compression testing

linmictil2 · Post by **linmictil2** » Thu Sep 08, 2011 8:03 am

Morning Gents: Question-- If I do a compression test on a cylinder and it is good do I need to test "leak down" also. Never done this before and wanted to check before going out and buying anything Thanks Mike

cknight · Post by **cknight** » Thu Sep 08, 2011 9:32 am

Performing the compression test first is the proper way to go. If you have good results, leave it at that. If there is an issue, the leakdown test will then help indicate where the problem is (leaky valves, bad rings, blown head gasket, etc.). The leakdown test will not help inform you if the compression ratio and cam timing are correct, but the cranking compression test will. Regards, Chase

DeeJay · Post by **DeeJay** » Thu Sep 08, 2011 10:00 am

I have never seen the need for a leak down test. If the compression test shows low compression,the engine will have to be torn down and the problem will be found at that time.

Goodysnap · Post by **Goodysnap** » Thu Sep 08, 2011 5:25 pm

Leakdown tests are ussually performed when compression readings are found to be at or below minimum specs. They will show you a percentage of leakage and possibly help you pinpoint where the leakage is occuring. By listening at the intake, exhaust or crankcase to hear air leaking from a certain area of the cylinder. With smaller leaks this may be challenging, as with a hole in the piston it would be quite obvious.

Keep in mind all engines have a certain amount of leakage to begin with. The question is, does the amount of leakage you have actually causing the problem your diagnosing. I personally runs a traditional dry test , followed by a wet test(one squirt of oil from a oil can).Then comparing numbers will give you a good idea the sealing ability of the rings.

"Running" (as in Engine running not performing)compression test's can help pinpoint intermittents that might not occur while cranking. Weak or Broken Valve springs , Worn cam ect.

Remember- Throttle and Choke Full open
All plugs out during testing
Charged Battery and maintain during test

Consistent cranking speed will yield accurate results

This goes for all Engines not just 305's

LOUD MOUSE · Post by **LOUD MOUSE** » Thu Sep 08, 2011 9:59 pm

Not a lot of info as ta what I know about Leak Down Test however I seem to assume this is for 2 Stroke not 4 Stroke. !!!????????. .........lm

linmictil2 wrote:Morning Gents: Question-- If I do a compression test on a cylinder and it is good do I need to test "leak down" also. Never done this before and wanted to check before going out and buying anything Thanks Mike

bsa lover · Post by **bsa lover** » Fri Sep 09, 2011 12:27 am

leak down is for 4 strokes, tests valves and rings
,crank case pressure test is for 2 strokes checking for bad crank seals, they do the same thing but in a different way, it can find a problem that is about to happen that a compression test may not pick up

teazer · Post by **teazer** » Fri Sep 09, 2011 1:02 am

We typically leak test a motor before it's stripped and after it's rebuilt. If leakdown is higher than we like, the top comes off to see why.

4 strokes we leak test at 100psi and 2 strokes with a small hand pump are tested at 6-9 PSI. Any more than that tends to blow out the oil seals.

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