Leak down testing for newby

[creia]

My son and I are experimenting with our new ("home made") leak down tester on some old Briggs 3 HP and 5 HP flathead horizontal shaft engines. We made it from a couple of gauges and some fittings. Full disclaimer- we are total rookies as far as this test. Any and all advice and pointers are most welcome! It is our understanding the the engine must be at TDC with both valves closed before performing the test (makes sense). My question is this: How do you verify that the engine is at TDC when it is completely assembled? Do you peer down the spark plug hole on the head and look for the piston to be at the top? (There is still a 50% chance it is out by 180 degrees, correct?) Or, do you have to take the side breather cover off (requires removing the carb) and look at the position of the lifters and the valve stems? What test air pressure to you recommend on these small engines and what is the most drop in air pressure (PSI or percent of air pressure) is considered acceptable? I noticed that the crankshaft wants to turn a soon as you pressure it up,(which would change the position of the valves and screw up the test) so the crank needs to be secured/locked for testing- what is the best (and easy)way to do that? Thank you all for any help. Sorry for the ignormaus questions.
Michael

[KE4AVB]

Sorry for the ignormaus questions.
Michael

Michael No question is out of order as long you truly need to know the answer.

First you're right as it a 50/50 chance of being TDC compression. Will be obvious if you have it wrong. As for when piston is at the top the stroke using the old wooden dowel method that we use for finding that 1/4" pass TDC on OHVs.

Most commercial tester are setup for 100 psi input so leakage would a direct read. On homemade testers it is a little harder as you need to do a little math. Now you don't need 100 psi for the test as 50 psi will work fine; you just got to double your readings. ie a 10# drop at 50 psi would be a 20% leakage rate which what most manuals I have seen suggest as a max leakage rate pass the rings.

When testing you remove the breather tube and the dip stick as you want the air to escape and not blow out the seals. What you're listening for is leakage pass the intake and exhaust valves, pass the head gasket, and of course the rings. The rings would be leaking into the crankcase so that why you be listening at the dip stick tube or breather tube. Now with OHV I found that if you remove the valve cover it easier to find a head gasket that is leaking into the push galley. Briggs 21,28,31, and 33 have head gasket that do this.

As for holding the flywheel a strap wrench or breaker bar works is why using a low pressure is safer. And you would need the blower housing off as to access the flywheel.

Now my home made leakage test don't even have the second gauge as common sense will work; unless, I actually need a leakage rate.

[KE4AVB]

BTW, I have an idiot brother that would never tell you anything you ask if he knew the answer and you didn't. I finally got fed up with that attitude and I fed the same thing then he started calling me an A-hole and shooting middle fingers at me for not telling him to go about repairing things or stopping him from making expensive mistakes.

He finally quit the business and when I continue to operate the business as customers were still coming in he started telling everyone that I stole his business that he quit. I feel that you can't steal something that someone had gave up on. Matter of fact he stuck with several unfinished repairs that he had already collect payment for.

Sorry had to vent a little.

[Luffydog]

You can put your finger over the plug hole turn to you feel the compression then stick a screw driver in and move the piston to tdc. They also make a flywheel holder block from Briggs. But can do it other ways to make it by as stated in the other post.

[creia]

KE4AVB,
Thank you very much for your detailed yet easy to understand reply. It is duly noted and very much appreciated. It seems very critical to get the engine at TDC, otherwise you get inaccurate or otherwise skewed test results. It seems like a real challenge to get that small Briggs 3 HP at TDC by looking through the spark plug hole, (you hardly see much of the piston) hence my question about pulling the valve breather cover on the side and noting the relative position of the valve lifters and valve stems.
Just a sidebar comment: I have followed your posts for some time with the many answers you have provided to those seeking help. An observation: You have the gift of being a "natural" educator sir, much like Brian (Bgsengines). You both have a way of adjusting the level of technical detail you provide in your answewrs to the intellect of those asking questions so as not to overwhelm them,nor speak too elementary, or offend them in some way for which I am very grateful.
Michael

[creia]

You can put your finger over the plug hole turn to you feel the compression then stick a screw driver in and move the piston to tdc. They also make a flywheel holder block from Briggs. But can do it other ways to make it by as stated in the other post.

Thank you Luffydog!
Michael

[Fulltilt]

IDK where you all get this TDC info from. In actuality, if the engine has any type of ACR mechanism, as most do, the piston needs to be about 1/4" or so past TDC so that the ACR is not acting on the valve.
You can also test the cyl with the piston at the bottom as well as in the middle of the stroke. Gives a better indication of cyl/ring wear. Not always necessary, but i'ts helped in troublesome cases.

[KE4AVB]

For me all I am interested in during test is to know where the leakage is as to know what area to look at. If the compression reading is low and there is high ring blow-by indicated I going to be pulling the head away for visual inspection and gauging. If is true if you have a valve leakage indicated too as you do need to see the actual source of the problem by pulling the head. I see no reason to over complicate the testing procedure. It is just to help point you in the right direction. This is why I subscribe to the Kiss program.

Seldom is the ACR active right at TDC though it might be slightly due gear wear for the valves should fully closed at TDC compression but if the valves are set correctly clearance wise they should be closed. All the ACR does is to partially bleed off the compression but you still need some compression for the fuel mix (fuel and air, plus oil if 2 cycle) to ignite. You don't want ignited fuel to slamming the valves close as this pressure can be high 500 psi even on small engines as the piston instantly accelerates up to 25 mph @ 3600 rpms. IF you have an intake ACR version you definitely don't want the valve open at TDC as the plug ignites the fuel mix.

[KE4AVB]

Here I might something else here. Many of us small shop just don't have budget for many of the nice to have tools myself included. If the mechanic is good at his job we can make do and experience is one the best teachers.

I am gradually buying the tools but I do need prioritize which I needs ones get purchased first. Actually I only have a 2K max yearly tool budget and with me adding the ATV / RTV line to the business it can be rough to buy those specialized tools. Just one diagnostic tool for the Cam Am is over $1000.00.

My leak down test is nothing more than a pressure regulator with a gauge and hose adapters which are share with compression gauge so I don't get a percentage of leak rate yet I still manage to find the problems on nearly every engine tested. It not fancy but gets the job done. I am planning on adding the second gauge after I get cross over tube with the .250 x .040 orifice made and T fittings; just haven't took the time to do it. Just don't use the tool enough to justify getting done as priority.

I even used just a large syringe and plastic tubing until last year for the carburetor pop off tests and crankcase leakage tests. Now I have a Mityvac pump which does makes things much easier.

[Fulltilt]

I see no reason to over complicate the testing procedure. It is just to help point you in the right direction. This is why I subscribe to the Kiss program.

Seldom is the ACR active right at TDC though it might be slightly due gear wear for the valves should fully closed at TDC compression but if the valves are set correctly clearance wise they should be closed. .

Might be slightly...should...?

Why guess?

By positioning the piston at 1/4" or so past TDC, you pretty much eliminate any shoulds, or coulds. Seems pretty simple to me...

Just "Keeping It Simple Stupid "...right?