Briggs & Stratton 20HP Intek Carb/Governor Issues

[bnwood]

Model: 407777 Type: 0124-E1

I have MTD mower with the above engine. It will not run smoothly at full throttle without 1/2 to 3/4 choke and of course is low on power. I got the mower used from friend and it was having these issues when he parked it. When I got it, I pulled the carb, cleaned it and installed a new kit. I also installed a new air filter, fuel filter, changed the oil, replaced the fuel pump and set the governor. It still ran the same, I used it most of the season like this. Recently I pulled it in for some deck and drive belt issues and decided to tackle the running issue. Below is a rundown of what I have done this time:

-Pulled the carb again and went through it again, no blocked passages, everything is clean, choke and throttle shafts are good (no excessive clearance) all new gaskets again. I did install the restrictor baffle this time just to see if it made a difference. (It didn't)
-I replaced the throttle/linkage bracket below the carb, the old one was pretty tweeked from my attempted adjustments before the last tear down
-Checked valves, all were out of spec. All were adjusted to .005" @ 1/4" Past TDC (initially I set them on TDC, not much difference)
-Compression check, both cylinders are @ 123-125 psi
-Leak down comes in around 8% on both cylinders
-New plugs
-I also removed the fuel filter and shutoff valve just to make sure that there was no fuel restriction.
-Checked for air leaks at the intake gaskets and carb to intake/intake spacer gaskets, all good.


Right now it will low idle, but you can still hear a slight fluctuation. Once you start increasing the throttle, it will start bouncing off the idle screw to full throttle constantly. I haven't bent any of the tangs on the new throttle bracket yet to set the low idle speeds and max speed, I will work on it this weekend. This is the same problem that I have always had, just a lot worse. Looking for thoughts on where to go next. I am going to remove the restrictor baffle and gasket this weekend. Also, does anyone know the max speed for this engine? The manual references another manual for the max speed, but I can't find that manual anywhere.

I have included a pick of the plugs after about 5 minutes of run time. I have a video of it running, I will try and get it posted or post a link to it soon.

 

[bertsmobile1]

Model: 407777 Type: 0124-E1

I have MTD mower with the above engine. It will not run smoothly at full throttle without 1/2 to 3/4 choke and of course is low on power. I got the mower used from friend and it was having these issues when he parked it. When I got it, I pulled the carb, cleaned it and installed a new kit. I also installed a new air filter, fuel filter, changed the oil, replaced the fuel pump and set the governor. It still ran the same, I used it most of the season like this. Recently I pulled it in for some deck and drive belt issues and decided to tackle the running issue. Below is a rundown of what I have done this time:

-Pulled the carb again and went through it again, no blocked passages, everything is clean, choke and throttle shafts are good (no excessive clearance) all new gaskets again. I did install the restrictor baffle this time just to see if it made a difference. (It didn't)
-I replaced the throttle/linkage bracket below the carb, the old one was pretty tweeked from my attempted adjustments before the last tear down
-Checked valves, all were out of spec. All were adjusted to .005" @ 1/4" Past TDC (initially I set them on TDC, not much difference)
-Compression check, both cylinders are @ 123-125 psi
-Leak down comes in around 8% on both cylinders
-New plugs
-I also removed the fuel filter and shutoff valve just to make sure that there was no fuel restriction.
-Checked for air leaks at the intake gaskets and carb to intake/intake spacer gaskets, all good.


Right now it will low idle, but you can still hear a slight fluctuation. Once you start increasing the throttle, it will start bouncing off the idle screw to full throttle constantly. I haven't bent any of the tangs on the new throttle bracket yet to set the low idle speeds and max speed, I will work on it this weekend. This is the same problem that I have always had, just a lot worse. Looking for thoughts on where to go next. I am going to remove the restrictor baffle and gasket this weekend. Also, does anyone know the max speed for this engine? The manual references another manual for the max speed, but I can't find that manual anywhere.

I have included a pick of the plugs after about 5 minutes of run time. I have a video of it running, I will try and get it posted or post a link to it soon.View attachment 27111

Hold the throttle open a fixed amount at the govenor linkage.
If it runs smoothly at a few different speeds then the problem is with the governor system, weak springs , worn linkages ets.
remove the air filter and slowly block off the air intake to manually choke the engine.
If the surging stops till you eventually start 8 stroking then you have a fuel supply restriction.
If neiher make much of a difference then you have an electrical problem .

For a fuel supply problem make up an alternate fuel tank ( pinch one off an old push mower ), suspend it above your mower and gravity feed it directly to the carbs.
Make sure you fit an inline tap to it.
You might not have enough supply to run full throttle but if the surging stops / reduces then reconnect to the inlet side of the fuel pump.
No difference or runs better the second time = crud in fuel lines or tank.
Big difference the second time ( runs worse ) = fuel pump problem

 

[bnwood]

Update:

Hold the throttle open a fixed amount at the govenor linkage.
-It will smooth out, but you can tell something isn't right, sounds like it is still struggling at most rpms
If it runs smoothly at a few different speeds then the problem is with the governor system, weak springs , worn linkages ets.
remove the air filter and slowly block off the air intake to manually choke the engine.
-Not much difference
If the surging stops till you eventually start 8 stroking then you have a fuel supply restriction.
If neiher make much of a difference then you have an electrical problem .
-I removed the grounding/kill wires from the coils, no change

For a fuel supply problem make up an alternate fuel tank ( pinch one off an old push mower ), suspend it above your mower and gravity feed it directly to the carbs.
Make sure you fit an inline tap to it.
-Pulled tank of mower and performed test with tank about 3' above carb with only a shutoff valve. I was able to get it to settle out around 3000 rpm, throttle lever was about 40%. With it running, it would backfire through the carb a every so often and eventually start to hunt again
You might not have enough supply to run full throttle but if the surging stops / reduces then reconnect to the inlet side of the fuel pump.
Same without pump, it would settle for awhile then start hunting
No difference or runs better the second time = crud in fuel lines or tank.
Tank is clear, line is new, pump is new genuine part
Big difference the second time ( runs worse ) = fuel pump problem

-I checked the valves again and they are fine.
-I verified the coil to flywheel gaps, both are 0.012"
-I performed the cylinder spark/balance test from the manual. I had a spark tester on both cylinders and saw no difference in the spark, when grounding each spark plug the rpm drop is less than 100 rpm


I am wondering if the fuel solenoid at the bottom of the carb is not staying in the run position? When I bench tested it initially, it didn't fluctuate then and the voltage to it is consistent.

I have ordered a crankcase breather assy as I tore the tube a little.

I ended up pulling the choke out about 50% and it will idle and run full throttle (about 3400 rpm unloaded). I tried to mow with it and once a load is applied to it, it will start hunting. It is a hydro static mower, if I went > half speed or the deck was engaged, it was the same. Playing with the choke would make it a little better, but it is still not right.

 

[bertsmobile1]

HAve a close look at the outlet from your fuel tank, leaves, grass clippings & dust accumulate there and form a plug.
A lot of tanks also hace moulding fins that protrude into the outlet hole which makes thigs even worse.

So to check the supply fom the tank properly in order to eliminate it you really have to use some form of substitute fuel tank.
At the very least use a long length of clear pvc tube syphioning fuel out of the filler tube into the carb / fuel pump.
A bit messy but can be done.
I have occasionally done this on big mowers that I an not physically get back to the workshop.

Now I have a fuel tank from an old dead Tecumseh 12 Hp that I use for this.
Before I was a mower mechanic I used an old juce bottle hangine from the cloths line when working on vintage motorcycles.

The fact that you can get it to run at full speed with the choke 1/2 on screams out air leak / low fuel supply

 

[bnwood]

The tank outlet is clear, I used a drill bit when I first got the mower and cleared out the hole and removed the plastic fins from manufacturing. Air leak is possible, I have ordered new intake gaskets. When they arrive I'm going to inspect the intake and spacer. The original fuel pump was metal and I could not locate a kit for it and bought a new style plastic one (part number superceded to it). I wonder if this may be an issue, it was a b&s part, not aftermarket.