John Deere lx288, 18hp briggs, engine boggs down

[usagun]

Hello, I am original owner of a John Deere mower, Briggs twin cylinder model 350777 1137-E1. The machine has 650 hours on it. This season the engine has offered challenges in that when I start off mowing, cold engine it will run well for about an hour then on occasion will start sputtering emitting black smoke from the exhaust and after a period of this nonscience will pick up and rpm and start running again. Most of the time.
To date: Only high-octane non-ethanol gas ever used in machine. New fuel filter. New plugs of correct heat range. Oil change, new air filter. Replaced ignition coil on left cylinder wire end burnt due to loose connection. Inspected plug wire on opposite side. Ok.
Checked valves for proper clearance, both cylinders. Replace fuel pump last fall due to leaking fuel. The carburetor has been replaced with new aftermarket carb from Amazon twice now, same results.
The problem occurs when engine hot, sometimes when pto is engaged, sometimes when on slight incline, and other times setting level at high rpms.

I pulled and checked both spark plugs yesterday, photos of plugs, (these were brand new plugs), about 3 hours run time on them. (Note: the #1 plug has thread insert on it, came out when I pulled plug for inspection. Gotta re-Loctite it.

Any ideas appreciated. Thanks.

 

[Auto Doc's]

Hello U.,

This problem suggests to me that the fuel tank need to be emptied, removed and cleaned out well. Along with that all of the fuel hoses replaced.

With use and age, dust, grass clippings and even bugs find their way into mower fuel tanks. This stuff lays in the bottom of the tank and eventually gets drawn into the lines. Grass clippings will form a loose "ball" of material inside the fuel lines especially where any bends exist.

It takes a bit of work, but the fuel tank and lines are where a vast majority of running problems like this come from.

 

[usagun]

Hello U.,

This problem suggests to me that the fuel tank need to be emptied, removed and cleaned out well. Along with that all of the fuel hoses replaced.

With use and age, dust, grass clippings and even bugs find their way into mower fuel tanks. This stuff lays in the bottom of the tank and eventually gets drawn into the lines. Grass clippings will form a loose "ball" of material inside the fuel lines especially where any bends exist.

It takes a bit of work, but the fuel tank and lines are where a vast majority of running problems like this come from.

Will investigate and for sure look into cleaning tank. Fuel filter(s) were replaced last fall when new fuel pump was installed and again during this last trouble shooting exercise. I have a new fuel pump that I'm going to install as also. Trying my best to nail down when this started last fall. What changed???, used mower for getting up leaves and pine needles when fuel pump started leaking.

 

[Auto Doc's]

Fuel filters are only good for small dust particles, but the chaff debris gets caught in the fuel line long before making it to the filter. Fuel lines also get dry and rigid with age and collapse the inner liner of them which will restrict fuel flow, especially under heavy engine loads.

B&S fuel pumps on this engine are a vacuum pulse pump, and they do fail from straining too much due to a line restriction.

 

[sgkent]

the plug that has the heli-coil or timesert - you may have been losing your plug ground with that if it was loose and soot was filling in around the threads. Aluminum expands more than steel. Not saying that is it but I would clean the threads well and put it back in - see if it changes anything. Might start looking for a good replacement head for it too. Usually once they start coming out, they keep coming out. Might also run a little Heet or something like that in the fuel to remove any water, unless you completely drain the tank and bowl, and refill with fresh fuel.

 

[usagun]

the plug that has the heli-coil or timesert - you may have been losing your plug ground with that if it was loose and soot was filling in around the threads. Aluminum expands more than steel. Not saying that is it but I would clean the threads well and put it back in - see if it changes anything. Might start looking for a good replacement head for it too. Usually once they start coming out, they keep coming out. Might also run a little Heet or something like that in the fuel to remove any water, unless you completely drain the tank and bowl, and refill with fresh fuel.

Thanks, that's one I did not consider, will look closely at it.

 

[sgkent]

my experience is that only rarely does a carb become "intermittent." A piece of debris can float around but usually that will get cleaned out if the carb is rebuilt. Electrical can be "intermittent". That would include anywhere there is a wiring connection, and switches. Tight valves can put an engine on edge, but it is not intermittent in the usual sense - unless a tight valve expands under heavy load and heat, causing a misfire. A bad fuel filter or tank sock can reduce the flow of fuel so that the bowl drains under heavy load and the engine then dies or misfires. Pay very close attention to the conditions it misbehaves.

 

[slomo]

Might also run a little Heet or something like that in the fuel to remove any water, unless you completely drain the tank and bowl, and refill with fresh fuel.

Just drain the entire fuel system. He's not working on a container ship here.

 

[slomo]

the plug that has the heli-coil or timesert - you may have been losing your plug ground with that if it was loose and soot was filling in around the threads. Aluminum expands more than steel. Not saying that is it but I would clean the threads well and put it back in - see if it changes anything. Might start looking for a good replacement head for it too. Usually once they start coming out, they keep coming out. Might also run a little Heet or something like that in the fuel to remove any water, unless you completely drain the tank and bowl, and refill with fresh fuel.

NICE JOB sgkent!!

The plug on the right is the offender. Look at all the carbon tracking trying to ground out to the base ring of the plug near the threads. Look at the porcelain on both plugs.

Notice the left plug looks normal.

Recommend making a ground wire for THAT one plug that has the insert. Wrap bare copper around the hex head of the plug clamping with a stainless hose clamp. Other end of this wire to the engine block. Spark is looking for a ground.

Again nice catch sgkent.

 

[usagun]

Just drain the entire fuel system. He's not working on a container ship here.

Yes indeed. Matter of fact I just had entire engine off machine to split upper and lower crankcase due to blown/leaking crankcase gasket that required replacement. Engine clean as new! Thanks!