Diagnosing a Small 4 Stroke Engine

hi this is john i'm out in the shop and today we're going to diagnose an engine that just doesn't want to start [Music] so customer brings this to me it happens to be an edger that's not what's important we're talking about a four-stroke engine here he had it for a couple of years it ran last year went to use it this year didn't want to work has no idea so it's a fairly new engine and i'm hoping that whatever i find it's going to be pretty obvious on a previous video a viewer had a comment and he sent me this acronym to diagnose an engine called fax and what does that stand for it stands for fuel air compression timing and spark now this is good on any engine however on small engines you really cannot affect the timing there's one time when timing can change and that's if you're say have a mower and it hits a rock and stops suddenly you will damage the shear pin or the woodruff key and then you'll have to get in there and fix that i did make a video about that but in this case i checked with the customer did you hit a rock or sidewalk or something he said no so we're just gonna get rid of that one so a small engine you're worried about can air get in there so i'm gonna take off the cover the air filter is here and it is not clogged i'll blow it out but it looks really good so i have confidence that air is getting into the engine so i'm going to strike off air next i'm going to go after spark this is the spark plug boot goes over the spark plug and pull it off and i have a spark tester so this goes in between the spark plug and the spark plug boot i'm gonna you can get these at auto parts stores very cheap i'm just gonna hook this into the boot and then when i give this a pull it should glow orange by the way as with most engines there is a cut off switch uh that dead man switch whatever it is but you have to hold this otherwise the spark is grounded so every time i'm pulling to generate the spark i am holding this closed like i would be operating it otherwise you're gonna pull it you're not gonna see a spark and you're gonna think oh there's something wrong well there's something wrong you didn't hold this handle down okay there now you can see it glowing orange what does that mean it means that the engine is producing electricity to make the spark and if that is getting to the spark plug it doesn't mean that the spark is actually going through the plug could have a bad plug but we can test that real quick i've removed the spark plug i plugged it into the boot i'm going to use a jumper wire and connect it to the side opposite the electrode so after it jumps the gap and then connect that to ground somewhere now when i pull i should see the spark come across the spark plug gap and indeed you can see that so the spark plug is good i am convinced that the engine is making spark and the spark plug is good so i'll scratch that off now we're left with fuel and compression i'm going to go to compression next since i already have the spark plug off this is a compression tester and you can get these also at your auto parts store you can also get them off of places like ebay i got a whole kit for under 20 very reasonable i selected the correct fitting i screwed it into the spark plug hole and then i just connect this and now i'm going to pull three four times and i would like to see this between 90 and about 110 so 100 is what you'd like to see well we may have found the problem he has no compression now we're going to focus on compression where am i going to go next i'm going to look up under here this is the overhead valve ohv it's a valve cover and there is a gap or a lash that has to be between the lifting rods and the valves and i'm gonna see if perhaps these need to be adjusted and if so it should fix the problem so we're gonna start by taking that cover off so after taking out the four bolts and carefully separating this from the gasket this comes off there's a breather tube that pops out this pulls off be very careful with this gasket unless you have another one to replace it with because it tends to get stuck over in the corners so this is what it looks like underneath these are gonna rock to push the valves in and out this is the intake side this is the exhaust side how do i know that because here's the exhaust tube coming up to the exhaust muffler get you at an angle where you can see the piston going up and down there it goes down and the valve moves comes back up and that's going for the intake stroke and there's top dead center on the compression you want this to be a top dead center these should have some gaps you hear that knocking around this one also and i'm just gonna check it with a set of feeler gauges and on the exhaust side i don't have the specs exactly but typical is eight thousandths so i'm just going to put this underneath on top of the valve and it slips right in it feels good so i think that's adjusted properly and then on this side six thousandths and that also feels good so now i'm scratching my head a little bit because this seems to be in adjustment but i'm not getting compression i'm gonna do something that seems a little unconventional and that is i'm gonna put some oil on top of the engine this hasn't been run for a year and i'm thinking well maybe the oil is drained out it's dry inside and it's not making good contact to to seal the rings against the cylinder so i'll put a little oil there and do another compression test see what happens i'm just going to use a straw and if you've got something more fancy that's great doesn't take much the oil will actually help to seal i'm going to tilt this up and move it about so it gets all the way around the rings and then we'll see if that improves things at all by the way i truly have no idea what's going on here so that's why i'm filming it so now it's gonna pull the cord again and see what happens ah look at there we got a hundred it has good compression so that may have been it the rings were just extremely dry let's try it again we'll push the switch the reset here we go there that ought to do it i'll put the spark plug in and this ain't going to start so before i try and start it i'm going to say we checked the compression that's done we didn't talk about fuel and so i'm going to do that right now one of the things you should always check is is there fuel in it and in this case yes there is plenty of fuel in it all the way to the top the other thing because it sits in the winter and there can be condensation so temperature goes up it goes down and air moisture will make its way in go to the bottom and then it winds up coming at the bottom of the carburetor which is right here so your air filter and the air goes through the carburetor picks up the fuel goes into the engine so right here is a little bolt and it's just meant to bleed off some of the gas so if gas and water was to come into here the water would go to the bottom and i'm just going to clear that out just in case and i'll i'll be able to see if there was water in it after i drain it off because the water will go to the bottom and it'll look like little bubbles at the bottom of the gas so i have a clean container i'm gonna put that up under it i'm gonna take this off just get a couple of ounces off of there if there was water in it you'd see these little droplets dancing around on the bottom and so all that looks clean so i should put some water in there and show them what it would look like brilliant i am brilliant that's why you married me putting in a couple of drops of water shake that up a little okay now if you can see that it's just different you see a little oval at the bottom and then you can tell hey that's that's different that shouldn't be in there that's water okay so this the gas looks good he says he puts stabil in it at the end of every season should be good i'm gonna go outside and give it a pull that's the oil that was inside it's going to burn that off it started first pull i should take the choke off is this your first time working on smalling that's my first time yeah all right so another successful repair the customer's gonna be happy to get this one back and he's going to be happy that it wasn't anything major i guess major point out of this video one go by this fax fuel air compression timing spark doesn't matter which order you attack it in but that's going to be a road map to diagnosis the other is look for the obvious stuff meaning this was a two-year-old engine hadn't been used very much so there's no need for me to tear into it and inspect the piston i'm going to look for the simple things and once i saw there was no compression but yet the valve lash was okay it had me scratching my head but then i thought well maybe maybe the piston is dry add a little oil and everything was good go about your diagnosis methodically and you're going to find a problem all right hey thanks for watching and we will see you soon on the homestead don't forget to subscribe bye 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Channel: Happy Wife Acres

Views: 2,684

Keywords: farm, homestead, small engine, small engine diagnosis, 4-stroke engine, 4 stroke engine diagnose, lawn mower engine won't start

Id: ZPFl7a-4kOM

Length: 12min 7sec (727 seconds)

Published: Tue Sep 14 2021