Topic: Dumb compression test question?

[jessezm]

Dumb question?  What happens if you do a compression test with the motor running?  (i.e., spark on 3 cylinders with the tester screwed into the 4th)?

I ask for two reasons....  first, I don't want to run down my battery doing the test since I don't have a kick starter.  Second, because I did actually accidentally forget the kill switch for the second cylinder, and the motor ran for a sec.  Now I may be paranoid but I thought I heard a new ticking sound...  Can that do any damage?

[KingCustomCycles.com]

No problem doing that.  Cranking pressure with the starter is the same as cranking pressure with the other cylinders doing the work.  As long as no combustion occurs.  When the mixture ignites, it goes to something like 800 psi.

[jessezm]

Sweet, so does that mean I can run a compression test without running down my battery using the starter motor? 

[KingCustomCycles.com]

I would just put a charger on the battery for the tests.  You won't get much recharge below 2500 rpm anyway.

[jessezm]

 Yeah, I know it makes the most sense to do that, but I run a 4-cell A123 battery, and I've been real careful not to discharge it all the way, and it's kind of a pain in the butt to get to...   I'm just being lazy, and looking for an easy workaround!  I know the bike wont recharge while it's idling, but at least it's not pushing the starter motor the whole time...