My John Deere did the same. I found some kid had put things in gas tank that vibrated to stop up the gas intake in the tank. It would start fine and then just stop some time later 15- 20 minutes. Next time it would start ok.
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Quantum 5hp dies after about 20 minutes
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Export threadYou've swapped everything but the deck paint.
My guess was the idle screw is set wrong at 2900rpm and the gov-na' slash throttle spring/s are not working proper. Summation, carb linkages and springs are a foul. But you've swapped carbs and everything else fuel related.... Still sounds like a dirty fuel tank or collapsing fuel line. This might be dumb but the fuel tank is higher than the carb? You mentioned it's better on a full tank.
You've removed the ignition coil kill wire completely right? Engine should run till the tank is dry. Next guess is clean/sand/polish to bare metal, ignition coil mounting pads, same coil mounting area and mounting screws. Again you've swapped coils. Thinking block was getting hot and expanding slash moving around breaking ground from the ignition coil. Shot in the dark.....
Are the cylinder or head missing cooling fins? You said they are all clean. As in you can see clean aluminum all around the cylinder and head?
Do a compression and leak down if possible. Something is getting hot and out of tolerance. But it will run for 3 hours on flat ground.
Still sounds like fuel. Back to the basics, air, fuel, spark and compression.
What else, that has remained ON the engine that hasn't been swapped?
slomo
My guess was the idle screw is set wrong at 2900rpm and the gov-na' slash throttle spring/s are not working proper. Summation, carb linkages and springs are a foul. But you've swapped carbs and everything else fuel related.... Still sounds like a dirty fuel tank or collapsing fuel line. This might be dumb but the fuel tank is higher than the carb? You mentioned it's better on a full tank.
You've removed the ignition coil kill wire completely right? Engine should run till the tank is dry. Next guess is clean/sand/polish to bare metal, ignition coil mounting pads, same coil mounting area and mounting screws. Again you've swapped coils. Thinking block was getting hot and expanding slash moving around breaking ground from the ignition coil. Shot in the dark.....
Are the cylinder or head missing cooling fins? You said they are all clean. As in you can see clean aluminum all around the cylinder and head?
Do a compression and leak down if possible. Something is getting hot and out of tolerance. But it will run for 3 hours on flat ground.
Still sounds like fuel. Back to the basics, air, fuel, spark and compression.
What else, that has remained ON the engine that hasn't been swapped?
slomo
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Just reread this entire thread and something caught my eye. Your differences in valve clearances, hot vs. cold, are contributing to the problem. When they are hot they will not open enough to get the proper amount of fuel into the cylinder. That amount of difference is most likely caused by overheating. I’m assuming that you have made sure that the cooling fins are clean, so that air can circulate past the flywheel, over the cooling fins, and out either over the head or under the cylinder. Many, many years ago a student had an engine that would quit after it warmed up, where we found that the exhaust valve would actually stretch. We thought that the guide clearance was two small causing friction, so we rebored to specs. Ran a little longer but problem still occurred. We then replaced the valve and that solved the problem. Never saw this occur since, but I thought I would throw it out there. What do the other tech here think, possibility??? Or is my age showing??
I too noticed the gap issue, didn't add up. We don't know how long the cooling process was in between checks. 5-10 minutes could cause a couple thou I would guess being off????
Being that he's replaced everything else and the EX valve is cheap, sure, replace the little guy. I would change both while I was at it. Sounds like it runs good other than the unexplained shutdowns.
Honestly I was thinking something internal but didn't post for fear of getting trampled on by you real pros. LOL Rivets called it though, full credit to him. I've never seen it so didn't post about it.
slomo
Being that he's replaced everything else and the EX valve is cheap, sure, replace the little guy. I would change both while I was at it. Sounds like it runs good other than the unexplained shutdowns.
Honestly I was thinking something internal but didn't post for fear of getting trampled on by you real pros. LOL Rivets called it though, full credit to him. I've never seen it so didn't post about it.
slomo
Valve clearance changes between hot and cold are normal. On most engines, if set cold, they're set a bit loose because the gap will tighten up as things expand with heat. I'd only be worried if the clearance was zero, which would mean a valve wasn't able to close. That's not the case. Besides, it'll run through a full tank of fuel at about 3,200 rpm without any stalling, it only stalls when its mowing grass, or being pushed or pulled around.
The carb has been gone through 10 times now, plus I've since swapped it with the other mower, and the carb off this one ran fine on the other mower. As did all the other parts, including the fuel tank.
The fins are clear, the motor is clean top to bottom and its not missing any fins.
I did clean the coil posts and the coil is new, and has been swapped back and forth twice now.
It does seem to run hotter than the other mower just like it.
I check the compression with a thread in gauge cold and got 87 psi, and got 91 psi hot.
I checked the other mower for comparison, and got 79 psi cold, and 88 psi hot.
Each time I pulled the rope till the gauge stopped gaining compression with the carb wide open.
It definitely acts like a fuel problem but I've swapped everything fuel and spark related and the problem didn't change.
I pulled the head to check the headgasket and valves after it seemed to get hotter than I thought it should but there's no cylinder damage, the valves weren't too tight, and the head gasket wasn't blown.
The point where it always seems to act up first is along one side of the lawn. These properties are each slightly higher than the next. It seems to be when the mower is leaning forward a bit or when I stop it pushing it back and forth around obstacles. That made me think there was something in the tank or carb bowl, but I've been over that a dozen times now and the tank is clean, and the carb runs perfect on the other mower. Yet when it dies, after a handful or restarts and finally won't restart, it won't fire up on carb cleaner shot in the carb. But 10 minutes later it will start and die a few more times. But it won't be ready to run any length of time until its cold again.
Something I did try today was to swap out a completely different carb, off a similar engine but not its twin. That mower had a throttle cable and movable throttle plate. What I found was that it will not run, even when cold below wide open throttle. It shuts off like a switch below full throttle. The carb runs fine on the mower it came off of, also a 5hp Quantum. I put an inline spark tester on it and it stops firing below full throttle. The kill wire is still off the coil, in fact its removed from the mower completely, plus the blade brake assembly is completely removed as well.
The coil, flywheel, intake, gas tank, and spark plug are all from the other mower. The carb on it now is off a different mower as is the throttle bracket.
I then tried swapping the coil and flywheel off the third 5hp mower but got the same results, it will only make spark at full speed once it gets warm. Those same parts idle just fine on the mower I took them from?
The carb has been gone through 10 times now, plus I've since swapped it with the other mower, and the carb off this one ran fine on the other mower. As did all the other parts, including the fuel tank.
The fins are clear, the motor is clean top to bottom and its not missing any fins.
I did clean the coil posts and the coil is new, and has been swapped back and forth twice now.
It does seem to run hotter than the other mower just like it.
I check the compression with a thread in gauge cold and got 87 psi, and got 91 psi hot.
I checked the other mower for comparison, and got 79 psi cold, and 88 psi hot.
Each time I pulled the rope till the gauge stopped gaining compression with the carb wide open.
It definitely acts like a fuel problem but I've swapped everything fuel and spark related and the problem didn't change.
I pulled the head to check the headgasket and valves after it seemed to get hotter than I thought it should but there's no cylinder damage, the valves weren't too tight, and the head gasket wasn't blown.
The point where it always seems to act up first is along one side of the lawn. These properties are each slightly higher than the next. It seems to be when the mower is leaning forward a bit or when I stop it pushing it back and forth around obstacles. That made me think there was something in the tank or carb bowl, but I've been over that a dozen times now and the tank is clean, and the carb runs perfect on the other mower. Yet when it dies, after a handful or restarts and finally won't restart, it won't fire up on carb cleaner shot in the carb. But 10 minutes later it will start and die a few more times. But it won't be ready to run any length of time until its cold again.
Something I did try today was to swap out a completely different carb, off a similar engine but not its twin. That mower had a throttle cable and movable throttle plate. What I found was that it will not run, even when cold below wide open throttle. It shuts off like a switch below full throttle. The carb runs fine on the mower it came off of, also a 5hp Quantum. I put an inline spark tester on it and it stops firing below full throttle. The kill wire is still off the coil, in fact its removed from the mower completely, plus the blade brake assembly is completely removed as well.
The coil, flywheel, intake, gas tank, and spark plug are all from the other mower. The carb on it now is off a different mower as is the throttle bracket.
I then tried swapping the coil and flywheel off the third 5hp mower but got the same results, it will only make spark at full speed once it gets warm. Those same parts idle just fine on the mower I took them from?
You can do want you want, but that big of a difference in valve clearance is not right. It means that the amount of time fuel is allowed to enter the cylinder and exhaust gases to get out is vastly reduced. This means that less raw fuel will be allowed to enter the cylinder, causing the engine to die. Second, you must be very quick to measure hot clearance, as the engine cools down quickly, which it may be 000 when the engine quits. Just the opinion of an old fart.
EDIT!!!
I’m editing this post as I’m 90% wrong in what I posted. Hot clearances that small will allow the valves to stay open much longer than they should, not closed. The both the intake and exhaust valves will open too soon, plus stay open longer than they should. You may be getting more raw fuel into the cylinder, but you will lose it out the exhaust, due to a longer overlap time. This will result in less power. If you understand what is happening in the cylinder during each stroke, you’ll understand what I’m trying to say. Intake gases actually help to push out exhaust gases during valve overlap. Sorry for giving out bad info.
EDIT!!!
I’m editing this post as I’m 90% wrong in what I posted. Hot clearances that small will allow the valves to stay open much longer than they should, not closed. The both the intake and exhaust valves will open too soon, plus stay open longer than they should. You may be getting more raw fuel into the cylinder, but you will lose it out the exhaust, due to a longer overlap time. This will result in less power. If you understand what is happening in the cylinder during each stroke, you’ll understand what I’m trying to say. Intake gases actually help to push out exhaust gases during valve overlap. Sorry for giving out bad info.
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Valve/tappet clearance will not affect timing or air/fuel passage enough to matter so long as the clearance is not zero. At the very worst, it could cause excessive valve tap.
The Briggs and Stratton shop manual says the clearance should be .005 to .007 for the intake and .007 to .009" for the exhaust valve. A couple thousandths of an inch difference won't amount to enough to make it run or not run.
The only reason I checked it at all was to make sure the clearance hadn't shrunk down to zero which could hold a valve open on the compression stroke. Valve clearances will vary with engine temp as the aluminum block expands as the engine warms up. Since aluminum expands at a different rate than steel, the valve clearances will change with temperature. This is normal.
What's not normal is it losing spark below full throttle. The coil for some reason seems to stop making spark below about 2000 rpm.
At first I thought maybe the 'fixed throttle' engine's carb maybe didn't have a full idle circuit but it runs completely normal and will idle on the other mower with a throttle control. The issue has to be spark to at least some degree, but I can't see how movement of the mower would affect that.
If anything was greatly amiss inside the engine I'd have noise. The cam timing isn't changing or moving, the crank doesn't have excess end play, and the valves are not sticking or held open by 'too tight' valve clearances, and its got good compression.
It makes me think that maybe it won't restart after its hot because the recoil can't get it spinning fast enough to make spark once its hot. But none of that explains why it'll run through a tank of fuel if I just let it sit there on the trailer till it runs out of gas. It never misses a beat. However, if I walk up to it, and push it forward and and back real fast it sputters and gets unstable, it starts acting like its running out of gas and never recovers, it will die even if I leave it sit again. After that point it will restart over and over running a shorter amount of time each time until it will only start and stall. Yet if I left it sit untouched it would have run till the tank was empty.
Like I said before, my first impression was that there was a puddle of water or something on the bottom of the tank, or in the float bowl being sucked up when the momentum forward stopped suddenly. But I've changed the carb, fuel tank, fuel, fuel line and tank several times. I've got a half dozen fuel tanks on the bench here, four carbs, 7 coils, four flywheels, and a box of spark plugs, all of which ran fine on other engines.
If there was something loose enough inside the engine where momentum would make it shut down, I'd hear it, and it likely wouldn't sputter and carry on for a minute before it died.
Someone local said check the float level but the floats in these are fixed, there's no adjustment. They likely don't use enough fuel to require or be affected by a minor difference in fuel level in the fuel bowl.
On the good mower, if I blip the throttle a bit on top of the carb it'll rev the engine higher and return to the fixed RPM.
If I do that on this mower, it will not rev, it dies with any throttle movement It bogs and stalls as the governor tries to return the throttle to the correct speed. On the good mower, I can easily over rev the engine and it returns to the set speed instantly. This mower revs ever so slightly then shuts off as I let go of the throttle.
Keep in mind that this problem came on slowly a couple of years ago when I stopped using it. It started acting up only a bit and got very sensitive to what fuel I used. It wouldn't run on older gas, it got hard to start on month old gas while the other one would burn anything I dumped in the tank. over the course of a few weeks it went from being finicky about gas to stalling every 20 minutes then finally it got to where its at now. That's when I put it away and used a different mower. It didn't happen all at once, as if something broke, it came on slow and got worse till it got to where its at now.
When it happened, I really just thought the carb needed cleaning. Which is what I did first a few weeks ago. That got it running again but since that nothing I've done has changed the situation.
Also, the point when it shuts off the first time isn't really long enough for it to get 'hot', weren't talking three to five passes across a 60ft wide lawn and around a few bushes when it dies the first time. The good mower will run all day tank after tank without any issues.
The Briggs and Stratton shop manual says the clearance should be .005 to .007 for the intake and .007 to .009" for the exhaust valve. A couple thousandths of an inch difference won't amount to enough to make it run or not run.
The only reason I checked it at all was to make sure the clearance hadn't shrunk down to zero which could hold a valve open on the compression stroke. Valve clearances will vary with engine temp as the aluminum block expands as the engine warms up. Since aluminum expands at a different rate than steel, the valve clearances will change with temperature. This is normal.
What's not normal is it losing spark below full throttle. The coil for some reason seems to stop making spark below about 2000 rpm.
At first I thought maybe the 'fixed throttle' engine's carb maybe didn't have a full idle circuit but it runs completely normal and will idle on the other mower with a throttle control. The issue has to be spark to at least some degree, but I can't see how movement of the mower would affect that.
If anything was greatly amiss inside the engine I'd have noise. The cam timing isn't changing or moving, the crank doesn't have excess end play, and the valves are not sticking or held open by 'too tight' valve clearances, and its got good compression.
It makes me think that maybe it won't restart after its hot because the recoil can't get it spinning fast enough to make spark once its hot. But none of that explains why it'll run through a tank of fuel if I just let it sit there on the trailer till it runs out of gas. It never misses a beat. However, if I walk up to it, and push it forward and and back real fast it sputters and gets unstable, it starts acting like its running out of gas and never recovers, it will die even if I leave it sit again. After that point it will restart over and over running a shorter amount of time each time until it will only start and stall. Yet if I left it sit untouched it would have run till the tank was empty.
Like I said before, my first impression was that there was a puddle of water or something on the bottom of the tank, or in the float bowl being sucked up when the momentum forward stopped suddenly. But I've changed the carb, fuel tank, fuel, fuel line and tank several times. I've got a half dozen fuel tanks on the bench here, four carbs, 7 coils, four flywheels, and a box of spark plugs, all of which ran fine on other engines.
If there was something loose enough inside the engine where momentum would make it shut down, I'd hear it, and it likely wouldn't sputter and carry on for a minute before it died.
Someone local said check the float level but the floats in these are fixed, there's no adjustment. They likely don't use enough fuel to require or be affected by a minor difference in fuel level in the fuel bowl.
On the good mower, if I blip the throttle a bit on top of the carb it'll rev the engine higher and return to the fixed RPM.
If I do that on this mower, it will not rev, it dies with any throttle movement It bogs and stalls as the governor tries to return the throttle to the correct speed. On the good mower, I can easily over rev the engine and it returns to the set speed instantly. This mower revs ever so slightly then shuts off as I let go of the throttle.
Keep in mind that this problem came on slowly a couple of years ago when I stopped using it. It started acting up only a bit and got very sensitive to what fuel I used. It wouldn't run on older gas, it got hard to start on month old gas while the other one would burn anything I dumped in the tank. over the course of a few weeks it went from being finicky about gas to stalling every 20 minutes then finally it got to where its at now. That's when I put it away and used a different mower. It didn't happen all at once, as if something broke, it came on slow and got worse till it got to where its at now.
When it happened, I really just thought the carb needed cleaning. Which is what I did first a few weeks ago. That got it running again but since that nothing I've done has changed the situation.
Also, the point when it shuts off the first time isn't really long enough for it to get 'hot', weren't talking three to five passes across a 60ft wide lawn and around a few bushes when it dies the first time. The good mower will run all day tank after tank without any issues.
From your post I guess what I taught for 30 years was all wrong. You keep on making assumptions about the internal workings of the engine, which my experience has taught me will get me in trouble. I would rather inspect, test and verify before stating this or that is fine. That being said I’ll back out of this thread, as I’m not helping you solve the problem.
First off, I've also compared specs measured on the good motor to the one with the issue and both are nearly identical.
If there was any internal engine issues it wouldn't run just fine sitting on the trailer, I let it run through two tanks of fuel now like that.
It rained all week and the grass this week was extra tall, a bit overgrown and a bit wet but I had to get it done today. Not wanting to move the car out of the garage to get at the good mower, I grabbed the one with the problem and mowed with that one. I started in the back yard first, which has about half the grass to cut compared to the front yard. I was able to cut the whole back yard and got one row done in the front yard before it stalled the first time. Again, it was while pushing the mower fore and after vs. pushing it just forward. It still has no kill wire on the coil and the carb, flywheel, coil, and muffler is off the other mower.
When it stalled, I hit the primer bulb about 6 times pretty hard, it fired right back up and sputtered a bit but stayed alive. I did three more rows across the lawn and it started to pop and backfire but it kept cutting. It then started to sputter a bit and out of rage I jerked the thing off the ground and bounced it a few times on the ground with it still running, It sputtered a bit more then smoothed out. It then ran fine for the rest of the yard. I finished the whole yard, shut it down in front of the garage and tried to restart it and it would not restart. A spark tester showed good spark, the plug fired just fine, yet it wouldn't fire on carb cleaner shot down the carb throat. Not even a slight sputter. I let it sit for 7 hours and it started up just fine and has been running on the trailer for the last 30 minutes just fine. What ever is going on its affected by sudden movement or sudden stops.
If there was any internal engine issues it wouldn't run just fine sitting on the trailer, I let it run through two tanks of fuel now like that.
It rained all week and the grass this week was extra tall, a bit overgrown and a bit wet but I had to get it done today. Not wanting to move the car out of the garage to get at the good mower, I grabbed the one with the problem and mowed with that one. I started in the back yard first, which has about half the grass to cut compared to the front yard. I was able to cut the whole back yard and got one row done in the front yard before it stalled the first time. Again, it was while pushing the mower fore and after vs. pushing it just forward. It still has no kill wire on the coil and the carb, flywheel, coil, and muffler is off the other mower.
When it stalled, I hit the primer bulb about 6 times pretty hard, it fired right back up and sputtered a bit but stayed alive. I did three more rows across the lawn and it started to pop and backfire but it kept cutting. It then started to sputter a bit and out of rage I jerked the thing off the ground and bounced it a few times on the ground with it still running, It sputtered a bit more then smoothed out. It then ran fine for the rest of the yard. I finished the whole yard, shut it down in front of the garage and tried to restart it and it would not restart. A spark tester showed good spark, the plug fired just fine, yet it wouldn't fire on carb cleaner shot down the carb throat. Not even a slight sputter. I let it sit for 7 hours and it started up just fine and has been running on the trailer for the last 30 minutes just fine. What ever is going on its affected by sudden movement or sudden stops.