CONCORD, NC - TECH MANUAL UPDATE 618.6 PRESSURE / VACUUM TESTING
Testing may be performed to insure extra air is not being pulled into the engine for a performance gain. Any means to bypass the intent of the class rules is illegal. Both pressure and vacuum tests may be performed -- engine must hold 5 psi for 60 seconds and/or 5 HG of vacuum for 60 seconds.
Blowdown checking procedure for Yamaha engines --- (601.9 and Figure 606.2)
1 -- By a careful visual inspection (light check) , identify the highest exhaust port and the highest intake port.
2 -- Using the Lad tool, zero the dial indicator on the highest exhaust port, taking care to hold the shaft of the tool against the cylinder wall.
3 -- roll the crankshaft backwards five turns (.500" on the dial indicator)
4 -- insert the Lad tool into the highest intake port, holding the shaft of the tool against the cylinder wall.
5 -- roll the crankshaft forward until the piston stops on the Lad tool and note the value.
6 -- the value must be between .380" and .420" to be considered legal
7 -- engine to be checked as raced