114 – How To Make A Vacuum Guage For Shop Testing Motors

HOW TO MAKE A VACUUM GUAGE FOR SHOP TESTING MOTORS

Measurements of crankcase vacuum in twin cylinder engines will often give a quick check on condition of piston rings, operation of breather, and proper operation of sump valves. Each repair shop should have a vacuum guage for this purpose. One can be made as follows:

Purchase two (2) three foot lengths of 5/16 O.D. boiler glass tubing having a one eighth or 5/32 dia, hole in center. These may be obtained at any plumbing supply shop. Fasten these on a vertical clean wooden panel or board keeping the tubes about 1-3/4″ apart, (See cut)

Use heavy wall (1/8″) rubber tubing (1/4″ I.D. ) to connecting the two lower ends together and support this rubber tubing at the bottom with a wood block cut out to fit the curve of the tubing.

Use another length of rubber tubing to fasten to the top of one of the glass tubes. This tube should be long enough to reach from the guage to the crankcase of the motorcycle. On the crankcase end of the rubber tube, use about 6″ of 5/16″ O.D. copper tube and solder on a cone and nut as shown. Slip the rubber tube over the copper tube. Use an elbow at the crankcase.

Purchase enough mercury to fill both glass tubes about 1/2 way up. After filling, mark on the board a line showing the mercury level at O and then mark inches each way from zero up and down. Dividing the inches into quarter inches will help the readings. The board may be marked before filling and then filled to the zero point if you wish.

After completing the guage, remove upper crankcase oil level screw. Connect rubber tubing at elbow. Start motor up. The vacuum in the crankcase should lift mercury up on one side and the opposite side will be down by the same amount. If your guage has no air leaks, it should be used as follows:

1. Motors in good condition will show a vacuum in crankcase throughout the range.
2. A poor working or badly installed breather valve will destory the vacuum and show a pressure (mercury on pressure will go down on motor side and up on opposite side.)
3. Bad piston rings will show a slight vacuum at low speeds and will break into a pressure at about half throttle.
4. A motor in good condition will never show a pressure.
5. A poorly working sump valve will cut down the vacuum and if motor is dirty or running hot, may show a pressure.
6. A bad seal between motor and primary on 74 cu. in. motors will show a vacuum in primary.
7. Remember – Cut down flywheels, holes in breather discs, slotted push rods, all reduce vacuum.
8. Experience in the use of the guage will teach you what to expect for motors in different conditions.
9. This guage is not for use on 4 cylinder motors.