Leak down test - stopping at TDC

In gear lock the rear brake. higher gears have less slop, lower leverage. With drum brake just tighten the adjuster nut.
 
In gear lock the rear brake. higher gears have less slop, lower leverage. With drum brake just tighten the adjuster nut.
K thx Gary, this will be on the Special so I’ll have to hold the brake pedal down. I’m sure I’ll be able to rig something up.
 
A socket and the longest breaker bar you have on the rotor nut is prolly your best bet.
I'm curious, what tester are you using to do this? And what criteria are you judging the results by?
In the airplane world I came from, we did leak down tests with pretty specific equipment and values to compare our results to. How's this being done on an XS?
 
Usually with an inscrutable HF tester.......... :doh:
I mainly do one by; zero the compressors pressure valve, plumb the air hose into the spark plug hole, slowly turn up air pressure til you can hear the hiss and listen at carb, muffler, and crankcase vent to see where the air is going. Cuz if you are doing this test it's not staying where it should. :cautious:
 
Usually with an inscrutable HF tester.......... :doh:
I mainly do one by; zero the compressors pressure valve, plumb the air hose into the spark plug hole, slowly turn up air pressure til you can hear the hiss and listen at carb, muffler, and crankcase vent to see where the air is going. Cuz if you are doing this test it's not staying where it should. :cautious:
Ah... so it's not a "differential pressure" test then. We referred to "leak down" and "differential" test as one and the same. This isn't the same as what we did on aircraft engines.... gotcha.
 
Ah... so it's not a "differential pressure" test then. We referred to "leak down" and "differential" test as one and the same. This isn't the same as what we did on aircraft engines.... gotcha.
Well in perfect world...............
Honestly; bought and unsuccessfully tried to use one of the HF testers "by the book". It's been near universally reported that ain't happening with that tool. shrug.
Think it's still here somewhere, wanna prove me wrong?
 
A socket and the longest breaker bar you have on the rotor nut is prolly your best bet.
I'm curious, what tester are you using to do this? And what criteria are you judging the results by?
In the airplane world I came from, we did leak down tests with pretty specific equipment and values to compare our results to. How's this being done on an XS?
I have an OTC leak down tester I use on my cars. With my recent discovery of loose head studs on the Special, I was curious whether any damage was done to the head gasket while it ran when setting the timing and carbs. Thought I could use the tester to pressurize each cylinder to detect head gasket leakage. Not looking for specific values, just seeing if there is abnormal loss of air. Not do scientific but also gives me an excuse to use an expensive tool that I rarely get to use. lol
 

Attachments

  • 684A3CBF-F74F-4157-B8FB-9EA83D941D84.jpeg
    684A3CBF-F74F-4157-B8FB-9EA83D941D84.jpeg
    277.1 KB · Views: 50
Most likely the same as HF but in a nicer case. 😀
 

Attachments

  • DE27A697-335A-4628-B4BA-1D4C4B9BDF40.jpeg
    DE27A697-335A-4628-B4BA-1D4C4B9BDF40.jpeg
    316.9 KB · Views: 71
Last edited:
Just did this with the Norton, noise was at carb. Removed carb, scrubbed intake valve seats, compression still nil, repeated test, air coming out muffler. Off with it's head!
 
Well in perfect world...............
Honestly; bought and unsuccessfully tried to use one of the HF testers "by the book". It's been near universally reported that ain't happening with that tool. shrug.
Think it's still here somewhere, wanna prove me wrong?
I have an OTC leak down tester I use on my cars. With my recent discovery of loose head studs on the Special, I was curious whether any damage was done to the head gasket while it ran when setting the timing and carbs. Thought I could use the tester to pressurize each cylinder to detect head gasket leakage. Not looking for specific values, just seeing if there is abnormal loss of air. Not do scientific but also gives me an excuse to use an expensive tool that I rarely get to use. lol
A differential pressure.. or leak down test on a Lycoming (for instance) involved using a tester that looks just like the one in your pic. Between the two gauges is a metered orifice (restriction)... a .060" dia hole iirc. The left gauge records the input pressure... 80psi. The right gauge (downstream of the metered orifice) shows how much leakage there is in the cylinder. A reading of 80/80 means no leaks... or none leaking faster than the .060" hole can replenish anyway. A leak of 80/60 would be excessive. Then you'd do as Gary said and listen to the intake, exhaust and crankcase vent to see where the leak was.
The factory determined the leak values... which was why I was curious.... I was pretty sure Yamaha never established any dif. press. values.
 
A differential pressure.. or leak down test on a Lycoming (for instance) involved using a tester that looks just like the one in your pic. Between the two gauges is a metered orifice (restriction)... a .060" dia hole iirc. The left gauge records the input pressure... 80psi. The right gauge (downstream of the metered orifice) shows how much leakage there is in the cylinder. A reading of 80/80 means no leaks... or none leaking faster than the .060" hole can replenish anyway. A leak of 80/60 would be excessive. Then you'd do as Gary said and listen to the intake, exhaust and crankcase vent to see where the leak was.
The factory determined the leak values... which was why I was curious.... I was pretty sure Yamaha never established any dif. press. values.
Your motor with a recent top should establish a decent baseline... ;)
 
That wrench on the rotor nut only works if you are sure you are at least a little bit past TDC.
Think I proved that once.
Actually, if you're dead on TDC, you don't need anything. I used to demonstrate to my students that you could set TDC and could let go of the prop... it wouldn't move. That was my preferred way of doing it. Never liked the idea of holding a crank (propeller) against it's will.
Go ahead and try it... shoot about 30psi into the cylinder and move the rotor back and forth until it stays still... the piston not trying to force it either direction.... TDC. Then go ahead and run your pressure up to whatever's called for. If you hit TDC, it won't move. Worst case... it spins. No biggie, try it again... you'll get there.
 
Ponderin' a 360° twin.... valve openings (cam) on the other cylinder might not allow you to set the crank on TDC.
Dunno.... never tried it. Now I'm curious. :umm:
 
Actually, if you're dead on TDC, you don't need anything. I used to demonstrate to my students that you could set TDC and could let go of the prop... it wouldn't move. That was my preferred way of doing it. Never liked the idea of holding a crank (propeller) against it's will.
Go ahead and try it... shoot about 30psi into the cylinder and move the rotor back and forth until it stays still... the piston not trying to force it either direction.... TDC. Then go ahead and run your pressure up to whatever's called for. If you hit TDC, it won't move. Worst case... it spins. No biggie, try it again... you'll get there.
Interesting. I get what you’re saying. Might be difficult rocking it back and forth with that much pressure in there. Looks like the long breaker bar would help. I guess there’s also the prerequisite of no leaks in the top end. Lol.
 
Back
Top