Posted by Max on September 25, 2002 at 19:00:52:
In Reply to: Acceptable COMPRESSION rate posted by Steve on September 25, 2002 at 17:45:37:
Compression tests are indicators of the rings and valves to seal against compression/combustion pressure losses. To run an accurate compression test, please observe the following. Compression testor gage is accurate (not been dropped or mishandled), engine warm and throttle and choke butterfly (na to Savage) open so there is no inlet air restriction, crank the engine over through 5 0r 6 compression events. Look carefully at the 1st (in the case of a Savage -after the decoompreeor has allowed the exhaust valve to shut) 2nd and last readings. Run at least 3 complete tests with a good battery. Average of you readings should be close to the book. 8.5 :1 compression ratio would be in the ballpark of 130 PSI at sea level. If it is low, a couple squirts of engine oil on the cylinder walls and a crank cycle to disrtibute the oil and and if the wet (oil) reading comes up the direction to look for compression pressure loss is rings. No increase then the loss is usually valves. A more definitive test is to use a cylinder leakge tester. It puts a specific quanity of compressed air in the cylinder at TDC compression stroke, and the percentage leakage is observed. Over about 15% one should listen to the air intake, exhaust outlet and crankcase breather to determine where the air is leaking from. (can be more than one and a stethscope or couple feet of heater hose can be a big help here eliminating outside air noises to determine where the air leaking from. As your bike is a 86 if it has been parked for a number of years the piston rings could be stuck in the pirton gooves and not be able to follow the cylinder wall and seal correctly rather than work out a such a low km if use. 48k is about 29 k miles and well within the service life of the engine rings. Stick rings can sometimes be loosed with out disassembly by using strong chemicals to loosen the varnish from the oil and then using a quality oil with lots of additives after that. An aside GM recommends 5-20 engine oil for reasons other that the improved mileage, they claim is does a better job of lubricating the rings and grooves minimizing the possibility of the rings sticking in the grooves.It also reaches the OHC faster on cold start as it flows better in cold weather. Max