Your leakdown test will do you absolutely no good if you don't adjust valve clearances first! Essentially you force air into a cylinder and listen to where it leaks out. If you have some intake valves hanging open due to a valve(s) sticking open due to lack of clearance and an exhaust valve also sticking open due to lack of clearance, you will get air coming out the intake manifolds, air coming out the exhaust and if the rings are also in poor shape, air coming out the crankcase breather. All you will be is confused! ADJUST VALVE CLEARANCES!!! Once you have those valves sealing, you will have a better shot at locating any compression problem. I also suggest that the lack of valve clearance might just BE YOUR COMPRESSION PROBLEM and you may find you have fixed the issue.
If you do a leakdown, it may be best to have an experienced hand or professional mechanic there as this method takes some practice to perform effectively. I personally don't bother do leakdown tests. With the valves adjusted, if you perform a NORMAL compression test with a $20 autoparts store compression tester, your compression will be within spec or not within spec. If it is not, say 3 cylinders have 130 psi compression and one cylinder has 80 psi, I repeat the test after putting a teaspoon of oil in each cylinder (do only one cylinder at a time or you will have an oil gusher!). Say the compression had been 130-130-130-80... if it goes to 140-140-140-135, then you can assume you have ring seat issues as the teaspoon of oil SEALS the #4 rings for the test. If the compression goes like this after oil testing 140-140-140-90, then you have valve sealing problems as the oil didn't do much for sealing the #4 ring. The advantage of doing the testing the way I do it is that you don't have to buy a leakdown tester (EXPENSIVE) or fabricate on AND you don't have to squat around the engine trying to figure out where the air is escaping... I am not a skinny guy and find squatting with my ear to an engine component undignified hehe
The reason that a bike shop uses a leakdown rather than my method? It is probably the best way to quickly diagnose the problem location and they are more interested in providing some sort of meaningful estimate of costs than I would be. My way of doing it, is that the engine will need to come apart anyway. If a shop finds you have ring issues, they will quote you for OVERSIZED PISTONS, ENGINE BORING and of course, NEW RINGS (and gaskets/labor, etc). If I did this, I wouldn't have my mind made up on the best (cheapest) course of action till I was sure what needed done. Like I already mentioned, I would just swap in another used block/pistons with the correct clearances and parts would be limited to rings and gaskets... labor would be about the same but I would be saving hundreds on the new pistons and engine boring. Again, I stress that if you have ring issues, NO ONE can tell if the ring issues are just ring wear or the piston to bore clearance is too great and new pistons/rings are needed by doing a leakdown or ANY other type of external test.
Post edited by: wiredgeorge, at: 2006/05/24 09:22
Post edited by: wiredgeorge, at: 2006/05/24 09:24
