Briggs& Stratton engine will not turn over
- Thread starter JimP2014
- Start date
Forest good call 35 PSI
Let me provide more context. So this is the very old cylinder head from a 14.5 horsepower Briggs& Stratton engine. The screw holes did align perfectly with a 19.5 horsepower Briggs engine, but maybe there's something different because obviously there is very little compression. The original cylinder had at least provided 85 psi so maybe look into that. Helicoil kit so that I could go back to the correct cylinder head because the spark plug is pretty much blown out.
Jim
There's a whole lot of jumping around and grasping it straws here throughout the thread.
A good diagnostic procedure is almost more important than knowing how to actually do the work to fix one. Lol
I think I read through all the highlights of what's been done but it all boils down to you have spark and the engine is spinning and when you give it an external fuel source it does not run so therefore it is not mechanically sound to run.
That's what I call it and every time an engine comes my way that won't start a run, the fastest way to diagnose it is to give an external fuel source and see if it runs or not for a couple of seconds.
In your case we know it's not mechanically sound at this time to run so the next thing would be spark which it seems you have ruled out and you have spark.
The next thing, and all too common on these Brigs single piston ohv engines would be valvetrain or head issues.
I just lump all this into the same category including camshaft problems.
You don't need compression testers or leak down testers etc..
All you have to do is take the spark plug out, and with a comments about the threads stripping out and it blowing out the spark plug, either a plug has been cross threaded and forced in or it's been over tightened many times.
Please don't have to be that type. In fact with a brand new plug once you tighten it and the gasket touches, one half turn is plenty.
I have had engines for the spark plug has been out literally hundreds of times and the threads are still fine so be careful about the installation and how tight you go.
All you do is take the plug out then take the tip of your finger and stick it into the tip of the hole.
Crank the engine over and it should go blow blow blow blow blow at a regular pace and blow your finger out every time.
It should never have a pause and then a double blow and it should never try to suck your finger back in the hole.
This is the first basic procedure and if it blows you'll figure out where you really can't hold the pressure in, it has enough compression to run.
If it still isn't running, which we assume it would not be but this test looks good, you have to consider some sort of timing issue which could be camshaft valve timing or it could be the flywheel position timing too with the key.
Seems you've checked the flywheel key and it looked fine.
Busy just have lousy camshafts. Mostly they have a lousy ACR on the camshaft but they can also have problems with the lobes of the camshaft ESPECIALLY the aftermarket ones..
I have one laying around here somewhere that was only in a mower for 3 months it was only used for about 8 or 9 hours total that you could take at least one of the lobes on the camshaft and spin it around and around on the shaft. It's got a little clickiness as it goes past each little spline but of course they're stripped off.
This is beyond pathetic when we're talking about a situation where the valve spring pressure is barely in pounds with almost no pressure and it shouldn't be hard for the lobe to stay exactly where it's supposed to be but this happens because people make junk and poorly design items.
It's really hard to tell by looking if the timing from the camshaft opening the valves is correct unless you have another engine very similar to it to sit side by side and watch the valves move because these engines are not timed like a car or like you would expect.
They have such low compression ratio and in order to make them able to spit over even with the acr, the intake valve is open far longer than you would expect after the piston gets to bottom dead center suck again in the air.
On most types of similar engines with valves like this, you would find that the valve closes when the Piston gets to the bottom so all of it's upward travel can result in compression but these small air-cooled lawn mower engines don't do that.
The piston starts it's upward travel and moves a whole lot in my opinion and it's sometimes over halfway up before that valve actually closes.
I'm not just talking about the ACR either which in this case is on the intake valve but even other engines where are the exhaust has been used for the ACR, it's just amazing how inefficiently they are designed but as I always say, they are low performance little turds.
If your engine will blow your finger out of the spark plug hole with a nice deal of force and your vows are set even somewhat close then I suspect you have a camshaft issue.
Make sure you are setting the valves the correct way as some people simply refuse to do it basically the Briggs & Stratton recommended way and insist on applying previous experience or car style setting to it.
Make absolutely certain you are looking at the proper valve and thinking about it the right way because the intake valve is not on top. It is counterintuitive. The exhaust valve is on top and the intake is on the bottom
Also, make sure you're actually on top dead center of the compression stroke and not top of the exhaust stroke.
Then go that half inch past top dead center which in effect you can go 3 in past if you want because nothing happens until you get all the way to the bottom and then the exhaust valve starts to open but you have to make sure you go at least that half inch down so you'll be away from the ACR ramp.
Then you set them both and then you need to turn it over a few times and put it back at the proper position again to double check them because they almost always move a little bit and actually they always almost move when you're tighten up the locking part of the adjusting nut.
No adjustment in the world though is going to solve your problem if the camshaft lobes have jumped.
I also forgot to mention that we need to get some universal terminology consistent here.
The title of your thread says Briggs & Stratton engine won't turn over but it does seem that the engine is turning over and spitting just fine through all of the posts.
People often use different terms and phrases to describe what their engines are doing and it causes great confusion and hinders the diagnostic process.
If when you turn the key or pull the rope on a lawn mower engine and the engine spins but does not start, it is spinning, turning over etc.
It is simply not starting, taking off, getting up to full speed etc.
Now if you turn the key and nothing happens or it just clicks or barely moves etc, then it is not turning over.
People often use the term "catch" also which should not be used regarding lawn mower or engines at all.
There is no catching.
There is a starter Bendix or gear engaging the flywheel gear and turning it or attempting to turning it but this is never what they mean when they use the word catch. They mean the engine is spinning over or turning over and over typically but not taking off and running on its own.
Catch should not be used for this description.
This has always been an issue among people when describing what engines are doing and it causes great confusion and slows down the diagnostic process.
It gets even worse. I will have customers call me who have figured out they have some sort of belt issue on the rider.
They sometimes have no idea that there's more than one belt but even when they do, they will often refer to the wrong belt.
They repeatedly talk about the belt for the blades when they have a mower sitting there that they can start and run but will not move around the yard!
I have to break it down and ask absolute specific questions to even determine which belt is messed up and walk the mower will or won't do at that particular time.
The title of your thread says Briggs & Stratton engine won't turn over but it does seem that the engine is turning over and spitting just fine through all of the posts.
People often use different terms and phrases to describe what their engines are doing and it causes great confusion and hinders the diagnostic process.
If when you turn the key or pull the rope on a lawn mower engine and the engine spins but does not start, it is spinning, turning over etc.
It is simply not starting, taking off, getting up to full speed etc.
Now if you turn the key and nothing happens or it just clicks or barely moves etc, then it is not turning over.
People often use the term "catch" also which should not be used regarding lawn mower or engines at all.
There is no catching.
There is a starter Bendix or gear engaging the flywheel gear and turning it or attempting to turning it but this is never what they mean when they use the word catch. They mean the engine is spinning over or turning over and over typically but not taking off and running on its own.
Catch should not be used for this description.
This has always been an issue among people when describing what engines are doing and it causes great confusion and slows down the diagnostic process.
It gets even worse. I will have customers call me who have figured out they have some sort of belt issue on the rider.
They sometimes have no idea that there's more than one belt but even when they do, they will often refer to the wrong belt.
They repeatedly talk about the belt for the blades when they have a mower sitting there that they can start and run but will not move around the yard!
I have to break it down and ask absolute specific questions to even determine which belt is messed up and walk the mower will or won't do at that particular time.
Just a hint.
If you decide to have someone fix the spark plug thread hole on your old head you might consider telling them to also fix the stripped out valve cover bolt hole.
Another hint. You need to get a feel for when you are about to ruin threads, especially in aluminum. It's quite common to strip them long reach spark plugs threads when REMOVING a plug because the bottom few threads have hard carbon and when you just keep wrenching out the plug the carbon caked on the end of the plugs threads is removing the threads in the head. Gently work the plug bolt, stud or whatever back and forth even when removing from aluminum.
Another hint:
What I suggested about feeling for compression W/O even removing the spark plug is a fast test for really weak compression. With 35 lbs you could have spun the engine easily by hand manually. With 85 psi you would have FELT THE manual force required to bring the flywheel up to 85 psi pressure.
Another fast way to test for weak compression With Out a leak down tester:
If you had a source of air pressure at about 20-25 psi to inject into the spark plug hole with the piston held at TDC you would be able to hear where the air is leaking. Example: air out carb = intake valve, Air out muffler = exhaust valve. Air going into crankcase past the rings is normal.
The camshaft that TobyU mentioned is really wimpy and a weak link on that model engine, cam lobe lift wearing down, compression release breaking and the $35 China clone replacements are not worth the effort. BUT I did see that you measured the lift and it appeared ok comparing Intake to Exhaust and the engine was not compression locking when cranking which indicates the ACR is probably ok.
Another hint:
When you remove the valve cover check and make sure that a push rod is not bent. (You do not want to re-use bent push rods) Whichever rod is bent is a hint as to the bad valve guide, If a valve guide is bad and pushed into the head this sometimes causes a push rod to bend and the push rod can fall into the crankcase. Most likely using that head give you a hint as to some of the donor engines problems.
If the grass is growing while you are flogging that dog:
You might have to invest in one of them new BATTERY OPERATED Briggs engines. (or convert that one to electric only)
If you decide to have someone fix the spark plug thread hole on your old head you might consider telling them to also fix the stripped out valve cover bolt hole.
Another hint. You need to get a feel for when you are about to ruin threads, especially in aluminum. It's quite common to strip them long reach spark plugs threads when REMOVING a plug because the bottom few threads have hard carbon and when you just keep wrenching out the plug the carbon caked on the end of the plugs threads is removing the threads in the head. Gently work the plug bolt, stud or whatever back and forth even when removing from aluminum.
Another hint:
What I suggested about feeling for compression W/O even removing the spark plug is a fast test for really weak compression. With 35 lbs you could have spun the engine easily by hand manually. With 85 psi you would have FELT THE manual force required to bring the flywheel up to 85 psi pressure.
Another fast way to test for weak compression With Out a leak down tester:
If you had a source of air pressure at about 20-25 psi to inject into the spark plug hole with the piston held at TDC you would be able to hear where the air is leaking. Example: air out carb = intake valve, Air out muffler = exhaust valve. Air going into crankcase past the rings is normal.
The camshaft that TobyU mentioned is really wimpy and a weak link on that model engine, cam lobe lift wearing down, compression release breaking and the $35 China clone replacements are not worth the effort. BUT I did see that you measured the lift and it appeared ok comparing Intake to Exhaust and the engine was not compression locking when cranking which indicates the ACR is probably ok.
Another hint:
When you remove the valve cover check and make sure that a push rod is not bent. (You do not want to re-use bent push rods) Whichever rod is bent is a hint as to the bad valve guide, If a valve guide is bad and pushed into the head this sometimes causes a push rod to bend and the push rod can fall into the crankcase. Most likely using that head give you a hint as to some of the donor engines problems.
If the grass is growing while you are flogging that dog:
You might have to invest in one of them new BATTERY OPERATED Briggs engines. (or convert that one to electric only)
Hi Toby, somehow I missed this. I just found it in my email a little while ago and I will read it. I mean it's pretty exhaustive and I'm sure it's complete. The one question I have for you and this got back to the crankshaft or the cam shaft possibly being bent or damaged and I think it was a camshaft. Keep in mind I'm not a mechanic but what I just read about an hour ago was if you're rotating the flywheel and the push rod for the intake. As you rotate the flywheel it has a little bump where it's going out and then it goes back in and I'm talking about the rock around. I think the rocker arm or maybe it's the push rod. But either way it's called the bump that bump happens. So I'm wondering if that proves anything that something is not broken or bent or along those lines. And I'm getting that cylinder head the original one fixed at a machine shop for the spark plug issue. But thanks again. I will read this with fresh eyes, everything you wrote.
Jim
So I thought about what I just wrote and I have a better question is if you see where the rocker arm makes contact with a push rod and that bump definitely occurs. What can you conclude about the internal components of the engine? Can you say well if you have the bump then that means this is okay. If you have the bump that means that's okay. That's really what I'm asking?
Thanks ,
Jim
We call that the compression release bump or the ACR bump.
The camshaft has an automatic compression release built on to it very close to the intake lifter lobe.
At low RPMs it kicks a small little finger out which protrudes out past the base circle of the camshaft in the spot where it's fastened.
This little finger causes the lifter to bump out right after the intake valve closes as the engine turns.
Has the engine gets up above a couple of hundred RPMs , centripetal force overrides the pressure of the little spring that holds this finger out normally and then it no longer protrudes past the camshaft love at any position so the valve smoothly opens and closes without any bump.
This is why on many of these engines if you take the air filter out and start them you will hear a little muffled popping sound coming out of the intake when they're cranking and when they first start to spin up to speed running on their own. You will also hear this when you turn off the key if the air filter is off especially and as the engine coasts down before it stops spinning.
This is simply that little finger being pushed back out by the spring because the RPMs are slow enough and it's allowing some of the air to puff back out of the intake valve which blows out of the air filter.
Looking for this compression release bump is the way you can tell if the ACR is busted but it doesn't tell you anything about the valve timing from the cam.
A camshaft could have been installed incorrectly if you don't know the history of the engine without the dots lining up or more commonly, one or both of the lobes could have slipped and are no longer in their proper position in relation to the camshaft shaft and gear.
Typically if you have one of these problems you will either hear an abnormally funny noise coming from the middle or the rocker cover of the engine and you will also typically have some sort of popping or backfiring either out the exhaust or the intake that just doesn't sound right either.
That's because the piston is still going up and down like it's supposed to and maybe one of the valves is operating properly but the other one is either closing way too early or way too late and depending on how this works out, you're either going to get some combustion blowing out the exhaust or the intake or some sort of kickback.
These engines really aren't that complicated so there's got to be something that we're all missing and overlooking because we're not there to check it out in person.
This is why I'm hung up on a consistent and systematic troubleshooting approach every time.
It gets me to the root of the problem the quickest way possible.
If you have spark and the Piston is going up and down and you have compression that will blow your finger out of the spark plug hole and it doesn't suck your finger back in the hole...and you give it an external fuel source like some carb cleaner spray into the intake....THEN it should do something that sounds like combustion, backfire, popping, trying to run, etc.
Once you know that the flywheel key is not shared and you have all of the above things, about the only thing it can be is the timing messed up on the valve train from the camshaft somehow not opening the valves at the correct time.
I really can't think of anything else that could do it.
You might try spending the engine over by hand with the valve cover off until the valve starts to open or even starts to close and then put some extra pressure with your thumb and a rag as hard as you can pushing down on the pushrod side of the rocker arm.
I didn't actually test it but I'm pretty certain the one with the Chinese camshaft that was slipping would have made a clicky ratcheting sound and the rocker arm would have been able to be pushed and pushed the push rod down had I have tried this.
I know once I had the camshaft out in my hand, I could easily rotate at least one of the lobes around the shaft even though it was knurled and supposed to stay pressed on in one spot.