Yard Works MTD

[Lloyd_E]

I have a Briggs twin 25hp mtd lawn tractor with 340 hours.
Oil changes are done every 50+ hours - new filter and oil.

Air filter is replaced each year and checked, cleaned with compressor if need be. I haven't replaced the plugs yet. It starts and runs fine and mows but recently after a few hours of mowing - in hot weather - it has started to run rough and back fire during operation. When I pull the choke out a bit the backfire goes away for awhile. New gas weekly. I run it 1/2 throttle when mowing, sometimes 3/4.

Any ideas?

Lloyd

PS: doesn't smoke on start up or during running time. Idles smooth.

 

[Scooby074]

Having to choke it usually means a fuel system problem.

Id bet dirty carb or plugged fuel filter. Could also be some debris in the tank getting sucked against the intake. Is the vent in your gas tank clear?

Pull the carb and soak it in dip. Run soft copper wire thru all holes. A visual inspection of the carb once the float is out would tell you if you really need to clean it, but Not a bad idea if it hasnt been done in a while.

Rebuild kit wouldnt hurt

Replace fuel filter. If ones not installed, install a new one as a preventative measure.

There have been issues with the coils and heat, causing spark issues, but i wouldnt look at this until i was sure the fuel system checks out. The fact that pulling out the choke smooths it out also points to other problems.

 

[jhwentworth]

If the motor smooths out when you pull the choke lever it's running lean. Probably some crap in the carb bowl. Drop the bowl and shoot some carb cleaner up the main jet. How does it run at full throttle?

BTW: Why do you mow at ½ throttle? Not a great idea; run the motor at full throttle.

 

[Bird]

Why do you mow at ｽ throttle? Not a great idea; run the motor at full throttle.

Yep, they're built to run at a constant speed at full throttle. Running it at less than that is sure not doing it any favors and in fact may account for it overheating, which in turn may account for part of the rough running problem.

 

[Welshman]

I agree with the above about not running it at part throttle - that's not doing the engine any good.

As to your problem - the fact that it runs fine for a couple of hours would lead me to believe it's a vapor lock problem, as opposed to a dirty or worn carb. The next time it starts to run badly, loosen the fuel cap. If it smoothes out within a minute or so, you've probably got a partially blocked vent (cap, or tank). Keep in mind that the carb bowl is also vented. Does it have a diaphragm operated fuel pump? If so, check the vacuum line and vent.

 

[jhwentworth]

As to your problem - the fact that it runs fine for a couple of hours would lead me to believe it's a vapor lock problem, as opposed to a dirty or worn carb. The next time it starts to run badly, loosen the fuel cap. If it smoothes out within a minute or so, you've probably got a partially blocked vent (cap, or tank). Keep in mind that the carb bowl is also vented. Does it have a diaphragm operated fuel pump? If so, check the vacuum line and vent.

I haven't seen a case where an engine would run for a couple hours then fail because of a plugged tank vent. If the tank were anywhere near full the problem will show up in a matter of 15-20 minutes. For most machines, 2 hours is a full tank of fuel burned. From what I've seen a blocked carb bowl vent will cause a rich condition and fuel leaking from the carb.
The OP should try running the engine at full throttle while mowing to see if he still has the same problem.

 

[Lloyd_E]

Thanks all,

I will try your suggestions today. Rain finally stopped - now I have lots of mowing to do.

Lloyd

 

[Lloyd_E]

Replaced fuel filter. Ran for about 2 hours no problems - idled smooth - ran smooth at fuel throttle. I let the tractor sit for awhile - had other things to do - restarted tractor to put back in building and it started back firing again at low idle. Increased throttle a bit to move tractor - same thing.

My next step is to flush carb...

lloyd

 

[Scooby074]

Like others have said, small engines like this like 3/4-full throttle. Is there any problems when running full?

Cleaning the carb certainly wont hurt anything.

 

[zman007007]

water in the bottom of the fuel bowl will also cause these symtoms. Drain the fuel bowl into something. If it has a drain screw use the top of a spray paint can to catch it, preferably a light colored top, so you can see if it has trash, dirt or water in it. zman