# Sears Lawn Tractor, No Start



## ggb715 (Aug 28, 2008)

I have a sears 18.0 HP 42" mower with a BS engine model # 917.273392
I cut my grass last week and pulled it into the garage and shut it off. I turned the gas off and left it. I went to start it today and when it turns over (full choke) all it does is back fires.When I try it with out choking it does not back fire but keeps turning over but does not start. I check the plug and it has plenty of spark. Full of gas and oil. what could be the problem and what could cause it?


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## 30yearTech (Sep 14, 2006)

Did you turn the fuel back on?

917.273392 is the Sears tractor model, not the engine model number. Sears shows that it should have a Briggs model 31H777-0297-E1 engine.

Could be a fouled spark plug, if a new plug does not help, then I would check the valve clearances.


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## ggb715 (Aug 28, 2008)

I did turn the fuel back on, I also get very good spark through the spark plug. I checked the shear pin and that is ok. I reset the magneto. It only backfires when I choke it. If I don't choke it then the engine just keeps turning over with no sign of trying to start. 
Yes your right the engine is 31H777

What is the valve clearances for this engine?


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## 30yearTech (Sep 14, 2006)

ggb715 said:


> What is the valve clearances for this engine?


Intake = .003" - .005" 

Exhaust = .005" -.007"

Did you try new spark plugs? 

Sometimes plugs will fail to fire under compression.


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## ggb715 (Aug 28, 2008)

Tried new plug, no go. Check the valves and the rockers were loose. I turned the flywheel clock wise until the cyl was tdc. I then moved it a little more and set the valves. This time the engine backfired more often when cranking. Should I set the valves with the cyl at tdc?


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## 30yearTech (Sep 14, 2006)

ggb715 said:


> Tried new plug, no go. Check the valves and the rockers were loose. I turned the flywheel clock wise until the cyl was tdc. I then moved it a little more and set the valves. This time the engine backfired more often when cranking. Should I set the valves with the cyl at tdc?


Set the valves at 1/4" past TDC on the power stroke. 
Rotate the flywheel and observe the valve operation, after the exhaust valve closes, then the intake should open. Once the intake valve closes, check the position of the piston once the piston reaches TDC then the power stroke begins. Set the valve clearance on both valves for the cylinder, and repeat this procedure for the other cylinder.


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## justin3 (Apr 10, 2007)

Almost sounds as if the coil isn't producing spark under compression. Try disconnecting the kill wire from the coil and try starting it. A safety switch may be causing intermittent spark.


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## phillipmc (Jan 27, 2008)

Try pouring a little gas directly into the carb (like 2 cap fulls of a soft drink cap) then try it


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## ggb715 (Aug 28, 2008)

30 Year Tech, I did everything you said and it stills backfires. I turn the flywheel by hand and watch the exhaust valve close, then the intake valve closed. I then took an pencil,erasure end first and put it in the spark plug hole, when the cyl came up tdc I marked the pencil then pulled the pencil out and made another mark 1/4" from the first mark. I then put it back in the spark plug hole and turned the flywheel just a bit to the last mark on the pencil. I then set the intake valve at .03 and the exhaust at .05. put the plug in and the valve cover back on and tried to start it. It would just back fire without starting. This engine has always turned over easy and still does. Could there be something else wrong besides the valves. I have checked the armature clearance, shear pin, put a new plug in it.


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## ggb715 (Aug 28, 2008)

Oh btw the backfire comes through the muffler.


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## 30yearTech (Sep 14, 2006)

Well, it's a puzzle that's for sure. 

A couple of things to check, if you have access to a compression gauge, then check the compression on each cylinder. 

Disconnect the kill switch lead wire from the tractor to the engine and see if that makes any difference. This would isolate any possible problems from the safety interlock system on the tractor.

Try priming the engine like phillipmc suggested.


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## 30yearTech (Sep 14, 2006)

ggb715 said:


> Oh btw the backfire comes through the muffler.


That usually is a result of an ignition problem. Are you sure the flywheel key is good?


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## ggb715 (Aug 28, 2008)

The flywheel key looks very good


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## ggb715 (Aug 28, 2008)

Pulled the flywheel off and the key is good cleaned everthing under the flywheel and put it back,haven't checked the cyl pressure yet, will have't to get a tester.Put gas in the carb and no luck. What should the pressure read? What is making this engine backfire and not starting? When I disconnet the kill wire what should it do or not do?


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## 30yearTech (Sep 14, 2006)

ggb715 said:


> Pulled the flywheel off and the key is good cleaned everthing under the flywheel and put it back,haven't checked the cyl pressure yet, will have't to get a tester.Put gas in the carb and no luck. What should the pressure read? What is making this engine backfire and not starting? When I disconnet the kill wire what should it do or not do?


If it runs alright with the kill switch wire disconnected, this tells us there is a problem in the electrical system of the tractor that is affecting the ignition on the engine. If it makes no difference then the problem is with the engine. This is just a way of narrowing down the problem.


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## ggb715 (Aug 28, 2008)

No go on the kill switch because it will not start.


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## 30yearTech (Sep 14, 2006)

You need to just unplug the kill switch lead, not the entire wiring harness plug.


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## ggb715 (Aug 28, 2008)

(kill switch) Thats what I did.
BTW I checked the compression with my finger, turn it over with the starter, it would not even blow my finger off of the spark plug hole.


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## 30yearTech (Sep 14, 2006)

ggb715 said:


> (kill switch) Thats what I did.
> BTW I checked the compression with my finger, turn it over with the starter, it would not even blow my finger off of the spark plug hole.


Okay, I thought you meant it would not crank over. If it won't blow your finger off the spark plug hole then there is a problem with one of the valves not closing all the way or a blown cylinder head gasket. A leak down test would tell for sure. 

When you checked the valves, did either one of them require quite a bit of adjustment to get set?


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## ggb715 (Aug 28, 2008)

How do I do a leak down test?


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## 30yearTech (Sep 14, 2006)

ggb715 said:


> How do I do a leak down test?


You would some special tools to do a leak down test. You could check with your local lawn mower shop and see if they have the equipment to do the test for you. You could also just remove the cylinder head and have a look at the valves and head gasket. The only part you would need to replace if you take the head off, is the head gasket, and if it's blown you would have to replace it anyway. Head gaskets are around $10.00 so it's not a big investment.


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## ggb715 (Aug 28, 2008)

I will check around to see if a friend of mine has a tester, don't want to buy one, they cost from $75. up to $250. or more. If not I will pull the head and see what's up, will let you know the outcome.


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## ggb715 (Aug 28, 2008)

So far this is what I've done. I pulled the head off and replace the gasket, ( it was blown)
I reseated the valves, reinstalled the head, the compression is great. I turn the flywheel clockwise until the intake just started to close. I inserted a pencil, eraser first and turn the flywheel until the cyl was TDC,marked the pencil at that point, pulled the pencil out and put the second mark 1/4" from the first mark, moved the flywheel to that mark, 1/4" past TDC. I set the intake at .003 and the exhaust .005. It tried to start but no go, reset the valves several times but still it will not start. I put a new spark plug and have plenty of spark, check the shear pin and armature setting. Everythig is fine.Any help would be appreicated.


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## 30yearTech (Sep 14, 2006)

Try priming the engine with a little fuel and see if it will fire off that way.


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## ggb715 (Aug 28, 2008)

Tried that.


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## 30yearTech (Sep 14, 2006)

Then were back to getting a compression reading to see if there is sufficient compression.


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## ggb715 (Aug 28, 2008)

Compression reading is 100psi


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## 30yearTech (Sep 14, 2006)

Well, I am currently out of ideas. I will keep thinking on it and if I come up with any other ideas of things to try, I will let you know.


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## ggb715 (Aug 28, 2008)

Thanks for the help so far.


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## 440s-4ever (Feb 23, 2010)

Are you absolutely, double sure your TDC determination was correct for the valve adjustment? When using valves to locate TDC on car engines I've seen people rotate the wrong way (finding the wrong side of the cam ramp) and/or look at the wrong valve. 

If it were on the bench in front of me, I'd have a wise man's words running thru my head......If you checked everything.......yet something is still wrong.......it must be something you already checked. 

Even without knowledge of your specific engine, I'd say the first valve adjustment was not required. When it started backfiring harder, that's a sign you misadjusted the valves on the first try. I'm not sure where the subsequent adjustments put ya, but definitely question what you're doing there as there may now be 2 obstacles to starting. Valves plus original problem

Again, just generally speaking- no starts don't usually require diving into oiled engine mechanicals unless it shut down with a bang. Now figure you got 2 almost independent cyls and neither are firing. What do they share? Carburetion and ignition. You put fuel in, still no start. That leaves spark. FWIW I've lost track of how many coils I've tested that ohm'd out good on the bench but would only spit and sputter during cranking. 

Good luck, 2nd time around is both the easiest and the hardest. Few things are more frustrating that questioning your own work, but all the threads and parts are nice and clean now


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## 30yearTech (Sep 14, 2006)

This is a single cylinder engine, that had a blown head gasket, he said it's got 100 lbs of compression ( a little low for an OHV) but still enough for it to start.

Whether or not the initial valve adjustment was needed, the resulting after fire is not an absolute indication of incorrect adjustment, but rather may a result of an increase in compression. The procedure he outlined that he used in adjusting the valves is correct. If the valves were not at least close to correct his compression would not be as high as he posted. 

I would suspect ignition issue as well, but with good spark and good flywheel key that's what's puzzling. 

ggb715
What procedure are you using to test your ignition spark?


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## ggb715 (Aug 28, 2008)

Good news!!, I went back and rechecked everything that I did and it all checked out. I then thought I'd better pull the flywheel off again and recheck the shear pin, sure enough it was sheared. I replaced the pin and it started right up,but was running rough. I reset the valves to .004 intake and .006 exhaust. It's running great now. I made a mistake that an instructor told me a long time ago. "if you pull the flywheel, put a new pin in." The one that I thought was good wasn't. Thanks for all the good info.


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## 30yearTech (Sep 14, 2006)

That's great that you got it going again. Make sure the flywheel is properly torqued or the key could shear again.


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## Maytag (Dec 10, 2007)

Let me throw a spear at it.......................Plugged air filter? Only fires (backfires) when its able to get air from the open exhaust valve? 
Hey what do I know about engines, but I'm curious to hear the end result of this one.

>Maytag:freak:


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## nnyparts.com (Dec 28, 2009)

As stated already...check the safety switches. If set-up right...the one under the seat, electric PTO and Brake peddle switch should not allow you to even turn the unit over if not open. The main cause with safety switch issues from what you have posted usually comes from the manual PTO lever switch in that its not all the way back or closed.

Another issue might be bad gas. If the gas is old or has any water in it....it will also produce the same issues you described.


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## 30yearTech (Sep 14, 2006)

maytag, nnyparts. If you go back and read post #32, you will see that he got is working again and that it turned out to be something that was suggested earlier but was missed.


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