Okay here's my problem. I have a Briggs and Stratton engine on my Toro Zero Turn riding mower. One day it decided just not to start anymore. I first checked the fuel, no water (at least non visible when I drained some off). I changed the fuel filter. I changed the spark plug. It does generate a spark when cranking. I tried putting a teaspoon of gas in the cylinder and then cranking it but it doesn't catch. The starter seems to be working fine as does the ignition. I thought it might be a carburator problem, but I would have thought the gas in the cylinder would at least make the engine cough a bit.

Any suggestions?

 

Does the engine have compression?

 

How do I check for compression? It's difficult to turn the flywheel.

 

Put your finger over the spark plug hole and crank the engine. If the compression is enough to blow past your finger, it has compression.

 

Yep, it's got compression. Any ideas?

 

Air, fuel, compression, spark at the appropriate time. This is what is needed for it to run. Put a bit more gas than a teaspoon full in the plug hole and see if it will start.

 

Still no dice. I put at least 2 or 3 tsp in.

 

How are you checking for spark, seems odd that with 2-3 tsp (I would not put that much next time) if you have spark and compression you are not hetting any ignition.

 

I used a spark testor. It seemed to show a healthy spark across the gap.

 

I went through a scenario JUST like this with my brother in laws string trimmer.
It had spark, but the flywheel has spun out on the crankshaft. They don't use a woodruff key anymore (well on this Chinese string trimmer motor anyways!) but a little nub that engages into the key slot on the crank.
Long story short is something jammed the flywheel and the nub sheared off, so the flywheel wasn't where it was supposed to be anymore. If it slips, the spark isn't happening at the right time. It HAD spark, but just no where at the TIME it was supposed to be there.
So check out a possiblity like that, especially if something jamming the flywheel the last time the engine ran (or on your first attempt to start it when it didn't start the first time) rings a bell.
Gilly
ps if you search my threads you will find my string trimmer story and how I resolved it cheaply)

 

Does anyone have any suggestions for a simple way to check the timing?

 

Check for a spark at the plug itself. I'm not convinced you have sufficient spark.

To check the timing, you remove the flywheel bolt and verify that the flywheel key is intact. Your timing is fine I'm sure, unless you've had the flywheel off at some time prior to this.

 

On the one I had to repair, it had a pair of magnets on the side of the flywheel, these triggered the coil pickup, which was part of the coil itself.
When the piston is at top dead center, the magnets should line up with the coil pickup, more or less.
The one I did it was obvious, the magnets were no where near the coil pickup.
It was thinking like Cheeses that made me work on that string trimmer for 2 nights before I found the problem, I thought the SAME thing "oh, the timing has to be right, it's only a single cylinder, no distributor" HAH!!

Gilly

 

The timing appears to be okay. As I manually turn the flywheel I can hear the valves open and the air suck then I can "feel" the compression, just at the peak (TDC) the magnets pass by the ignition coil. A primitive check but I think the timing is okay.

I replaced the ignition coil in case I was getting insufficient spark (no dice), and then removed the carburator and cleaned it as best as I could. There was about a teaspoon of debris and corrosion in the little reservoir overtop of the shut-off solenoid, cleaned all that up but she still won't start.

This has really got me stumped. I think I must be missing something very simple here.

Just as history that may help you expert diagnose the problem, for the last year (the mower is only 2 years old), when I would start the it I would have to let it idle for about a good 5 minutes. If I didn't it would cough, lope and backfire a bunch when I started to move. The last time I used it about a minute before I shut it off I just had a feeling that it didn't sound quite right, it was almost like it was just starting to run out of gas. It was getting dark so I just drove it my driveway and turned it off. It's been there every since.

 

Sounds like it has carb problems, but I'm not sure it's limited to just that.

I'm still wondering if you have spark. Again, check for spark at the actual plug, not with a tester.

 

Checked the spark at the plug, looked good. Nice strong blue spark. Changed the fuel, removed the carb again and cleaned it real good and blew out all the passages with compressed air, the thing looks almost like new. But still nothing. After trying to start it the plug is wet when I remove it, so fuel is at least getting to the cylinder.

Man I am really stumped on this one. I'm thinking about GillyWi's comment again about timing. Does anyone have any suggestions for an easy way to test this?

What else could it be?

 

I do not think there is an EASY way out to determine what the problem is with your engine. The flywheel key controls ignition timing. To check the key you may be able to remove the flywheel nut and visually determine if the key has moved/sheared. Sheared keys are rare on engines in riding mowers. If it is the key is sheared, you will need to pull the flywheel to replace the key.

The most important and definitive test has not been done and that is a leakdown test. The links below explain the test and give instructions on making a tester.

The thumb method of testing compression is OK as a cursory test but accuracy is ZILCH. You really do not know the condition of the cylinder and valves. You never listed the model number of your engine so I do not have a clue which engine we are dealing with. Most engines have compression release, which makes a compression test not very helpful.

It just may be more cost effective to take it to a shop for repair.

http://en.wikipedia.org/wiki/Leak-down_tester

http://vmaxoutlaw.com/tech/leakdown_tester.htm


Good luck

 

The engine is a Briggs and Stratton model: 286H77-0165. I'll check for a busted flywheel key. The leakdown test looks too complicated for my garage. Oh well, I guess I'll need to find a trailer and drag this thing to the shop. Sigh.

 

Remove the valve cover and inspect the valvetrain components. I'm thinking a pushrod may have dropped out of place, since fuel and spark are proven.

 

Since this is a single cylinder engine, the valve train, as Cheese points, out could easily be the problem.

Posting model numbers in the beginning eliminates confusion and guesswork. If a valve pushrod is loose, it is not too difficult to correct.

 

How difficult is it to inspect the pushrods? I presume this would show up in a leakdown test?

 

No, it won't show up in a leakdown test. All you have to do is remove the valve cover. 4 bolts.

 

Simply remove the valve cover and you will see the valve pushrod out of the rocker arm if that is the problem.

If a valve pushrod dropped from its position, the valve stays closed. A leakdown test, tests the cylinder for loss of compression. With the valve closed and not leaking the leakdown test will show the cylinder good.

 

You guys are awesome! Took the cover off and whammo! One of the pushrods was sheared in half. Now I got all the bits of it out, can I just install a new one? What would cause that and could it happen again?

 

It worked! Replaced the push rod and she fired up right away! Thanks for everyone's help! There's no way I would have figured that one out on my own.

 

Great! Glad we could help!

 

I just dealt with a similar issue, I removed the cut deck to sharpen the blades and had lifted up the tractor onto the deck and inadvertantly knocked the intake valve seat loose, now in this instance the engine would have failed the finger over the spark plug hole test. After removing the head I was able to put the seat back in place and lightly tap it back into the block. Now I plan to rebuild this engine over the winter and will no doubt be replacing the seats and I am a little curious why they are such a loose fit in the block.

 

They don't fit loose at all, in fact, they aren't made to be removable or replaceable. If yours fell out, the head overheated and expanded far enough to let it fall out. You should peen the old seat back in place or it will come out again. Peening it in place is a permanent repair.

 

I'm having the same issue as CybrPNK was having, Now I pulled of the intake while I had carb off to clean, turned the key over and valve seems to move good. Did the same visual test on the exhaust side and also looks to be moving good. Is this enough confirmation that they're working or not?

 

I would say so, especially if you feel you have compression or at least feel air being pushed out the sparkplug hole. I made the post on this thread about the ignition timing being off due to a slipped flywheel, have you checked that?