721 runs for 5 minutes then runs poor

BradBrad1

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I have a 721 grasshopper with a 52 inch deck. It runs great for the first 5-6 minutes of mowing then it sounds like it starts missing and looses power. It almost dies. I shut off deck and after a few second it winds back up but, not to full power as before. I can keep mowing to get the lawn mowed but, it just limps along the remaining time. I have ran it for 10 minutes with out deck going under no load and it runs fine. As soon as I put deck in motion 4-5 minutes later it will start missing even just sitting still under no load. I have replaced both air filters, fuel, oil, hyd filters. Also i am not sure why but, when I bought my mower it has a in line fuel filter on the return line back to the tank. I have replaced that also but, not sure why that is there? Any thoughts on why I am losing power after 5 minutes? things to look at? also do I need in line filter on return line?
 

bertsmobile1

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Unless it is a fuel injected engine there is no return line to he tank, but there is a tank vent that plugs into the manifold .
Your symptoms are concurrent with a cylinder dropping out , kill wire diodes failing , a coil failing or floating debris in the fuel tank.
The quickest way to diagnose which one is to pull the blower housing off and remove the kill wires from the coils .
The carb solenoid should be able to stop the engine but it is prudent to start the engine then check you can turn it off.
If all is good slip the cover back on and go mow.
Remember this is not a repair, just a diagnosis
Let us know what happens
You can also check the plugs, if one looks good & the other is fouled then the fouled side is a bad cylinder.
Again let us know what you find .
 

BradBrad1

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Sorry should have mentioned this is a diesel engine. Would the things you mentioned be the same for diesel as a gasoline engine? would it be an injector going bad?
 

bertsmobile1

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No
Check the return line is returning
Check thelift pump is actually getting fuel to the injector pump
\and in particular check the fuel filter
a small restriction can cause it to run out of fuel
 

tew45

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My fuel pickup line was partially stopped up. Cleaned it out and no other problems!
 

bertsmobile1

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Thanks for coming back & closing the thread.
Glad to hear it was a cheap fix
If you have not done it already , good idea to drain the tank then let it dry out completely.
Once dry with the fuel cap off thoroughly blow the tank out to get rid of all of the debris.
Dust gets sucked into the tank during use then you get grass clippings that fall in during refuling .
The fuel replaces the water in the clippings & then they become almost transparent
 
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