I know...I know..., what´s a leak down test?
Thanks,
I have written about this before and I actually bought one and sent it back with a friend to Bristol because they seemed to be so rare over there. The crudest way to do the test would be to modify a spark plug to take an air fitting so you could force regulated air into your engine, say 100 lbs. While the air is still connected to the front cylinder, for example, with that cylinder at top dead center (Valves closed) you can listen at the carb and hear if the intake valve is leaking, then listen at the exhaust to check the exhaust valve, finally, listen or feel at the breather and you will probably feel a blast of air because the rings are allowing the air to slip past at least in the above case. Repeat on rear.
The tool itself is only a little more sophisticated. It has two dials and after you set the regulator at 100 lbs. the second dial will tell you how much of that 100 lbs. is leaking past your valves and rings. On a new build it is usually less than one percent, but it can be at 5 % until the rings bed in. When racing in Texas in 2011 the bike became quite slow with no other noticable distress. I did an leak down test and it read 60% with air coming out the breather. A tear down showed a broken compression ring. A deglaze and new set of ring was done. The valves had been done the year before (2010). In August 2014 another leak down test was done by the new owner who has been racing the bike and it read 4%.
On the single the engine will want to run backwards rather than stay at TDC when the air is hooked up. I just remove the pushrods and let the piston stop where it wishes. This would work with the twin also and is fine unless you think for some reason that the rings are worse at the top of the bore than the bottom. At the track I would put it in gear with the piston at the top and use the tire to keep the engine from spinning. Best if done warm, but still useful if cold. The benefit of frequent testing is that you can anticipate the problem. For the street I would think you could do it at the end of every riding season. When you start edging up to 20% you should just plan around a top end freshening for the winter. I find it to be a cheap ($50) and useful tool that eliminates much speculation.
David
David
