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The mower is a 2006 Simplicity Javelin RZT2044 with about 125 hours on it. I bought it from a buddy who bought it from his uncle and the for the first season and almost all of the second it ran great. It has a Briggs and Stratton 20 HP V-Twin model number 441577 - 0118B1 engine in it.

I have fought with this for a bit and have a lot of detail. When this first started to happen I had some help diagnosing it and thought it was a magneto. I bought one and then went on to diagnose which of the two was faulty. Pulled the plug on one side, still ran the same, pulled the plug on the other side, still ran the same..... Swapped out first one then the other magneto and there was no change.

I limped through the season with the mower working for about an hour then sputtering and coughing when under power and surging when just sitting still. I would shut it off for half an hour and it would start right up and work fine. The problem disappeared completely for the last three or four mows so I assumed it was bad gas or dirt in the system somewhere that had worked its way through. I had already cleaned everything and replaced the fuel filter but when the problem solved itself I assumed it was just something that worked itself loose.

Anyway this spring I mowed the lawn about a week ago and it went fine, the little bit of conditioned gas that was in it would have been all gone so was probably burning almost entirely brand new gas this week and for the first hour it ran fine then sputtering and coughing again. I limped it to the garage determined to diagnose it during the problem this time and tried the spark plugs again. No change to the way it ran with one or the other off. It was surging, the choke or governor or whatever you call it was going up and down steady along with the surging. Then I took out the air filter, no change. Then I clamped my hand over the air intake to force a vacuum it almost died out and when I removed my hand it smoothed out and ran fine, finished the lawn, about another 45 minutes with no problem......... I am thinking it must be an air intake issue, crack line or something but I can't see anything the matter. What would make the air intake being sealed for a second do to make it stop surging and run fine?

My buddies suggested it might be the fuel solenoid acting up but I can't see how cutting off airflow would fix an electrical problem which I would assume the solenoid would be..... Any advice or guesses welcome as I don't really have the cash right now to pay for a shop and the grass doesn't give a crap about covid.

Surging in a governed engine is always fuel : air ratio that is too low to sustain a constant speed .
So either air is entering the system after the fuel is added
or
There is not enough fuel being metered into the engine by the carburettor.

In the first case that will be caused by an air leak so you test for it by saturating the engine from the carb to the inlet with WD 40 or similar from a trigger pack with the engine running flat out
If there is an air leak the WD 40 will get sucked in, the engine will slow down & white smoke will come from the exhaust.
Some like to use propane or spray cans but too much of a chance of singed eyebrows for me to do that .
Putting your finger on the governor and accelerating and backing off the engine can help increase the manifold vacuum and highlight this problem

Once that is ruled out pull the carb off and clean it manually according to the instructions found on the outdoor power Equipment site
Remove the fuel line at the first item from the fuel tank
Drain the tnak and watch the flow
It should be smooth and continious
If you get spurts of full flow then not flow then full flow you know there is an obstruction in the fuel outlet
Check the drained fuel for water as that will cause surging randomly as water gets sloshed around & enters the carb.
Look very closely at the inside of the fuel line, rotate a phillips head screwdriver in it.
If it comes out with sticky rubbery muck on it the inner lining of the fuel line is gone, replace all of it tank to carb.

Remove solenoid, clip plunger, install fuel shut off valve. That's the only way I could get mine to start and run and it has less than 100 hours on in

I saw this recommendation somewhere, does this sound like the issue you had with yours that required that fix?

bertsmobile1 said:

Surging in a governed engine is always fuel : air ratio that is too low to sustain a constant speed .
So either air is entering the system after the fuel is added
or
There is not enough fuel being metered into the engine by the carburettor.

In the first case that will be caused by an air leak so you test for it by saturating the engine from the carb to the inlet with WD 40 or similar from a trigger pack with the engine running flat out
If there is an air leak the WD 40 will get sucked in, the engine will slow down & white smoke will come from the exhaust.
Some like to use propane or spray cans but too much of a chance of singed eyebrows for me to do that .
Putting your finger on the governor and accelerating and backing off the engine can help increase the manifold vacuum and highlight this problem

Once that is ruled out pull the carb off and clean it manually according to the instructions found on the outdoor power Equipment site
Remove the fuel line at the first item from the fuel tank
Drain the tnak and watch the flow
It should be smooth and continious
If you get spurts of full flow then not flow then full flow you know there is an obstruction in the fuel outlet
Check the drained fuel for water as that will cause surging randomly as water gets sloshed around & enters the carb.
Look very closely at the inside of the fuel line, rotate a phillips head screwdriver in it.
If it comes out with sticky rubbery muck on it the inner lining of the fuel line is gone, replace all of it tank to carb.

Click to expand...

This sounds like good advice. Does any of it makes sense given that it runs fine for an hour or more, and the problem gets fixed by starving the air flow for a second and then it runs fine again.....?

Yes.
It tells me that the fuel supply can just not quite keep up with demand .
Choked for a few seconds allows the fuel supply to catch up then it starts falling behind again .

hav you cleaned the carb according to the instructions linked to back in post # ?

If you are expecting a "Yes I know that, fold tab C it slot B then turn it upside down" type of an answer , don't hold your breath.
Fixing these types of problems requires a methodical approach , checking each bit as you go .
It could be a dozen things from a crack in the impulse line to the fuel pump, the fuel pump itsel ( very rare ) , a blocked tank breather, crap in the fuel lines , faulty fuel lines, crap in the carb, a sticky float needle , crap in the tank etc etc etc
So one at a time we need to check each & every one of them till the guilty party is found.

When you shove your hairy mitt over the intake, you reduce the air flow over the venturi which reduces the amount of fuel sucked out of the float bowl.
However the engine is still pumping fuel into it at the same rate so the fuel level rises back to the full mark.
From then on it slowly drops till there is not enough fuel in the float bowl to sustain a constant high speed .

Now if that came into my shop, I would fit a temporary gravity fed tank to the mower and go mow .
Normally I would start with it hooked up to the carb direct.
No problems then move it back to the fuel filter
Then back to the fuel pump .
If all of them showed no problem then I know it is in the tank or the lines from the tank to the pump.

If the problem remained with the temporary line on the carb then I would know the problem is in the carb.

It is a process of elimination.
Tedious & boring but very satisifing when finally isolated & fixed.

People by & large do not understand engines or combustion theory
You have a 500cc engine
It will run if anywhere from 100cc through to a full 500 cc of a combustable Air: Fuel mixture enters the cylinder,
The actual volume is not important, the ratio of fuel to air is
If your engine gulps 500cc of air but only gets enough fuel for 100 cc of air it will not fire
The ideal ratio is around 1:14 for modern unleaded and anything from 1: 12.5 throught to 1: 16 will go bang when the spark happens

bertsmobile1 said:

Yes.
It tells me that the fuel supply can just not quite keep up with demand .
Choked for a few seconds allows the fuel supply to catch up then it starts falling behind again .

hav you cleaned the carb according to the instructions linked to back in post # ?

If you are expecting a "Yes I know that, fold tab C it slot B then turn it upside down" type of an answer , don't hold your breath.
Fixing these types of problems requires a methodical approach , checking each bit as you go .
It could be a dozen things from a crack in the impulse line to the fuel pump, the fuel pump itsel ( very rare ) , a blocked tank breather, crap in the fuel lines , faulty fuel lines, crap in the carb, a sticky float needle , crap in the tank etc etc etc
So one at a time we need to check each & every one of them till the guilty party is found.

When you shove your hairy mitt over the intake, you reduce the air flow over the venturi which reduces the amount of fuel sucked out of the float bowl.
However the engine is still pumping fuel into it at the same rate so the fuel level rises back to the full mark.
From then on it slowly drops till there is not enough fuel in the float bowl to sustain a constant high speed .

Now if that came into my shop, I would fit a temporary gravity fed tank to the mower and go mow .
Normally I would start with it hooked up to the carb direct.
No problems then move it back to the fuel filter
Then back to the fuel pump .
If all of them showed no problem then I know it is in the tank or the lines from the tank to the pump.

If the problem remained with the temporary line on the carb then I would know the problem is in the carb.

It is a process of elimination.
Tedious & boring but very satisifing when finally isolated & fixed.

Click to expand...

Thanks very much for the reply. I am going to go at it thoroughly today. I was just checking to make sure the "fix" from my test was taken into account because it just baffles me how it could have solved the issue for another 45 minutes of runtime last mow.

Jpow said:

Remove solenoid, clip plunger, install fuel shut off valve. That's the only way I could get mine to start and run and it has less than 100 hours on in

Click to expand...

Mine just would not start; it was firing but just would not turn over. Clipping the plunger worked like a champ