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Discussion Starter · #1 · (Edited)
When it comes to testing a mower engine to see if it’s time for an overhaul, one of the most accurate test is for crankcase vacuum. It’s much quicker to perform than a compression test, or a leak down test. Both Kohler and Briggs say -4” is the minimum for an engine to operate properly. When testing for vacuum, most guys will try to use whatever vacuum gauge they have in their toolbox, such as a fuel pump vacuum tester. The problem is that 99% of vacuum gauges you can buy in an auto parts store read in inches of MERCURY (Hg), while both Kohler and Briggs list that critical -4” vacuum specification in inches of WATER.

Mercury is about 13 times heavier than water. If you try to use a standard Hg gauge to read crankcase vacuum the needle won’t normally even move. The correct tool to use in looking for that -4” is a WATER MANOMETER. You can buy a cheap water manometer on Amazon for less than $12. Water manometers are actually a PITA to use in a shop. You have to keep them filled with water, so you can’t easily store them flat in a toolbox because they will leak. You usually need to add food dye to the water to be able to read it and eventually the food dye stains the clear tube and makes an accurate reading almost impossible. Both Kohler and Briggs require their dealers to perform crankcase vacuum test for warranty claims with either a water manometer, or an electronic vacuum tester that reads in inches of WATER (part # 19495, or DTI-33)

That DTI-33 is kind of pricey for the average DIY mower mechanic, but it’s worth every penny if you buy a lot of mowers to flip, or take trade-ins like I do. Just plug it into the dipstick tube, fire up the engine, and turn it on. It will instantly tell you what you need to know about that oil-soaked piece of junk that Honest Earl says “Runs Great” and he can’t argue with the results because he probably doesn't even know what you're testing with that "thingy"…..

Water Manometer

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