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Riding mower bogging down after using old fuel


triumph74's Avatar
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10-16-09, 09:59 PM   #1  
Riding mower bogging down after using old fuel

I have a 2004 Husquvarna YTH2148 with a 21 HP Briggs and Stratton ELS 656 V-twin engine that ran problem free for roughly 200 hours. Last weekend I found the tank empty and was too lazy to drive into town to fill up a gas can, so I siphoned some out of a project car that was probably 2 years old. I ran the mower for around 2 hours including through some very high and heavy grass and everything was normal, but a couple of days later it was hard to start and when I did get it started it bogged down whenever I engaged the blades, and when I disengaged the blades it took several seconds to get back to normal and coughed black smoke during the recovery, where it usually is an almost instant response. I drained the old fuel and replaced it with new, cleaned the (sooty) plugs, replaced the fuel filter, cleaned the mower deck, replaced the blades and checked that all were spinning freely and still no improvement. I checked all the linkages and gave it several sprays of carb cleaner and still no improvement. Finally I gave the crankcase pressure driven fuel pump a cosmetic cleaning and a bit of a shake and when I replaced it the engine seemd to have recovered some but not all of it's performance and it still bogs down and doesn't self recover if I go into thicker grass. My next step is probably a carb teardown and cleaning, but I'd prefer to do that over the winter. Any thoughts on other things to try short of that?

 
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10-17-09, 01:41 AM   #2  
Sounds a bit like the carb or one cylinder not firing. I'd say start by replacing the plugs.


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10-17-09, 07:17 AM   #3  
Plugs replaced but the issue remains. How would I tell if a cylinder wasn't firing?

I think they are both firing since it runs smoothly at idle and all the way up to max throttle on a slow accel with and without the blades engaged (roughly a minute to gradually move the throttle from idle to full) as well as slow and fast decels from full to idle, but any sudden change in throttle from about midpower up stumbles and takes a few seconds (and some puffs of black smoke) to recover; longer if the load is engaged. On grass sometimes stopipng forward progress lets it recover, other times I have to cycle the blade engagement to clear it. It seems to be an issue regardless of where the command to increase fuel flow comes from (throttle or governer).

Thanks

 
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10-17-09, 08:07 AM   #4  
With the engine running and using and insulated tool, remove each spark plug wire one at a time. If the engine dies with one wire removed, the opposite plug is not firing.
Sounds to me like you have introduced some debris in the fuel system and has likely made it to the carb. Either from an empty tank or contaminated fuel. if both of the old plugs looked similar when you changed them, I would suspect the carb needing cleaned.

 
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10-20-09, 05:26 PM   #5  
Thanks for the idea on the cylinder not firing, that seems to be the case. Right plug off - engine runs fine. Left plug off - engine dies. A bit more troubleshooting revealed that at similar gaps between the plug and the plug wire the left plug had a strong blue spark but the right plug had a weak orange spark. I'm guessing that the issue could be a weak coil?

How (or if) that is related to using old fuel remains another question, but having a non-firing cylinder certainly doesn't help!

 
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10-21-09, 01:53 AM   #6  
Pull the plugs and verify spark at the plug tip.


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11-05-09, 05:19 PM   #7  
Troubleshooting continues... after a bit more wandering down the electrical route didn't really help matters. Ran a compression check on both cylinders. Left cylinder - consistantly around 150 - 170 psi. Right cylinder - sometimes 150 psi, sometimes nothing. It seemed that if the left cylinder was checked and stopped on a suction portion of the stroke, then the right checked, the right had the same compression as the left. If the right was checked again then nothing.

Now I'm starting to think there is a valve issue on the right cylinder, but not sure what would cause it to check fine sometimes and not others besides user error on the check, but I'm pretty sure that isn't the case.

 
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11-06-09, 12:38 AM   #8  
Sounds like a valve seat came loose. Sometimes it will fit in place and you'll have compression, other times it will fall between the valve face and the head, holding the valve open, causing no compression. Head removal is required, and usually the seat can be peened back into place.


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11-06-09, 08:10 AM   #9  
I think it is time to post the engine model, type and code number.

I suspect the engine is an OHV and the problem began as valves too loose and has progressed to valve push rod coming loose and possible cylinder head damage.

 
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11-13-09, 07:39 PM   #10  
Problem found: a frozen exhaust valve. After taking the valve cover off a bent pushrod was visable, looking for what caused it to bend led to finding the valve frozen.

 
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11-14-09, 06:08 AM   #11  
Use of old fuel, varnish.

 
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11-14-09, 10:30 AM   #12  
I agree, old fuel causes valves to stick.


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