The Service Department
THE SERVICE DEPARTMENT
JOHN DUNN
GP2 TT TIPS
I recently purchased a slightly used Amal GP 2 to be installed in my 1966 Bultaco 250 TT, which I use primarily for flat tracking. I bought it from a BSA dealer who had removed it from a Spitfire Mk II because he thought it was causing the machine to perform below par. It was taken from the right side of the bike, along with the float bowl.
The carburetor bore is 1 3/16-in. except that it necks down to 1 5/32 in. from the engine side of the slide on. I plan to have this turned out to 1 3/16 in. and to have an O ring groove made on the mounting surface by a machinist who is good at this type of procedure. Do you approve of the plan?
There is an air bleed on the carburetor, which I was told should riehen the mixture when approaching maximum speed in fourth (top) gear to keep the engine from frying and allow more speed. I’ve noticed that the air bleed assembly is completely removed from some drag and road racing machines. Why is this done? What is the purpose of the air bleed, and how should the carburetor be tuned to take full advantage of this device?
Should I use this carburetor for TT and scrambles or should I revert to my Monobloc for this rougher use? On which side should the float bowl be mounted to take advantage of the left turns when flattracking?
Scott Erickson Elk Point, S.D.
The GP Amal carburetor is a precision instrument, designed for the ultimate in maximum output.
I advise that you do not enlarge the diameter of the choke by machining the bore between the engine side of the throttle slide and the outlet. The reduction in diameter at this location is essential, unless the engine is going to operate on full throttle at all times. Increasing the area here will result in poor throttle response and carburetion.
When the carburetor is correctly jetted and at normal operating temperature, the air jet control should be fully open at all times. Closing the air jet will
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result in a rich mixture which is necessary for cold starting and operation until the engine has warmed up. If it is necessary to partially close the air control under sustained full throttle, it is likely the main jet is too small or there is an internal or ignition malfunction.
The air control can be a useful device for determining correct main jet size. To do this, run the machine in top gear at full throttle on a good straight section. Then close the air control slightly. If rpm increases, it is a sure sign that the main jet is to small. If the rpm drops, the jet is either correct or too large. When you have found the best jet size, insure that the engines does not tend to overheat when held on full throttle. In this case go one jet size larger to be safe. It should have little effect on rpm. The air lever on standard Amal carburetors cannot be used for this purpose because it is situated in the effective choke area and even if partially closed will choke off air flow through the carburetor.
The GP 2 carburetor you have may be far too large for TT and scrambles applications, where throttle response is more important than maximum power. For flat track applications I suggest you mount the remote float chamber on the right side of the carburetor to minimize the chance of leaning out when the machine is acutely banked over for relatively long periods.
With regard to tuning the GP 2, the procedure is basically the same as for the Monobloc. A small booklet is available, entitled, “The Amal GP 2 Carburetor, as Fitted to Your BSA,” that fully covers this model. It should be available at your nearest BSA dealer.
IRON OR ALLOY?
What are the advantages and disadvantages of cast iron cylinders as opposed to aluminum cylinders and please explain why most manufacturers favor aluminum. Also, which type can be expected to last longer before reboring becomes necessary, assuming a comparison is made between two four-stroke engines of equal displacement and maximum piston acceleration. You state the advantages of chromed bores (CW, Oct. ’69) in the test of the Moto Guzzi. Why is this feature omitted in many of the new powerplants? Finally, would it be feasible to rubber-mount the engine of the Triumph TR6R without altering basic frame configuration?
Paul Hess Bridgeton, Mo.
The aluminum cylinder barrel has many advantages when compared with its iron counterpart. An aluminum cylinder will be approximately two-fifths the weight of an identical cylinder made from cast iron. Aluminum has much better heat conductivity than iron, as an aluminum cylinder will conduct heat three and a half times faster than an identical iron cylinder.
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Also, as heat always travels from a hot area to a cooler one, the aluminum unit will achieve more equal distribution of heat because of its higher rate of conductivity. This aspect minimizes distortion of the cylinder, which is an important consideration relative to piston ring sealing efficiency and effective skirt bearing area. With this in mind, it is possible that bore and piston wear will be less with the aluminum cylinder, particularly when engine output is high. For a given displacement, modern engines deliver twice the horsepower of their predecessors 20 years ago. Therefore a large proportion of the engines manufactured today have aluminum cylinder barrels.
Another approach is to apply chromium plate directly to the aluminum cylinder bore. For regular production bikes this method has been only partially successful, for several reasons. When the bore becomes worn or damaged (chromium torn off due to seizure), it is usually necessary to purchase a new cylinder barrel; it is difficult to have the cylinder repaired because of the complexity of the plating process. An iron sleeve in a cylinder can be bored several times and then be finally replaced using conventional and available equipment.
The existing Triumph TR6R frame could be modified to allow a rubbermounted engine/gearbox unit. It would be necessary to design and construct mounting points on the frame to accept the chosen flexible mounts. foi
