Lawn Mower Compression Test
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- Опубліковано 6 жов 2024
- repairs101.ca/...
How to do-it-yourself instructional discussing troubleshooting an ignition with a fouled spark plug and demonstrating both dry and wet compression tests on a four stroke (four cycle) Tecumseh lawn mower engine. #lawnmowers #gardentools #mechanic
Transcript provided for the hearing impaired:
[No dialogue] [Music]
This sparkplug is a little bit blackened, a little bit fouled, you would say. Now the most likely cause of this is overfuelling or a rich fuel condition. Let me compare it here to a sparkplug out of my car, which is a very clean burning machine, especially alongside of my lawn mower. However a machine like a lawn mower sees such dirty service and has kind of a crude air filtration system. So you're going to have problems like that.
Reasons for a blackened or carbon deposited spark plug would include things like too rich a fuel mixture or a sticking choke or a clogged air cleaner, you know, anything that's going to increase the fuel in the fuel : air ratio. Also a good possibility is oil seeping past the rings on the compression piston and getting up into the combustion chamber and fouling it. So what we'll do is a quick compression test and have a look and see what kind of numbers we're pulling.
OK so check it out, on the left we have my low-range compression tester, on the right my high-range compression tester. The high range of course is for high compression diesel engines and the low range one is for a wide range of vehicles and machines including this tiny little four stroke engine we're going to give a test to. OK you just push on this spot right here and that releases the lid... just remove these two screws. There's another one in behind here. That's why you need the long extension on your socket wrench. And as that one comes out it just falls away like that.
OK so we'll have a look in here. There's your money shot right there showing the throttle plate so the procedure here is to put a screwdriver in this hole to block that open. Whatever you use make sure that it can't fall in and it won't break apart and pieces of it fall in. Put my screwdriver on in there past the throttle plate and make sure it's blocked open before I pull on the cord. Thread it into the spark-plug hole, just do it about like that: hand tight. It's a good idea to just put it down somewhere safe out of the way.
OK so I looked around online to see if I could find some technical specifications for this engine. A new machine like this will make about eighty psi in pressure. Sixty psi on the other hand is generally agreed to be acceptable. Twenty psi is only a quarter of eighty psi so that's twenty five percent. Most mechanics I've worked with agree on a range of acceptability of around fifteen to twenty percent. So twenty five percent is quite generous, now keep in mind that it would depend on the type of machine. Consensus seems to be that it'll still cut grass acceptably under normal conditions even with a twenty-five percent loss in compression.
[Engine turns over]
Sorry about the bumpy ride.
OK I've done a dry compression test and the results are in we've got eighty psi and its been holding eighty psi for five minutes now.
OK we're just going to put a drop of thirty weight oil in there. Maybe two shots. Put the compression tester back in.
OK so now with a drop of oil in the combustion chamber we'll do a wet compression test.
OK now the wet compression test is eighty psi. This ten year old lawn mower of mine is making the same compression it did the day it came out of the factory.
[Mower starts]
Most, if not all small 4 cycle engines have a built in compression release on camshaft. If you get from 45 to 60 lbs, you are good. They won't show any higher because of compression release. Quick troubleshoot: Spray some flammable carb cleaner (what I use) into carb intake. Pull rope. If it runs and dies right away, you have spark and compression. You have instantly eliminated those as a problem. It's not getting fuel. No need for comp. test or spark test. Continue on to carburetor.
It's only been 12% of a century, but thanks for this information; I got 60psi on my Murray push mower and thought it was bad, but now I have hope :) Edit: It didn't make sense that it did run on Brake cleaner, but not on its own fuel system, again, thank you :)
The fact that the compression reading stays for five minutes has nothing to do with the condition of the engine. It means that your compression tester valve is holding the pressure.
Exactly. Heard that too and laughed.
You'd want to know that, though, eh?
He probably thought it works like the tire gauges
3 things, after adding the oil leave the compression tube off of the engine and pull it several times to distribute the oil around the cylinder to fill in temporarily any cylinder wall scratches. Reattach the guage and run the test again if compression goes up the oil has blocked pressure from leaking past the piston through the scratches. 2 The pressure reading will stay at you last reading for several minutes or until you push the relase valve the extended hold of the pressure had nothing to do with the engine. 3 remove the air filter and set the throttle to full open and choke to full open that will allow maximum air flow into the cylinder no need to hold it open with a screwdriver
Just wanted to clarify a comment you made that may mislead some. You said that the compression "held at 80 psi for a few minutes" This is not really true. What held pressure was your tester and hose assembly. No engine "holds" compression for any length of time after it stops turning and it doesn't have to. Whats important is the peak pressure during the test. Testers have 1-way valves so the peak reading is held. In fact you could unscrew it after test and it would still show the peak reading, as long as you didn't press the relief valve.
An engine is basically an air pump, as air makes its way thru the carburetor into the cylinders and out the exhaust. Over the years, I have noticed that little attention is given to the muffler and spark arrestor as a problem area. Not only can you have a restricted intake system with a dirty air filter, you can also have a restricted exhaust system. Recently, my chainsaw was not running properly and by simply removing and cleaning the spark arrestor screen, the engine ran like new again.
"It's been holding 80 PSI for 5 minutes." An engine won't "hold compression". It might for a brief moment if it stopped in the right place, but certainly not for 5 minutes. Compression gauges have a one way check valve at the end that screws into the spark plug hole. It allows pressure to enter the gauge but not exit. Without it you couldn't even do a static compression test. You could screw the gauge out of the engine and it would still read 80 PSI. That's the reason for the little schrader valve next to the gauge, to vent the pressure after you've finished the test (or between cylinders if you aren't checking lawnmowers).
You are correct, typically 80psi is compression found off of push mowers off the showroom floor. Also, yes compression testers have the check valve (tire valve stem-like valve in it). So yes, if you left the compression gauge in the mower, it will read 80psi all day long, and is not a function of how well the engine can hold compression.
Thank you for a little test that can be a real labor saver. Excellent!!
Thanks Dave. All my videos are closed captioned for the hearing impaired as well as for translation in 58 different languages using the UA-cam translate captions feature.
I thank you for the closed captions I'm hard of hearing much respect for you sir you have a subscriber & most of you tube channels doesn't even think about this & again much respect to you sir. I'm working on a 2007 snapper 150Z 20up Briggs & Stratton zero turn lawn mower it'll turn over it has spark to the pugs but no crank it did run before but I stop running so this is what I'm going threw when I got it as its not running
My Snap On Compression Tester also has the release valve.
Also, within the flexible hose, it has a check valve too, that will prevent from the pressure being released.
The only way to relieve the pressure in my compression tester is to press the release valve, or to disconnect the compression gauge from the flexible hose. If I disconnect the flexible hose from the lawn mower (with gauge still attached to hose), the compression gauge will still show pressure.
If it had 80 PSI when dry why would you do a wet test with the oil? I'm not trying to be a smartass. I've just started working on small engines and I'm trying to learn as much as I can. Great vid by the way. I got to get me a compression tester.
Thank you! My mower won’t start. I found a few of the head bolts kinda loose. I’m thinking (hoping) that’s the problem. It’s always been a hard starter. Anyway, I’ll put it back together when my torque wrench arrives. 🤞
It sounds like you wore your battery down trying to start it - charge it if required.
If it burns Diesel you'll need to bleed the lines to the injectors after running out of fuel.
Gasoline burner - check AIR filter/passages are clean/clear - check FUEL is present at the carb - check SPARK is present/sufficient. If your battery is good (+13 volts) it will likely be one of these three things.
Check choke operation by looking down the throat of the carb (see video). Good luck!
One correction: You noted that your compression tester "held" the pressure for some time. They make them like this so the highest reading stays long enough to read it. This is due to the valve in the tester - it has nothing to do with the engine being tested. You mentioned the engine held this pressure - that's not true. Any engine would leak down over seconds and would never "hold" pressure after stopping
This guy needs some help. 80 lbs is damn good considering these engines have a compression release on cam. That's why it didn't increase w/oil (by the way, the compression will "hold" because the tester has a tire valve stem-like valve in it)
To perform test, there's nothing to do as far as removing intake, etc. As long as carb throttle shaft is open wide, which it will be w/engine off.
His plug actually looked pretty darn good. Probably all he needs is a new air filter.
Keep it simple...okay?
Wow that's a well built machine!
I did an oil change and ran it for an hour yesterday.
20 years this summer and still running perfect on the original spark plug!
if you do not have a can of starter fluid get one it is hand item to have it will help start many balky engines you dont need to take eng apart just spray starter fluid into filter it crank right up if you are having a fuel problem to start if it dont start with this fluid you prob. have a spark prob.
Thanks. frank! Great tip!
Yes, I wore my battery down and just went an bought a new one. I also put new plugs in and a new fuse. I took the air filter off, cleaned it and looked into the top of the carb and it seems to be getting gas and the choke does open and close. Can this be just bad gas or dirt from tank got into system or what?
@D12caterpillar - You're not giving me much to go on. Battery +13 volts? Fresh fuel? Airway clean and clear? Choke & throttle plates operating? Maybe perform a leak-down test to see if a problem is revealed.
When you make a video speak up
I heard about half of it
And my hearing is fine.
Good video, I have a very old Briggs and Stratton engine that makes 75PSI on compression and is almost like new inside. The spark is bright blue also but the thing just won't run. I am running it on propane and it was working brilliantly but I think some curse has been set on it as all it does is kicks occasionally or just backfires. Will not even run for a second.
+Shaun Dobbie - if it kicks or backfires it wants to run. Sounds like the timing is out but I doubt it. See if you can get it to start on a TBSP of gasoline prime.
+Repairs101 It turns out the spark plug was dirty and the gap was too wide. It works now.
Glad to hear you got it! Rock on!
Shaun Dobbie thanx for the update...too many people fix their problem and never come back and let us know what it was...this helps everyone out in their diagnosing skills
@LawnManCL ... Hi, I tested the B&S motor of his new lawn of my neighbor, very very very brand new and read 60 PSI, I tested lawns with new rings I saw 65 PSI and they work very good. Is this normal depending the machine? or Why this is happening?
Thanks I'm just curious.
The compression is held within the valve, not in the mower and hose
Why do you prop the throttle plate open when you do a compression test? What happens if you don''t?
Nice video. Thanks for posting.
How do you know if you have a headgasket leak in a motor like this? I have no signs of smoke, but a loss of power and checked my intake and fuel. Hard starts and its cuts out alot. Its over ten years old.
Liked the video. Thanks for posting.
:)
i got 52 psi I guess that's why it's not running craftsman 6.0
well i finally got it running.but when i put if in reverse it stalls.i determine that i have a blown head gaskets.
My question is... Is 60 psi good enough for compression on a huskee LT4200/which is a 420cc riding lawn tractor the year of made is 2014.. I'm only getting 60 on compression tester 🤔
you seem to really care about that mower.
Like, a lot.
I look forward to keep my Law Mower for 10 years!
16 years now and still mowing strong!
Same mower i have. The thing was great and could run tree Limbs over shredding them up with that blade on it. Until i hit a green 2inch stump bent and ruined the blade tore the shit out of the frame and finished off the rusty wheel mounts.
Set for 3 years and isnt running now. Recently put it in a new frame and put a new Chinese carb and ignition coil on it. Just replaced the flywheel pin and it fired right. So happy. =D
Do you need the gas tank attached to run compression test?
Nope.
Hi, My little weed eater shows 100+ psi when cold (3 pulls) then after its hot for a while it starts to lose the idle and wont run any more. I quickly tested again and only got 70-90 and coudnt even reach 100 psi after 5+ pulls. Is the engine too worn now to make the 2 stoke cycle work anymore? all coil and fuel and carb has been substituted and bypassed so its not that. also crankcase is airtight after pressure testing it to 9lbs and replacing all gaskets ect. only thing left is a worn engine? also idle Low screw setting wont settle in one place anymore and have to keep using choke to start it even when hot. got full power and the l & H screws can give me more or less fuel , its just after a time 10-20 mins its dies and wont run any more like its starving or air being introduced but i already eliminated all that with total strips and replacement and 3 carbs and emptied tank twice into a container to be sure it wasnt blocking up or the vent valve was closing. I even gravity fed the carb when i removed the pump gasket and blocked off the pulse channel to see if that was losing pulse when hot and it worked great and still stopped exactly the same, so fuel delivery cant be it with 3 diff carbs.
Thank for anyone who can help out!
So how do you fix it ????
How to know if Choke is sticking or always open maybe? My tractor tries to fire up and the motor spins but will not start. This started after running out of gas and upon refueling I thought I flooded it. Now it has been a week and will not turn over or fire up. Help!
Iam getting 90psi on a 20 year old 3.5hp briggs and stratton classic mower engine. that was a dry compression test i took pics to prove also
Sounds like you take good care of it.
oil is changed twice a year with SAE 30 oil :-) head is decoked once a year :-)
that sounds great! but id bet you could get more on a 3hp! :)
Those engines are indestructable
Why does the butterfly valve need to be held open? I have an old Murray like that and it has been loyal for decades. This year though it wants to bog down in thicker grass and I'm wondering if its finally giving up the ghost. Tested 80 psi though but I didn't do the butterfly thing.
It's what they taught us in Mechanic's school.
Wouldn't that only work if the cylinder happened to stop at TDC on the compression stroke? ;)
What would cause over compression? I've got a 2 cycle tiller that's so hard to pull start and I know it's over compressed somehow. I also hear a slight hiss when pulling
Water
Some two-stroke engines come with a compression release button you can pull to make it easier when starting. It automatically goes back to a normal position after the engine starts.
I’ve seen a plugged exhaust cause similar problems
Lawnmower Week continues on Repairs101 with
ua-cam.com/video/U0DuLtoHbWg/v-deo.html
May I know what is the u r working on?
typos sorry.did a carb rebuild.
@repairs101ca Oh, it's t-ed into the hose. Damn, that's modern stuff ! Mine keeps the pressure reading after it's detached from the cylinder. Learn something new everyday. Time for me to get a better compression guage, even if I only use it once every 2 yrs. or so. Ha, ha,...
Anybody know how if engine temperature affects a compression test?
Could it fail at cold but then pass when warmed up?
Yes if the compression gap is closed by the metal's thermal expansion.
I Like your logo are you on the level brother?
smib
won't that oil in the compression chamber make the mower smoke? ???
+King - yes but only for half a minute
what if the motor has to much compression and the fullly chared battery cant start it? (starter is good i already check, it also has good plugs and plenty of oil
Valves need to be adjusted
I was givin a craftsman riding lawn mower. The previous owner siad he had it running and it just died! He never had it running since. I first checked for spark from the plug - nothing. I got a new plug and then a new ignition module. Still I didn't get spark. I then tested it for spark with a simple screwdriver up in the end of the plug wire while being grounded on the engine. I only get a FAINT spark. I no doubt had cleaned the monting bracket for the module and the flywheel. I do not know what to be looking towards? I even took off the flywheel and cleaned it under that. I saw it only had 4 magnets and then seen other ones, the same model number on ebay with only 4 magnets. Another told me the the manets under the flywheel are for charging the battery and nothing to do with the spark for the plug. Any ideas??? Thanks
Well got to have the proper gap between the magneto poles and the flywheel.
Usually between 6 and 10 thousands. Then all the connections, have to be clean. Magnets on flywheel may be weak. They do not just charge a battery, they predominately make the spark the plug needs.
I did adjust the gap to 10 thousands (a credit card) cleaned all connections w/ wire brush for the coil grounding purposes of coil. Is there a place I can order new magnets from then?
How do you compress it or what do i my motor is dead
Got a 4 cycle Toro 20332 that won't start. Compression is only 30psi but I did not open the choke. Will that make a difference in the compression reading? I have another mower that starts on the first pull and it's compression is 45psi also did not open choke.
rfn944 Choke open or closed will give nearly the same reading - the issue is with fuel being pulled in. I think your 30 psi motor may be hooped. If it's supposed to be 45 like your good one that's a 33% loss of compression, well past the 25% mentioned in the video. Good luck!
@@repairs101 f
Alot of answers but,there is alot of questions, first of all there is a decompression valve on the cam,second of all to do an accurate compression check you have to remove the lifter off the exhaust,otherwise you won't get an accurate reading..so what is every body babbling about???
Is there anyway to measure the temperature of the air ? (Dry)
Pyrometers do exist for some extreme duty engines but I doubt you'll ever find one on a mower. Maybe you can get a reasonable approximation from a hand held Laser Infrared Digital Thermometer/ Infrared Pyrometer unit aimed or placed in front of the open exhaust.
I'm getting a reading of like 30 on one mower about 90 on another. The one with 30 doesn't seem to be seating well. Is it difficult to grind them?
foreverwood1963 Sorry but you have two mowers or you have a 2 cyl mower? More details required. Makes, models, engines, service hours (years?) history and symptoms.
Well I have a craftsman 4 cycle mower that has 30 on compression, the valve seats don't appear to be seating well. I was wondering how difficult it might be to grind them down
foreverwood1963 Have a look at this excellent resource from Briggs & Stratton on valve removal and lapping.
www.briggsandstratton.com/us/en/support/faqs/valve-repair-maintenance
Ive been trying for a while now to repair my moms mower.
It spits gas into the air filter. Will no compression cause this?
BlackDynamite901 Dude, not enough info. Does it run at all or are you saying you pull and pull and all that happens is the air filter gets wet with fuel? If it's gravity fed and it floods while running then perhaps yes but more likely a broken line / cracked fuel hose kind of thing. If it floods because it won't start that's normal and you just have to solve why it won't start. ua-cam.com/video/gPb64N01AAY/v-deo.html Good luck!
Repairs101 Doesnt run at all. Spraying carb fluid into it gets it to stutter more but never fires up.
Spark plug is fine
Gas has been drained and replaced
Oil level is fine
Carb is brand new
BlackDynamite901 Try priming the carb with a splash of fuel (1/2 oz) and the air filter out. Keep the choke wide open for a few pulls (if so equipped) then try 3/4 choke and back to full choke if you can get it to fire. Obviously put the air filter back in once it's running. Any chance the new carb is wrong or set up wrong?
Repairs101 Thanks for the help!
I replaced the gaskets(they were all worn and broken), and looked at a diagram for the linkage and it fired right up.So yes, it was setup wrong, and i dont know if the gaskets helped but lets say it did.
Now ill check out the tune up video.
Need to see if it will hold 80 psi for like 5 hours is a more conclusive test.
I can remove my compression tester and it will still hold the reading.
The guy in this video posting seems to know a hell of a lot more than most of these "UA-cam mechanics".
I saw one guy, "golfcartractor" show how to get spark on an old points style Briggs by wiring in place a wet cell battery on the deck and automotive-style coil with wires running everywhere. Unsafe as hell.
I told him just replace coil w/Magnatron and he got all mad. His mouth was so foul UA-cam 86'd him. What a maroon!
I think there's a problem here , if the first test indicate 80 psi , With oil added to cylinder should have shown higher pressure . 80 psi is high for a dry test but an old mower or even a new one usually shows a higher value . Your mower isn't that good .
Fronzi
I have a Briggs and Stratton 12 hp ic its not ohv it seems like it has got to much compression plz help me
Timmy Weller I'll need more details to be able to form a theory. Tell me what it does (or doesn't do) that makes you think it has "too much compression".
have your rocker arms adjusted
Keep in mind that Briggs states that one (1) ounce of oil per cylinder per hour is normal.
It turns over hard like it gets stuck somewhere
Timmy Weller Electric start? Clean battery leads and full voltage? Turns over freely with spark plug(s) removed? Again help me out I need details. So far I suspect you have a low battery and/ or poor cables/connections (especially bad ground) and/ or hooped starter.
my riding mower back fires.hard to start.i adjusted the valves.clean the gas lines and tank.didva crab rebuild.and still no luck.any ideas?
Sounds like valve timing is way out.
+Repairs101 i did 600 on the intake and 400 on the exhaust.
That's valve clearance - valve timing is the precise timing of the opening and closing of the valves
Check to see it isn't supposed to be the other way around - usually in is smaller and out is bigger due to thermal expansion
+Repairs101 the valves?
hey i was wondering if i do not have that washer on the spark plug will i have low or no compression. becuase i have an honda gx200 and it is incredibly hard to start (have to pull it like 50 times) and i think that might be the problem becuase there is carbon residue around the outside of the spark plug and when i do finaly get it started i can hear sort of pshs pshs pshs noise on each stroke, is this normal?
Div Bro Your plug is definitely letting go of compression ( pshs pshs pshs ) and the evidence is the carbon build up. Sounds like this may be only one of the problems with that mower. Good luck!
Dude get to the fucking point
Did not like his technical teachings.