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Useful data

Calculations and useful numbers:

Calculate CC: Bore X Bore X Stroke X 0.0031416


Calculate cylinder CC:
(Bore X Bore X Stroke X 0.0031416) divided by 4
Deck height CC: (Bore X Bore X Deck height in mm X 0.0031416) divided by 4


Head CC: using a measuring jug (one in c.c.s and in single units) fill it with 100 c.c.s of a liquid (I use ATF 'cos it's red, but you can use anything that is easy to see) put the head in a vice with the bottom of the valves facing upwards (you will need both valves and a spark plug in), make sure it is level and pour in your liquid until it is level with the surface where the cylinder mates with the head, once there you know how much fluid it took to fill it up.


Compression ratio: (Cylinder CC + Deck height CC + Head CC) divided by (Deck height CC + Head CC)
Calculate carb size: (Square root of (Cylinder CC X Max RPM)) divided by 40


Calculate Choke size: (Carb size X 40) divided by 50
Calculate Main Jet size: Choke size X 4


Oil temperature: Don't go over 120 degrees C too much

Other data:

To set up a 009 or 050 distributor when using a degreed pulley wheel, make a mark on the 32 degrees before TDC with a felt pen or liquid paper, then run the engine using a strobe light attached, slowly raising the revs, then when the engine is at full advance (you will know when that is as the mark you made stops moving) turn the distributor to move the mark until it lines up with the case centre.

To balance a set of dual carbs is not all that difficult, if you have a carb synchronizer this can be used to measure rather than listening for evenness. Either way, firstly disconnect the linkage to both of the carbs, then screw in both idle adjustment screws in about two turns. If you are going to use a synchronizer, remove the air filters. Now start the engine, it should run relatively high, now by adjusting only one idle screw up and then down until the engine sounds sweet and smooth or measures the same on both sides. Turn off the engine. Reconnect the linkage to both carbs and loosen the adjustments on one side. Now adjust the linkage on that side until both throttle arms rest on their stops. Open the throttle from the cable link (centre of the cross-bar) noting which carb opens first, adjust the linkage to raise or lower that throttle rest and continue until both carbs open together, then tighten the linkage lock nuts and re- fit the air filters. Re-start the engine and unwind the idle screws evenly until you have a nice running engine.
To set up the idle mixture, wind all four screws in gently until they stop, then wind them out about two turns (as long as it is the same on all four carbs). Restart the engine, once it has warmed up it should be running smoothly.

Thanks for reading. I hope it has been useful.

 

 

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Last modified: February 11, 2007