I just replaced the connecting rod in my Briggs Stratton 15.5 OHV lawnmower engine. I did not do anything to the valves. Lubed it up, set the timing, put it back together, and now it won't start. It will crank for what feels like one cycle, then stop. Tried jumping it from my car -- same thing. It will turn over once, then stop. I turn off the key, wait a bit, try again, and get one more crank. One time I kept the ignition on and it slowly pushed and then turned over again -- once. Starter was pretty hot. Feels like too much compression/resistance. Any ideas what might be wrong, or what I might have done wrong? It ran fine before it threw the rod.
It sounds like your valve clearance is too high and the compression release is not working. When that doesn't work, your poor battery will not have the power to crank the motor past the compression stroke. We call that stopped on compression.
Anyway, simple fix. Remove your spark plug. Remove your valve cover. Hand turn the motor until the piston is 1/4 past top dead center (TDC) after the compression stroke. Now with a feeler gauge, ensure that the clearance between the valve and your rocker arm is:
intake valve = 0.003 to 0.005 and exhaust = 0.005 to 0.007.
If adjustment needs to be done (I suspect your intake clearance is currently much higher then 0.005, causing all your problems) There will be a torx screw that is used to loosen the lock and a 10mm wrench is used to make the fine adjustments. Once adjusted, torque down the screw to about 45 inch pounds and you are done.
Should crank fine after that.
(Usually the intake valve is the one on the bottom but verify by watching the 4 stroke pattern to be sure).
Thanks -- that sounds good. I had a feeling I should have checked the valves before putting everything back together. I probably can't get to it for a day or two, but I'll give it a try and let you know. Thanks again.
It could also be gasoline leaking through the carb into the cylinder, If you have a Wiresbro carb, that might be it. I had that problem.
Sid
Rats. I set the intake valve to .004 and the exhaust to .006. Still stopped on compression. I really do think that is the problem (it turns over fine when I take the plug out), but the valve adjustment didn't fix it. Do I maybe have a problem with the cam and tappets?
Should I take the base off and see if the compression release is OK? (I'm not exactly sure how I would tell, other than to make sure it sticks up a little bit past the cam lobe...) Ideas?
Take the spark plug out and try it. It might blow out gas and or oil. Is the spark plug wet?
With the valve cover off and turning the motor over by hand you can usually see the compression release working. After the intake stroke you will see the intake valve close. If you keep turning the flywheel slowly and pay close attention you should see a little bump in the valve. If that isn't happening, then that is your problem. Perhaps you could attempt to tighten up the tolerance a little lower then 0.004 and see if it eventually bumps. If you tighten it up too much (close to zero) you will run the chance of the valve not closing completely, loosing compression and creating all the problems that ensue after that. Also, when you are taking your clearance measurement, make sure you give the rocker arm a good wiggle to be sure that your are actually reading 0.004 clearance and not something larger but with crud or obstruction giving you a false reading.
The other consideration is that your battery has drained down from previous starting attempts and needs charging. Look into that.
Also, sidny made a suggestion about a leaking carb. Sometimes if it leaks too much fuel into the combustion chamber it can sort of hydrolock on you. To test this all you need to do is take out the spark plug. Crank the engine for 5 or 10 seconds to blow out the leaked fuel. Put the spark plug back in and start it up quickly. If it locks, then I doubt a leaking carb is the problem. If it doesn't lock but takes quite a few cranks to start and 1/2 starts and goes back to cranking etc., before it starts, the you most likely do have a leaking carb inlet needle. Easy fix. Install a manual fuel shut off valve, right between the fuel filter and the gas tank.
I appreciate the help, guys. I took out the plug and cranked it for several seconds. It didn't blow out gas or oil. Put the plug back in -- still stuck. I looked for a little bump in the valve from the compression release mechanism, and saw nothing, even at .0025 clearance. I tried a few times, watching the pushrod, and saw no bump. So -- do I need to disassemble and take a look at the compression release mechanism? I'm wondering if it might have been damaged by a piece of broken connecting rod in the couple of seconds that things were flying around in there. If it is bad, do I have to get a whole new cam? I don't see the release listed in the manual or anywhere else as a separate part... Again -- I really appreciate the help.
Yes, the whole cam will need replacement if the comp release is broken. It commonly gets broken when the rod breaks.
Yeah, sounds like a cam issue. Even if the clearances were too high, you should still see the push rod bump the rocker arm as you slowly turn the motor. If you are not seeing that, it must be the cam. You could try reversing the push rod first to see if that makes a difference, before you dive into the camshaft. I have never done it but I believe they can be simply flipped around. Maybe others have some thoughts.
Well, it's been a while, but I wanted to thank OptsyEagle and Cheese, and sidny too, for your help -- and to let you know how it all turned out. The cam did need replacing. I put in a new one, and the comp release worked fine. It still wouldn't start, even though it cranked fine. I would get a backfire. It seemed like a timing problem, even though I had timed it when I put it back together. I took the flywheel off to see if the key had sheared -- and it had. The flywheel was about 90* off. Put in a new key, and I'm back in business. Mowed my one acre lawn this week. Thanks again for the help!
Tags: mower, briggs, stratton, only, turn, compression release, spark plug, intake valve, back together, leaking carb
