How to do-it-yourself instructional on basic tune up of Tecumseh mower engine. Demonstrates troubleshooting a fouled spark plug and solving by increasing operating speed.
Today on Repairs101 Iíve got my ten year old mower on blocks and Iím going to show you how to pim it out. Nah, Iím just kidding.
 You donít have to block yours up as high as I did. I just did this for the sake of making it easier to film. All you really need to do is put it up on a couple of bricks or on the edge of your workbench or something like that. So the first thing Iím going to tell you but not show you is ďRun it dryĒ. OK I run it dry every time I use it.
The next thing you want to do is pull the spark plug out of it so Iím just going to pull the wire off, push it aside. Put a socket wrench on it nice and easy, spin it out and have a look.
Itís quite clean, itís got some blackness to it but thereís no large deposits. Looking at the insulator around the electrode in the center, thatís not cracked, thatís very important. Inspect the insulator, hereís the insulator on the outside. Thatís the electrode.

Lawn mower service data tag / decal  Filling crankcase with fresh 30W oil

This sparkplug is a little bit blackened, a little bit fouled, you would say. Now the most likely cause of this is over-fuelling or a rich fuel condition. 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 thatís 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 weíll do is a quick compression test and have a look and see what kind of numbers weíre pulling.
I had the good sense to hold onto the literature that came with the lawn mower when I bought it ten years ago. See it says right here the gap on the spark plug should be thirty-thou so weíll just check that with the feeler gauges. Itís pretty much a perfect fit for thirty-thou so thereís nothing wrong with our gap. Clean it up with a nice soft wire brush and reuse it.
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. Consensus seems to be that itíll still cut grass acceptably under normal conditions even with a twenty-five percent loss in compression.
OK Iíve done a dry compression test and the results are in weíve got eighty psi and its been holding eighty psi for five minutes now. 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.
 
               
 
OK so weíre going under and weíll have a look. While weíre in her of course weíll take a look at the main seal on the end of the crankshaft and it looks nice and dry, as you can see so thereís nothing to worry about there, weíre not losing any crankcase oil. Oil residue built up here around the drain plug. So either my drain plugís been weeping a little tiny bit or itís spillage from above. Which s just as likely, I think. So itís a three eights inch square drive, Iíll just take this old toothbrush and clean up the...
Always going to see a little bit dripping out of course but when it gets down to a slow drip like this itís time to put the plug back in and fill up the crankcase with fresh  oil. Iím going to clean up the surface where the plug seats here, make sure thatís nice and clean, in spite of the fact that itís still dripping. And Iíve already cleaned up my plug so Iím just going to go ahead and drop it in. There we go. Weíll just tighten that down a little bit.
Just give it a little cleaning. Make sure that you canít get this dirt accumulation here down into your crankcase. Thatís the main thing here. Give it a little shot.

Carb bowl location

Be sure not to overfill the crankcase. One of the problems you can get is too high of a base pressure and what will happen then is the engineís not breathing properly so itíll start misbehaving on you. OK.
OK so the next thing you want to look at is your air cleaner. OK you just push on this spot right here and that releases the lid. So we just pop that off. Pop the cover off and here it is right here. Now the air cleaner is this piece of low density foam that you can of course just replicate. Grab another similar piece of foam and cut a new one. And you know weíve got some garden debris thatís been picked up in the top of it, Iíd say all we need to do is vacuum it real quick, blow it real quick and Iím ready to reuse it.
And of course this is the air intake screen right here and you want to keep this free of debris. When youíre mowing leaves and things, grass clippings often will start to come up inside here and it travels through the body and into that filter housing that I just showed you right there and into this filter.
OK so weíre going to pop the air cleaner housing off and itís just held on by two screws one on either side of the carburetor intake right here. OK now Iíve got the screws out I just need to take the lid off and you see that, it just falls right away. OK so thatís what the air intake system looks like. From this fan just under this grate here and it picks it up through this tube which is inserted in here.
Iíll just do a little general clean up here before I put it back together. As you can see the springs and things have a little bit of garden debris on them. This is the governor mechanism right here. This is a breather tube which I try and keep clean and be certain to reconnect when I put it back together.
Anyway, so there is no adjustment here. The only adjustment I can make is I can bend this metal tang, right here that the governor spring is controlled by and that will speed up my high end. This is the governor spring right here and this metal tang here is its anchor point. Stretch the spring further to speed it up or compress the spring a tiny bit to slow it down. In this case, Iím burning a little too rich so Iím going to add a tiny bit of speed to the top end. There we go. I just bent it a tiny, tiny bit.
 
                
 
So more of the same for this, of course, you just want to clean that up, especially the breather. Breather vent right there. Make sure that thatís clear and you can take out the sponge Ė you see thereís a separate section cut in the sponge just for the breather. I can put my finger in there, itís certainly working. I can see light coming through it so itís certainly clear. Iím just going to take a rag. Thatís got a nice thick rubber gasket, we want to preserve that so it gets a nice seal here on this aluminum deck. Tuck the pick-up into this here, swing it over towards, line up my hose for the breather. OK so thereís the breather hose and thereís the port for it on the air cleaner and you just want to mate them up and insert it into that hose there Ė there we go. Line up my breather hose, there it goes, it just pops right on nice and easy. Then weíre ready to put those screws back in.
The best reason of all to keep your machine clean is that these fins are the cooling surface area of the machine. Itís air cooled and it depends on these fins to provide it with a larger surface area with which to exchange heat with the air outside. So if you pack that with lawn clippings and other organic matter itís going to become insulated and not be able to do the heat exchange it needs in order to operate efficiently.
 Here we go: Iím just going to put in my spark-plug after giving it a quick check. Now you definitely may want to change your sparkplugs. Theyíre cheap and sometimes if you have a no-start/no-run problem it could be as simple as a dead plug. Itís such a common problem that if you still have a no-start/no-run problem: itís not a bad idea to go get a second new plug and make sure that the brand-new new plug that you just tried isnít also dead.

Draining crankcase oil  Fouled spark plug

And, well be prepared for a good deal of smoke in the first couple of minutes if youíve done a wet compression test like I have. Itíll be a little smoky at start up and it might even take a couple of pulls because itís harder to ignite the oil but itíll start and itíll burn off. So the other thing you would consider is if this is a winterization Ė a lot of guys will tell you to leave the sparkplug out and just put a rag covering the hole but I respectfully disagree. You know, Iíve got a lot of internal combustion engines and I store them all with the spark-plug in it. Whether itís out on the street or in the back yard or in my basement or in my garage, all my internal combustion engines have the sparkplug stored in them.
So if youíre doing a spring tune-up youíre ready to go. Get some fresh fuel in it pull the cord and it should start right up for you. If it doesnít, well check out some of my other videos where weíll discuss some of the basic problems people have with getting their mowers running in the springtime. My philosophy on almost any machine is: if it didnít quit on you while it was running Ė if you shut it down, then it will run again. But if it quit on you while it was running, wellÖ
 [laughs]
 you could be in for a world of pain.

Mower burning wet compression test oil at start up

[Mower starts]
OK now Iíve run the machine for an hour after the tune up and Ė check it out. Itís starting to whiten. OK, itís obviously hotter over on this side, you can see that. Spark is hotter on that side but itís starting to whiten up which means somethingís worked. Either the tune-up or the raising of the operating speed has leaned out the mixture enough that weíre starting to get a nice clean burn.
 


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