Drop Test
Once the piston and dashpot sections were free I cleaned them up so I could perform a drop test. This gives a general idea of the piston wear. As shown here the piston is put in the cover and a small nut and bolt fixed temporarily in one of the mounting holes. The trick is to put a large washer under this bolt head so that it overlaps the edge of the dashpot cover and will stop the piston dropping onto the floor. If you use a metal washer you should be able to get an audible click when the piston drops and this helps with timing.
Unless the air bleed holes are blocked the piston will just drop without resistance. If you have some 6mm bungs then use them- I found that the end covers from filler gun cartridges fitted perfectly- but you have to find the sort that aren't ridged all the way down. Its then just a matter of inverting the dashpot and timing how long the piston takes to drop onto the washers- I had to use fibre washers to get them large enough which took away the nice "click" and made timing trickier but it was OK with a bit of practice. My piston dropped in 6.5 secs, just within the 5-7s window cited for carbs of this size. Slower dropping could mean oil or other obstruction on the sides (so clean it and repeat); faster probably means the piston is worn and needs servicing.
Once the piston and dashpot sections were free I cleaned them up so I could perform a drop test. This gives a general idea of the piston wear. As shown here the piston is put in the cover and a small nut and bolt fixed temporarily in one of the mounting holes. The trick is to put a large washer under this bolt head so that it overlaps the edge of the dashpot cover and will stop the piston dropping onto the floor. If you use a metal washer you should be able to get an audible click when the piston drops and this helps with timing.
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| Cartridge gun caps used to block air holes |
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