How to do a compression test on small engines.

Metal in Motion shows you how to do a wet and dry compression test on small engines or powersports. Keep in mind that the compression release mechanism on most 4-stroke small engines will bleed off some of the compression during cranking for easier starting. Once the engine starts, that mechanism is disengaged. For this reason I prefer to do a leak down test, which we will be demonstrating in the next video.

[@mosfet500]

Small engines need a compression tester with a VERY short line to the gauge, even the volume in the gauge itself affects readings. You will always get a low test on trimmers and small engines like the Honda GX 25 and 35. The reason is that the cylinder volume may compress to the 130 to 150 lbs Honda says is good but a long hose to the gauge will never indicate correct, it will always be low because the volume of air in the line has a high ratio to the volume of air in the cylinder. Ask me how I know! I just rebuilt a Honda GX25 and never got the compression over 90 even spinning it with a drill past the compression release rpm. My engine runs perfectly, the compression is fine but I probably changed the rings and reground the valves for nothing. If you want a truly accurate reading what you have to do is calculate the volumetric ratio with respect to gauge and line to the volume of the cylinder. Remember the smaller the engine the greater error and the lower your compression will be regardless how good your true compression is.

[@GoldSeals]

Its absoulutely necessary to have a shrader valve inside the end of the hose when checking small engines.Otherwise you will get a low reading.Some compression testers dont come with shrader valve inside the hose.

[@mosfet500]

@@GoldSeals I think you're misunderstanding what I'm saying. The volume of air in the cylinder relative to the volume of air in the hose to the gauge is to great. The release valve has nothing to do with it because it doesn't change to ratio of air in the hose to the air in the piston. When a small engine compresses air at TDC there is more air in the hose than the engine and this reduces the compression radically. Think of it this way, let's say we put a balloon in the spark plug hole. When the piston is on TDC of the compression stroke the compression would be very low. This is an exaggerated example but basically what is happening. Because of the hose you can expect readings to be close to half of the normal compression. The best way to check compression on small engines is with a gauge that screws right into the cylinder head where the gauge sits right at the head.

[@spac1565]

@@mosfet500 Good day. Could you advise me how to calculate the volume ratio? Alternatively, recommend the meter that screws directly into the cylinder head?

[@mosfet500]

@@spac1565 I wish I could recommend a gauge with a short line, I have a small machine shop so I made my own short adapter just this morning, guess what? I got over 120 psi. so my theory works, here's the math: Let's say you have a small tiller engine with a 1-1/4" piston and a 1-1/4" stroke. So the volume (displacement) is (3.14 x (1.25 / 2)^2) x 1.25 = 1.534 cu/in. Now let's say you have a compression gauge with a 12" long tube and a 1/4" hole thru its length. (3.14 x (0.25/2)^2 x 12 = 0.589 cu/in. That's 1/3 of it's volume dissipated by the line to the gauge, now you're thinking - geeze I have bad rings which is exactly what I thought before I sat down and thought about what was happening.

[@leonscogno1914]

Thank you, by adding teaspoon of 2cyl oil to cylinder head and opening to full throttle raised the psi during my compression test, my first reading of 85 psi, this prior to your most helpful suggestion which then raised the psi to 120. Unbelievable helpful I quickly subscribed. Thks again Lee Old Bridge NJ

[@MrThedocholiday]

Very helpful info for engines that are a little more difficult to diagnose upon startup, but may seem to run fine once they do start. Thanks a mint!

[@alister9556]

well explained , thanks

[@chuckdawg2799]

Good stuff. Got a zero turn with a kohler motor, replaced everything, and I mean everything. Still has issues. Now I'm really curious if I got a bad valve or head gasket.

[@mowtoegee7049]

Nice video. I got heaps of different equipment gonna get a tester and try this now

[@scmafia7515]

What about when a cam has a compression release?

[@AIainMConnachie]

For saws trimmers etc. Air filter out. Choke off. Not sure you mentioned that. Throttle full. Pull until it stops climbing. Sounds right? 😊

[@abbaskhan3888]

Great video.thanks.Make a video on small generator overheating causes and problems.thanks

[@pastorlugo121]

Outstanding video. Thanks.

[@MowerMike]

Excellent video! I find that any riding mower with compression under 100lbs has issues.

[@hopfrogy3773]

I think you MAY have just saved me

[@larrytyler8601]

Thank you for your video. It helped me.

[@matthewpuryear8009]

Any recommendations on a decent compressor tester? Looking for one

[@thomashatfield5628]

Hi I have 14.5hp Briggs engine which I bought in 2003 I have replaced head gasket? But engine keeps back firing spark plug is black after 10 minutes keep changing plugs same thing ? No power in engine? I have done head pressure test 112 p.s. i . I have kept Westwood mower going for nearly 20 years but I am stumped by all the smoke bellowing front of mower. Will be grateful for your help. Thanks Tom Mixer.

[@johnmitchell8925]

Could very well be your carburetor

[@geogeduggan6579]

Good explanation, I'm a beginner . whats a good compression tester ?

[@metalinmotionshop]

I always just used the ones from the auto parts store. They usually have a cheaper one with no rubber case around the gauge and a slightly more expensive one with a rubber case. I like the rubber cased one better. Also, I get the one that has the 3 adapters for different size spark plug holes. I used those quite a bit.

[@ricksanchez3711]

Great info!

[@martindesmarais7158]

Do you know what compression should we get on a Tecumseh 10hp ???

[@freshmaker5]

Nice video

[@MrChickensnatcher]

My weedeaters run but the compression tested doesn't read anything... and ideas other than buy a new tester?

[@genecenora7176]

Awesome video

[@charlesronk2989]

My dad has a 2012 17.5hp Briggs and Stratton. A bought a leak down tester and it says the engine is bad, but noticed when not hooked up no air comes out of the tester. Yet, if you put your thumb over the end it slowly builds pressure and goes to good. I was not sure if the tester is broke or not as I never used one. So I just put 30lbs pressure in the cylinder and it blows oil out the dipstick hole. I tried a compression test and it was 70psi dry and 90psi wet. The mower smokes blue smoke like crazy if you can get it to start. My thought is it needs a ring job. does that sound right.

[@harigopaladamus9481]

Sound aright

[@jmg36344]

Help!! 2012 Polaris 800xp. Hard to crank over when sparks plugs are installed. Easy to turn over when one or no plugs installed. Compression 175psi both cylinders. Fuel 145psi. Spark at both plugs. Only has 600 miles. New battery. New starter. (Burnt out starter due to hard crank when both plug were installed). Thank you.

[@metalinmotionshop]

I haven’t looked at the manual but the first thing that comes to mind is the compression release mechanism on the camshaft. If it’s broke or if the valve adjustment is too loose it may not engage that mechanism. It’s supposed to bleed off compression on startup for easier starting.

[@rick3090]

compression tester hose is too long, i got bad readings on mine like that, by the time the little piston puts enough pressure in the tester its inaccurate , try a short gauge with a rubber tip, mine went up a bit.

[@colleendryden7563]

YA YA YA , Two out of every three patients whose doctors chew gum recommend gum for those patients,leave the difficult task in life to us highly trained professionals,in the scheme of things you will find that in most cases you will be money ahead of the entire game. Trust me there is a particular reason why folks willand often have driven 75 miles to drop their disabled lawn equipment off at my house to be repaired for nearly 45 years,abilities come with a lifetime of experiences.

[@bobjackson3442]

All my weed eaters read 25PSI think my gauge is broken.

[@johnsonbobo2376]

I would think so bud

[@777gearhead]

How long should an engine hold compression? Cranking I'm getting 90psi but it bleeds off. Is that normal?

[@metalinmotionshop]

There is a valve in the compression tester that should hold the pressure. The engine isn’t holding the compression, it’s just measuring how much force the piston, rings, valves and gasket can create. If it’s bleeding down it’s because the hose where it screws in at the spark plug hole is leaking or the valve in the tool is leaking. It will hold the pressure reading in the tool until the button is pressed.

[@metalinmotionshop]

What are you compression testing? For push mowers 90psi would be fine. It’s could be OK for riding mowers. I’ve seen 90 up to 150 for riding mower engines. Did you do a wet compressor test to compare?

[@777gearhead]

@@metalinmotionshop it's a riding mower and it des get up to 90 so it sounds like I'm good. The valve in the tool must be worn and leaking. Thank you

[@peachsncream5808]

Thank for the vid brother . I have been asking around to find out if there was a different compression gauge for car engines and 2 stroke small engines . I had asked 2 - Small engine repair stores and several tool supply stores … and not a single place could answer the question ….they simply didn’t know If their was a difference 🤯 . Question answered after watching your vid 👣🦘👍

[@homemadebuilds8873]

I have a gx390 when i do compression reads zero and have another gx390 runs excellent nothing wrong with it did a compression test on that one and it reads zero is there a specific way to check compression on these gx390 engines?

[@mosfet500]

See my answer above as to why this happens.

[@mrhomefinder1]

I don't about others but jumping around on all the numbers and makes and models confused me. I would suggest simplifying the readings to just two stroke and four stroke engines. I'll watch again perhaps it get simpler the second time. Thanks

[@metalinmotionshop]

You cant really. Different engines and their designs have higher or lower specs. That’s what made this video a little tricky.

[@dinto_louis]

Hi, the wet reading on one of the cylinders is lower than dry reading..what could be the inference..? I have done dry & wet compression test on my VW Polo 1.2 TDI..the reading is lower in the wet test than the dry test on cylinder number 3..? Dry: 1 - 26.7 (Bar) 2 - 26.5 3 - 27.8 Wet: 1 - 27.6 2 - 26.7 3 - 26.3

[@timd5597]

Briggs big block smoking at start up found oil in intake before carb. Did leak down test 55% leak found out head gasket was blown between piston and push rod Gally. Clean up head, valves, lapped valves and replaced head gasket. Tried leak down test again now at 28% most air coming out of carb what could it be?

[@metalinmotionshop]

Two things come to mind. When I lap my valves I pour gas into the intake and exhaust port and see if it drips out around the valve seat. I keep repeating until the valve is sealing the gas off with no leaks. You might have already done that. The second thing is the valve adjustment could be too tight. You can slide the rockers off the valve springs or just loosen the rockers and redo the leakdown. This way you will know if the valve itself is leaking or if the valves are misadjusted and being held open by the rocker arms.

[@timd5597]

@@metalinmotionshop thanks so much for your help...after lapping valves I put brite flash light in ports no light what so ever past valves but I will try taking out push rods so there won't be any pressure on springs then re-test. Thanks again

[@Sensei948]

@@metalinmotionshop 140 psi good compresion 372 xp oe ?

[@PopaAlexandru.]

Only dedicated devices must be used, otherwise the measurement is erroneous. And smaller measurements due to the cold engine... all engine manufacturers recommend measuring the compression with the engine cold, but it seems that they are all wrong.

[@daver2964]

nonsense! air filter will affect all cylinders. BTW, if your engine is running smoothly should you even be looking at compression? IF your engine is starting like a champ , should you do the test at all? Warm engine???? If it comes in warm, then it will start better than cold...but doesnt mean you dont have a problem...but seeing the engine from cold to me is better.