This is especially important if you will be storing your bike indoors. Gasoline vapors are extremely flammable, even in minute amounts. If you cannot blow the tank dry, then dry the inside with paper towel or clean rags and a long screwdriver or length of stiff wire. Afterward, leave the cap off the tank for at least 24-48 hours until the gasoline odor is gone. If your bike has a steel tank, then a light spray of fogging oil (sold at most auto parts stores) will prevent rust while the tank is empty. This is also a good time to check the filter screen located in the petcock on some model years.  That's about it...a 30 minute job that will save you hours of grief.



  • ENGINE...A single cylinder engine is easy to prep for extended storage. At a minimum, the oil should be drained while the engine is still fairly warm. This will get all of the dirty oil out of the crankcase. Normal combustion residues, when mixed with the oil, will form acids as normal atmospheric moisture condenses inside the engine. This can result in rust, corrosion and/or sludge formation. For less than the price of a single quart of oil, you can prevent this from occurring in the first place. If you want to really give your engine the best shot a a long and happy service life, then this is the time to clean the centrifugal filter. On stock CT70 and Z50 engines, the filter is located on the outside face of the clutch. You will need to remove the RH engine cover, then the three (auto clutch) or four (manual clutch) retaining screws with an impact driver. Two things to note: buy new gaskets for the RH engine cover and the fliter cover plate before you begin, make sure that the impact screwdriver bit fits the screws tightly or you'll simply damage the screw heads. Cleaning the filter simply requires wiping out the accumulated particles and giving a light spray of cleaning solvent.  These engines don't have much in the way of oil filtration, yet what is there works surprisingly well when kept clean. If you've never seen one of these disassembled, then you'll be amazed by how much crap accumulates in them. We've seen a lot of low-mileage engines in need of rebuilding, all of them had clogged oil filters; most were heavily sludged from lack of oil changes as well. The oil screen should also be cleaned at this time, although it's really only there to prevent large debris (such as logs, discarded tires & sunken ships) out of the oil pump.   After reassembly, the last thing left to do is remove the spark plug and spray a small quantity of fogging oil into the cylinder, then spin the engine over a few times to distribute it. Replace the spark plug, applying a small dab of high-temp antiseize compound, then turn the engine over until you feel compression. This leaves both valves closed, effectively sealing the upper end and keeping moisture and dirt out. We refill the crankcases on our engines with fresh oil at this time. However, if you choose to leave yours empty, then it's a good idea to place a reminder tag on the engine someplace where you can't miss it. There's little sense in taking a chance of seizing an engine after it starts on the first kick. 

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  • RUST PREVENTION...This is the time to clean all the accumulated dirt from your pride & joy. After a thorough wash & dry, simply give all of the polished metal and painted surfaces a good coating of automotive type wax. We prefer paste, but liquid is okay too. If the bike is going be stored in a non-climate-controlled location, then we recommend leaving the excess wax on the bike, polishing it off in the spring. That's all it takes to keep the chrome shiny and pit-free, the polished aluminum from dulling and rust raspberries from erupting through the paint over the winter.






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