Using leakdown to diagnose valve life?

Just curious if any of the briggs engine gurus use leakdown testing as a diagnostic tool for valve life?

If so, what parameters are you using? How many PSI, how long do you let it sit to measure leakdown?


100psi on a block off plate. It’ll hold constant pressure forever.

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What about on the engine? Is it a good diagnostic tool?

Not as helpful on the block side.


Derek is correct. You will have some inherent leakage around cylinder that can make diagnosing a valve leak more difficult. It’s the test I do, but only because I haven’t found a blank plate and now we are looking at 2 stroke for next year, so no worth chasing.

Leakdown testing is a must. And if you are running with the fast guys, it matters. A leaking exhaust valve is pretty common on these motors and my son says he can notice a loss in mid rpm and top end power when it starts to leak. Almost every time he reports it, I find a leak. When I check it when he hasn’t complained, no leak.

For what its worth, I’m going to design myself a little block-off plate with a M14 threaded hole in the center so I can do a leak down test with the head off the engine. And I’ll be able to do leak tests on each valve individually by inserting air through the ports rather than the combustion chamber. That should make isolating any leaks easier since you can rule out your block.

Might be able to make several extra sets if anyone on here is interested in them!

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You’re making it way too complicated. You can hear and feel where the leak is.

Get a piece of scrap plate and mark the holes with a head gasket and drill them. Send your air through the spark plug hole.


At the end of the day, it doesn’t matter what valve the leak is coming from because if you have the head off and you have a leak, you need to work both seats anyway by either cutting or lapping and drop in new valves and springs.


True true, I didn’t even think about making a plate to cover the combustion chamber.

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I’m with you, if I’m going to do any valve work, I’ll be doing both valves. I’ll dig out some scrap aluminum this week and make a plate, then order the Neway valve cutters.

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Is there a method to determine whether the valves need cutting or lapping?

That is the purpose of the leak-down test. You force a bunch of air into your combustion chamber, and check for any air leaks (which you will be able to feel and hear). If you hear/feel any air coming out of your intake and/or exhaust ports, then that’s your indicator that you have a leak at your valves. then you look into lapping.

some leakage is expected though, especially on a cold engine. But as long as it’s minimal you’re okay. A leakdown tester kit will tell you exactly how much air you’re losing.

Sorry bad question phrasing on my part I meant cutting vs lapping how do you determine which to do?

I’m fairly new to engine building, but have 20 years in tool and die making. Lapping is reserved for fine fitment work, on parts that are almost perfect, its not really of any use for material removal. Ideally you’d cut the seats, then give the valves a quick lap to confirm the new seat face and valve fit perfectly.

Another thing you can do with lapping, is mark the valve seat with something like a blue sharpie, then lap your valve on it for a few seconds, and check to see if the blue has been rubbed off consistently all the way around the seat face. If you’re virtually there, you can possibly get away with lapping compound to clean it up, but if there is a lot to remove, or there is visual damage/pitting of the seat or valve, they will need to be cut.


Jacob, how did your block-off plate fabrication go? Any issues?

I am looking to do the same thing to my LO206, only thing left is a missing block-off plate.

FWIW, listening to the ports when pressure is applied to the cylinder should be sufficient. If you can’t hear air, there’s no leak worth worrying about (In my humble opinion). Putting your ear close and/or listening with a tube in the port. Covering and uncovering the port by hand can also help diagnose.

You can even put the piston at BDC to avoid needing locking the crank at TDC.

The only thing about applying pressure to the back of the valve is that it’s going to attempt to lift the valve off the seat and open it. Emphasis attempt because it may not make a practical difference depending on how much pressure you apply.

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I used to use a leak down tester tool by OTC. Repeatable and decently priced.

It’s a fairly quick test to do once you know what you’re doing for at track diagnostics. I’d test at 50, 80, and 100psi. I used different pressures as I found over time you’d uncover different leaks at different settings.

One thing to be careful of with lapping compound is the grade of it. A really coarse grade compound will actually make your problems more challenging rather than solve them and will take a long time to ‘lap out.’ I learned this the hard way.

I’ve seen people on forums talk about putting a valve in a cordless drill to lap them and I cringe every time.

Generally for most racers I’d say every 2-4 race weekends doing a leak down is a good idea. Over years of working with customers you also become a deep believer in the importance of regular oil changes and pulling to TDC consistently on shut off.


This, more isn’t always better. At times it seems like higher pressure are less likely to reveal the leaks.

James, just so I understand correctly, that would be the BDC of the compression stroke? To best find BDC would I first find TDC and then back it down to BDC as to make sure the valves are still closed before testing.

Logic would suggest more doesn’t equal better. That said, just my experience was sometimes at various pressures you’d discover leaks in either valve.

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