I have an briggs and stratton 18.5hp. OHV engine on my lawn tractor. After the engine locked up on me while mowing I took it completely apart. The connecting rod somehow came loose and shattered. Now I just replaced the piston, rings, connecting rod, and all accompanying hardware. I put it all back together, torqued it all to specs, and it won't start. It has lots of compression. It will turn over freely when I try to start it with the spark plug out. With the plug in it will sometimes turn over freely but won't start. It has spark, it has gas. Sometimes it will backfire very loudly, but won't start? Please help!! I have double and triple checked everything!!

How did you adjust the valves?

I did not adjust the valves. I left them just as they were

It does sound like a valve or timing issue. Liguid fuel is making it difficult to turn over. You can't compress a liquid.

You did line up all the timing marks too, right?

If you need a little "starting fluid" use carb cleaner and not starting fluid.

I did line up timing marks. If in fact is is a valve issue, how would they have come "undone" since I did not mess with them at all. What can I check to eliminate this as the problem source or not? Your replies are much appreciated.

What is the actual engine model? If you can find the exploded view it at Home - Briggs & Stratton that would be great.
Model and type numbers are usually stamped on the blower housing. Posting the link to the exploded manual would be great.

You may be able to find the lawn mower model at Parts & Accessories | Shop & Find Lawn & Garden, Appliance Parts at Sears PartsDirect | SearsPartsDirect.com from Sears with exploded views.

Checking the valve clearance is just one step you should do. It may have been the cause of your failure. I'm sure you checked and/or replaced the flywheel key.

You should make sure that when going from intake, compression, power and exhaust, that the valves are at least opening and closing in the right order. That's a gross step.

If it's a belt driving the cam, then the engine could have skipped time.

Once you find the specs, it's easy to check and adjust. The adjustment is usually done when the piston is down 1/4" in the cylinder on the power stroke. Both valves are adjusted then.

These are just some ideas. Briggs sells repair manuals for particular series. The parts breakdown is free.

Where should the piston have been when I did the initial timing ( matching up the markings on cam gear and gear on crankshaft)? Now that I think about it I have no idea what position the piston was in when I did this I just turned the flywheel so that the marks would both line up.

Well, go back to grade school when you learned about 4 stroke engines or at least review it.

For fun lets think about a few things:

What happens to the piston when you rotate the crankshaft? Leave th valves out of this, for now.

The piston goes up and down - once. Is that enough to go through a full cycle of the 4 cycle engine? No!

It may be the intake and compression strokes OR the power and exhaust strokes.

Lining up one mark without knowing what the valves should be doing, your in trouble. You set the stage for bent valves etc.

When you don't have a manual, you have to understand the principles.
You could be firing in the intake stroke, rather than the power stroke and exhausting when your supposed to be compressing. And intaking on the power stroke.

Something that's been added for cars is that you can fire the plug in the power stroke and also in the exhaust. This fact allowed 3 coils for a 6 cylinder engine.

I do have the manual for the engine I have. It does not say anything about piston position when lining up timing marks. Unfortunately I am not a mechanic in any way shape or form, I was just trying to save some money. I understand how 4 strokes operate and I understand it takes two crankshaft revolutions to get all 4 strokes ( intake, compression, power, exhaust). I just don't have a clue to know where the crank or piston should be when I line up timing marks. My guess would tell me at the top, but I don't know. How can I go about finding this info.

When you rotate the crank shaft look at the piston and the direction of the piston and at the same observe the valves. The valves are closed when the lifters are loose. You need to know what valve is the intake and which is the exhaust.

It might not be much, but you can feel them.

Do you agree, that their MIGHT be two possible positions when you take the valves into account?

Adjusting the valves (technically valve clearance) in a vehicle, you do it a top dead center. Briggs, I believe, has chosen to adjust BOTH valves when the piston is 1/4" down on the power stroke. Both valves would be closed and thus the rocker arms would be loose. Moving it 1/4" probably makes it easier to adjust.

When it's there, you try to insert flat feeler gages or combinations (about $8.00 at Sears - guess) with out it hanging up going through the opening.

Mechanical valves need to be adjusted periodically. In one of my cars, I did it yearly. It's also BEST to do it with the engine hot.

You must be doing good so far. I rebuilt my first engine when I was 10. I threw a rod because the oil was low. Dad made me rebuild it.

Thanks for the info K.I.S.S. I just removed engine again from mower so this weekend I will have time to work on this. I will keep you updated on my progress. By the way, I have never taken anything apart to try and fix it. So trying to tackle an engine was probably not the smartest thing!