Quantum 5hp dies after about 20 minutes

[yardiron]

I've got a pair of Quantum 5hp mowers, both on 20" mulching decks.
The one will start and run fine for about 20 minutes, then it starts acting like its running out of fuel, then it dies.
If I pump the primer three times, it starts right back up and goes for another few minutes, it'll repeat that die/restart behavior as long as you care to keep restarting it.
I've had the carb apart, even swapped it with another identical engine. The fuel line is new, the tank is spotless clean, (Even swapped that with the other mower).
I eliminated the kill wire and switch, which is the only kill circuit its got.
I've even swapped the coil and flywheel from the other mower but it persists.
I went so far as to pulling the head off after it stalled for the 8th time the other day to make sure the valves weren't hanging up but they're seated tight at TDC.
Judging by the look of the bore, its got a lot of life left in it. When it runs, it runs fine and cuts fine, but once it starts stalling, it keeps it up over and over till it sits overnight.
The gas is fresh, I replaced, then swapped the gas cap with the other one, and no change. The other one runs perfect and will run all day but that one has a lot more use on it now.
This started about two years ago, and I put it away to deal with later. I dug it out the other day, and started trying to troubleshoot it but have gotten no where.
When it dies, it acts like its running out of fuel, it starts running bad, surges a bit, then shuts off. I have to prime it like a cold start to get it to restart, but it fires up on the second pull every time but dies after another few minutes, and I have to keep repeating the same thing. It acts like it stops feeding fuel after about 20 minutes, but after it stalls, the fuel bowl if full. I replaced the float, needle, and cleaned the carb but it didn't change a thing. Then swapping the carb and fuel tank should have proved out both parts, as they don't act up on the other mower and the other parts do the same on this one.
Cold the valves have about .005", hot they go to about .002". So they're not hanging open. Besides, if they were, it wouldn't start right back up so easy. The motor doesn't get 'tight' and its got good compression. I does seem to run hotter than the other one but not enough to say its overheating.

It runs too long to be a mixture issue or vacuum leak, it also runs too long for it to be heat related. I generally get 3/4 of the way done mowing the yard before it acts up. I've owned this one since new in 1990.
(Motor # 122702 3131-01 90022129). The second mower was bought used a few years ago as a back up, but its always puffed a bit of oil smoke and used some oil. Its likely got far more time on it then the one with the running issues.
Time wise it really acts like a tank not venting but I've both swapped the caps and left the cap loose as well with no change. And it dies faster the second time and each time its restarted after that. Filling the tank makes it margninally better after the first time it dies but not enough to say fuel level has much to do with it. A full tank would last me four cuttings in my yard.

 

[bertsmobile1]

Just about the only thing you have not swapped over is the long induction tube.
They tend to get cracks on the under side from heat ( thermo set plastic ).
Usually it will surge .
Some are alloy which do get hot & leak big time .

 

[yardiron]

I already swapped the intake tube with the good mower, it made no change. The other mower still runs fine and this one still dies after 20 minutes.
It definitely seems like its running out of fuel, hitting the primer button three times gets it started again every time but the run time is less with every restart, eventually getting to the point where it won't go more than a few feet without dying.
I thought at first there was something floating in the tank because it seemed to begin acting up when I stopped the mower when going forward. As if there were some water or something that was getting drawn in as I stopped the mower, and shaking the thing back and forth seemed to stop it from stalling when it began to act up but eventually nothing helps it. The tank has been off, washed out with fresh gas, and even swapped with the other mower but nothing changed.
The carb bowl shows nothing. I did check today when it stalled for the 11th time, if there was fuel in the bowl, and I found if I unscrewed the bottom jet, fuel flowed steady, so its not out of fuel, and the needle and seat are working. Yet it won't restart unless I pump the primer as if its a cold start every time. It only resets and runs fine if its cold or has sat for a few hours. Then it starts all over again. When it first starts, and is warmed up, it runs perfect, but only for about half the lawn, or about 10 80ft rows or so back and forth. It starts to act up the same way and at the same point every time.

This mower has a fixed throttle, the plate is bent and locked at full speed. The only control is the blade brake and primer bulb.
It never surges or runs rough, it just goes right into the stumble then stall behavior.
When it dies and gets really bad, after a dozen or so restarts, at the point where it will only start and stall, if I spray carb cleaner down the carb throat I can keep it alive, so its definitely not moving fuel when it happens, but the fuel bowl is full.

I pulled the intake and gave it a really good look and no cracks, the carb oring is fine, and the gas is fresh, and the same gas I run in the other mower. If it were dirt in the carb or tank, I doubt it would die in the exact same place or time every time I mow the lawn.
It begins at a point where I sort of push the mower down a slight grade at the end of a couple rows, when I push it down and stop, that's when it begins, and from there it progresses till it won't stay running.
The second mower will run all day, even with parts swapped off this mower.

 

[slomo]

Remove blower cover. Clean the engine block cooling fins all the way around the cylinder. Any dirt or oil, REMOVE it.

Spray some flammable wonder juice, looking for vacuum leaks around carb and intake pipe.

Either the coil is getting hot or the fuel is percolating like back in the old 350 Chevy days (not likely). Get an inline spark tester. Watch it at the 19 and 20 minute mark. See what you got for spark when it dies on you. Harbor Freight is your friend.

Air, fuel, spark and compression.

slomo

 

[Rivets]

Loosen the gas cap the next time it happens, leave it loose, and see if that solves the problem. If it does, vent on the cap is plugged, replace the cap.

 

[slomo]

Thought he did the gas cap trick? Maybe not for 20 minutes? Like Rivets said, leave the sucker wide open and see what you got.

slomo

 

[bertsmobile1]

If it stops dead like it has been turned off , then that is what is happening, it is turning off
Only a couple of things that can cause this
1) faulty coil
2) faulty kill switch
3) faulty kill wiring
4) faulty high tension lead
On fixed throttle machines with a flywheel brake I have found the switch to be he problem.
Usually the mounting actually gets hot then allows the switch to move just enough to shut down the engine.

The requirement for priming the carb could be a lot of things.
Some engines just require a prime hot or cold and some don't.
All depends upon the effiency of the idle circuit in the carb , float level , valve lash , piston ring wear etc etc etc

 

[yardiron]

I tried a few things on Sat. I tried running it with the cap loose, (had to put a bit of tape on the cap so it didn't vibrate off). No change, it died at the same point.
I bought a new coil, set the gap at .010" and gave that a try, no change, it died at the same point.
I've already swapped the intake pipe out and looked it over for cracks, I even wiped it down with RTV figuring if there were any cracks hiding they'd be sealed, or at least it would change something but it does the same thing.
When it dies, it acts like its running out of gas, it happens at the point where I mow one corner of the yard, where I'm pushing the mower back an forth to get to the edge of the lawn that borders the neighbors hedge row. Regardless of which way I mow, that section ends up at about the half way point time wise.
When it dies it starts surging a bit, then it sounds like its about to stall but it'll sputter along for a bit before it finally dies out completely. Sometimes if I stop, and shake the thing a bit it'll smooth out and keep going but never for more than a few seconds.
Loosening the cap, pushing the primer bulb, tilting or shaking it won't help once it gets to that point, it dies out and it'll take three or four pulls and 3 pumps of the primer to restart it. It'll restart and run fine for a couple more passes and repeat the same thing. The longest I've gotten it to keep it up is 11 restarts, then it won't run no matter what I do. By the time I push it back to the shed to check for spark or fuel, its already able to start on its own again. Since it starts back up, I doubt if its spark, (I have an inline spark tester but its hard to see on a bright sunny day, and if it were spark, its cured itself by the time I walk it back to the garage or shed.

If I push it till it won't stay restarted, spraying carb cleaner into the carb won't get it running. All it does is make the eventual restart harder and give me a ton of black smoke.
When it dies, there's no black smoke, so its not flooding out.
The kill wire is removed from the coil, it has been since this started. If I need to kill the engine I just pull the plug wire or wait a few minutes and it'll die out on its own.
I've swapped in the following new or 'known good' parts off another perfect running mower:

• Coil, tried two used one's and now a brand new one (New style p/n 590454)
• I drained and flushed the fuel tank, no change, so I swapped it with the other mower, still no change.
• Carburetor, cleaned it first, it didn't help, so I swapped it with the other mower.
• Intake tube, removed it, changed the gasket, then swapped it with another mower, then coated it just in case, no change
• New spark plug, then went back to a known good one from the other mower, no change. (Briggs 5095K, or Champion RJ19LM)
• New air cleaner,
• Checked and swapped out the gas cap, no change. (B&S p/n 691034)
• Checked the flywheel, swapped it with the other mower. Key way was fine.
• Checked valve clearance, valves are not open or 'tight' when it stalls.
• Fins are clean, its been apart 30 times now, but it does seem to be 'hotter' than the other mower when this happens and it stays hot longer.
• Oil is clean and fresh, (SAE 30 non detergent).
• New fuel hose, 1/4" ethanol resistant black fuel hose.
• Hot valve clearance is Exhaust - .003", Intake .004, cold is Exhaust .010" Intake .008"
• Compression feels the same hot or cold, the motor doesn't feel 'tight' and it don't rattle or knock when hot.

I've had the head off this thing, the cylinder looks like new, the head has minimal carbon on it, and the valve guides feel fine.
The exhaust valve looked like its been super hot, lots of blue heat swirls and a hardened white top surface. The intake was just black, the ports are clean. The muffler is clear, I've even swapped that and run it with a plain old tube type muffler just to see if opening up the exhaust helped anything, but it didn't. It just make it louder.

When its running and not acting up, its hard to kill, I can push it through heavy grass as fast as I want, once it acts up, it'll stall super easy.

Sunday morning I drained the fuel tank, drained the fuel bowl, went and got a fresh batch of gas, treated it with Sea Foam as I always do, and filled the tank. I started it up for the first time that day and let it run, (the throttle is fixed, no choice on the RPM), it was running at or around 2,900 rpm using a contact type tachometer
I let it sit there on the trailer and run till it ran out of gas, it ran fine for 3 hours and 4 minutes without any signs of stalling. I refilled the tank and tried to mow some grass, and it stalled in less than a minute, I'd restart it, and get another 30ft or so, it kept that up till it wouldn't restart again.
I put it back up on the trailer, tank still full, pumped the primer 5 or 6 times and restarted it, and let it run, it ran there with the blade brake tied back for another 2 hours and 10 minutes before running out of fuel.
Movement definitely affects it. It seems like it'll run fine if run with no load and no movement.
I refilled the tank, restarted it, and again it stalled even as I pushed it down the ramp off the trailer.
It did the same thing, once hot, if I move it or put a load on it it dies. If I let it sit, it runs for ever.
The fuel is fresh, I just had the fuel bowl off and rechecked the jet, and there's nothing floating around, the kill wire is not on the coil, and I even taped over the terminal just in case.

Meanwhile, its twin, an identical mower, runs fine with all of the original parts off this mower that I swapped from the bad one. The only thing I've not 'swapped' is the short block, deck, and shroud. At this point, the blade brake is removed completely, as is the switch as well.
I don't know if it matters or not, but something I did try with it running was to start it with the drill, run it without the shroud on it, and I poured water over the oil to see if it had a bad lead but I can turn the hose on it and it won't die when wet. I can wiggle and move the plug wire around and can't make it stall. From the start, I felt its running out of gas, it acts like its running out of gas or got water in the fuel when it acts up, but all that's been checked and proven. The carb is off the other mower, if I swap it back, the other mower still runs fine. I've gone so far to swap the coil, carb, intake pipe, tank, and spark plug all at the same time from the hot mower to the other mower and there's no change, once it acts up, it keeps it up till its cold again. Regardless of the parts bolted on it. When its cold, it starts with three pulls every time. I pump the bulb three times, pull the rope twice, it starts on the second pull, then stalls, I prime it three more times and its off and running. Both do the same thing and have since brand new. When the bad one is hot, it takes a three to five more pulls to get it going after it dies.
Once it sputters, there's no stopping it from eventually dying. I can't just stop, and let it run, its going to die no matter what. I tried just letting it run after the first restart, not moving it or putting a load on it but it dies just the same as if I was mowing grass. Yet if I let it run untouched from dead cold it'll run till the tank is empty every time.
I'm getting close to the point of just taking my shotgun to it and solving the problem once and for all but I'd really like to know what on earth is causing this before I blast it to bits or waste a couple good slugs on it.

 

[beerslayer12]

u know way more about this stuff then i but i have been watching ur post hoping to c a success story . not sure if u tried cyl comp. test cold vs stall temp ? did u rotate engine while hot with valve ( is it ohv ?) cover off ? maybe a spring is weak . i know it doesnt explain running well at idle but who knows ! good luck hope u solve it

 

[yardiron]

I haven't put a compression gauge on it but really didn't see the need since it doesn't feel like its lacking any compression.
I did check the valve clearance and guides, both hot and cold and there's nothing I see wrong or different from the other mower.
The clearances vary a lot with heat, but I'd say thats normal. The valve tappet clearance gets tighter when it warms up but it don't go to zero, so the valves are not hanging up. With the motor hot, and to the point it won't stay running, I zipped off the head and checked the valves and at TDC, both are seated tightly.

The thing that gets me is that it will run fine if left sit. There's no idle or full speed, the speed is fixed by the throttle bracket. There's no throttle, just a safety handle and blade brake, and the primer bulb. No other controls on this mower. I did try bending the tab up on these to add a manual throttle control but neither one will 'idle'. They shut off dead below full throttle. Its all or nothing. There's no kill wire or switch on the throttle plate, only on the blade brake, and that's now removed till I fix this. I have a similar mower, a slightly older 4.5hp Quantum that also runs fine, but that one has a throttle control and different primer bulb location. Its got a kill switch on the blade brake and throttle plate.

The two identical mowers are both older Troy Bilt mulching mowers with the rear bag option.
I've been trying to keep these going because I like how fine they mulch the grass, plus they're super light to push with handles suitable for my height. The third one has a real low handle set up on it and is super uncomfortable to use. The deck is also super flexible on that mower. These two Troy Bilt decks are very solid and they never rusted.

What I may do is just toss the Quantum motor and put an older style motor on there. I've got a good running 3.5hp I took off an old rusted out deck that will fit that deck. That one runs perfect.

I actually had a third one of those Troy Bilt mowers with the 5hp Quantum motor on it, that one died early on, it was mowing just fine and it suddenly lost power and died. When I tried to restart it I could hear something rattling inside. The piston is moving, so I figured it must of broke the came or governor inside. I just put it on the shelf and stuck a used Honda GX motor on it I had trash picked. It still runs but its at my other house, the Honda runs good but has cost me a dozen spark plugs and a couple of coils over the years.