Briggs Vtwin giving me a fit

[RED-Z06 2,491]

I picked up a really clean, well maintained Z mower a couple weeks ago, 48" 18 or 20hp briggs V-twin, ive put 12 hours on it and it still has the same problem as the day i got it.  Runs smoothly, no smoke, no noises, plugs come out clean with clean ceramics.  Idles perfectly, great governor response, dips on deck engagement but recovers in 1 second so thats reasonable, my problem is...under any additional load, it gets choppy, and drops rpms.  

 

Ive replaced the carburetor, both coils, plugs, serviced it, verified fuel flow, verified valve lift on all 4 valves, checked valve lash, and done a leakdown test which it passed...it also runs evenly with either cylinder isolated.

 

Im stumped...nothing makes any difference, it seems completely healthy up until i get into like...3" of grass, the governor opens and it starts to stumble

[OutdoorEnvy 1,750]

You could verify what RPMs you are running at full throttle and if it's not maxed out adjust the high jet on the carb to make sure you're getting full power.  Or sometimes they list a peak torque rating as well at a certain RPM level.  Or you can ear adjust and raise the RPMS a little at a time until you get where it's handling the load.  Other than that maybe verifying the governor and spring etc. is all normal and not wore out or out of spec.  Verify the air filter is clean and maybe try a different brand one.  I've heard odd things with air filters where one brand is just a little more restrictive and that makes a lot of difference under load for certain engines.  Seems odd but it happens.   

 

Bogging down would mean it's either not getting enough of something or both between air and fuel when it needs more.  Or the compression is just low enough to not perform well.  But your leakdown test should have ruled that out. 

 

Other than that you get to mow twice or more often!  You get to pick!   

Edited July 28, 2022 by OutdoorEnvy

[RED-Z06 2,491]

Just now, OutdoorEnvy said:

You could verify what RPMs you are running and if it's not maxed out adjust the high jet on the carb to make sure you're getting full power.  Or sometimes they list a peak torque rating as well at a certain RPM level.  Or you can ear adjust and raise the RPMS a little at a time until you get where it's handling the load.  Other than that maybe verifying the governor and spring etc. is all normal and not wore out or out of spec.  Verify the air filter is clean and maybe try a different brand one.  I've heard odd things with air filters where one brand is just a little more restrictive and that makes a lot of difference under load for certain engines.  Seems odd but it happens.   

 

Bogging down would mean it's either not getting enough of something or both between air and fuel when it needs more.  Or the compression is just low enough to not perform well.  But your leakdown test should have ruled that out. 

 

Other than that you get to mow twice or more often!  You get to pick!   

3650 no load, 3560 with the deck on, pretty typical numbers on these, sadly no adjustments...fixed jets.

[OutdoorEnvy 1,750]

Just now, RED-Z06 said:

3650 no load, 3560 with the deck on, pretty typical numbers on these, sadly no adjustments...fixed jets.

 

Well shoot...these modern engines just get better all the time.  Well if you could find a cheap air filter like a Stens or something it might be worth a cheap experiment.  Or try a quick strip of mowing with no air filter on and see what it does.  That would probably confirm it for free if it handles it fine. 

[RED-Z06 2,491]

17 minutes ago, OutdoorEnvy said:

 

Well shoot...these modern engines just get better all the time.  Well if you could find a cheap air filter like a Stens or something it might be worth a cheap experiment.  Or try a quick strip of mowing with no air filter on and see what it does.  That would probably confirm it for free if it handles it fine. 

Yeah i wish we had some adjustment on them.  I put the deck in some water to simulate a high load and it all but stalls out, never dies but..so slow you can hear the compression release kicking, the meter shows 480hrs on it.  Filter out doesn't seem to make any noticeable differencem

[OutdoorEnvy 1,750]

hmm...I'm not a zero turn expert by any means but is there a clutch pulley that might be losing tension and slipping(worn out belt or other part?) or has a friction disc that could be worn down?  Other than something like that I'm running out of ideas...  It looks nice though

[RED-Z06 2,491]

20 minutes ago, OutdoorEnvy said:

hmm...I'm not a zero turn expert by any means but is there a clutch pulley that might be losing tension and slipping(worn out belt or other part?) or has a friction disc that could be worn down?  Other than something like that I'm running out of ideas...  It looks nice though

Its an electric clutch, no binding anywhere.  If i turn the deck on and butt the mower against something and add load with the tires..it boggs way down.  Im going to get some carb cleaner and spray around the manifold...i dont think its lean but at this point im out of ideas

[Rick3478 434]

I've seen some carbs with an "automatic choke" that works off vacuum, and when the throttle plate opens all the way there's not enough vacuum to keep the choke open.

[Handy Don 13,465]

Any pollution control plumbing that might not be up to snuff? Disconnected vacuum hose?

Throttle plate opening fully?

Choke opening (and staying open)?

My guess would also be "lean" and so not getting full "juice" when the engine is demanding it. You've already replaced the usual suspects, though, so it seems like it has to be something still untouched.Any way to load it while watching the governor action?

[RED-Z06 2,491]

22 minutes ago, Rick3478 said:

I've seen some carbs with an "automatic choke" that works off vacuum, and when the throttle plate opens all the way there's not enough vacuum to keep the choke open.

This is an older manual choke, its opening all the way, cant stand the auto-chokes

[RED-Z06 2,491]

10 minutes ago, Handy Don said:

Any pollution control plumbing that might not be up to snuff? Disconnected vacuum hose?

Throttle plate opening fully?

Choke opening (and staying open)?

My guess would also be "lean" and so not getting full "juice" when the engine is demanding it. You've already replaced the usual suspects, though, so it seems like it has to be something still untouched.Any way to load it while watching the governor action?

No emissions stuff on it, the throttle does flip to 100% on a load, i setup my camera and turned on the deck...it instantly went wide open.  Usually when these engines get any air leaks they surge, or if it's only on one cylinder you get a lazy engine that has a hollow pop sound as it misfires.  The plugs being "clean" says lean but the way it runs says normal.  

 

Im hoping the carb spray test reveals some air leak.  Its worth putting a $1000 new engine on..but id rather not

[wallfish 17,644]

Old gas will do that (and right from the pump too. I got 5 gallons of crap gas and checked everything else first. What a PITA that was! )

Bad plug wires

Deck spindles spin free? Like nothing binding them. String twisted up in there can bind them up

Just kinda throwing $#!t against the wall and think'n out loud

[RED-Z06 2,491]

42 minutes ago, wallfish said:

Old gas will do that (and right from the pump too. I got 5 gallons of crap gas and checked everything else first. What a PITA that was! )

Bad plug wires

Deck spindles spin free? Like nothing binding them. String twisted up in there can bind them up

Just kinda throwing $#!t against the wall and think'n out loud

Gas (should) be fine, i got 80 gallons in cans in April and treated it, I keep rotating 20 gallons up to the mowers so i never have less than 60 in case we get a hurricane, it runs in everything else but ill grab 5g of marine gas...at this point why not try it.  Plug wires new with the coils, spindles spin free, pulleys too. The deck itself really puts only a small load on it, get into 3" of grass and it struggles like its dropping a cylinder.

[gwest_ca-(File Mod) 11,284]

Try it with the muffler removed. May be restricted.

[RED-Z06 2,491]

I called my former boss and he said ive covered pretty much all the bases except slipped flywheel key causing an out of time situation. Id considered it but figured Iike he said...sheared keys on riding mowers is virtually unheard of because theres no direct drive. But ill check it in the morning...if not that ill see if it can be run no exhaust without blistering the frame 

[Handy Don 13,465]

8 minutes ago, RED-Z06 said:

except slipped flywheel key causing an out of time situation

Excellent thinking.

The drives are hydrostatic, but aren't the blades direct belt via the e-clutch?  I HAVE seen two sheared keys on v-shaft engines--one Cub refused to start and one Craftsman ran badly.

[RED-Z06 2,491]

Just now, Handy Don said:

Excellent thinking.

The drives are hydrostatic, but aren't the blades direct belt via the e-clutch?  I HAVE seen two sheared keys on v-shaft engines--one Cub refused to start and one Craftsman ran badly.

Hydros are belt and blades are belt off the clutch, conventional thinking is the belt or clutch would slip/break before that much shock was delivered.  But at this point...not alot left.  He admitted it would probably be the last thing he checked too...the huge tapered journal holds really well

[Bill D 2,130]

Is it possible the governor arm is slipping or has slipped on the shaft?  If the governor is improperly adjusted or can't respond properly when the engine is under load that could be causing your issues.   Are there multiple holes for the linkage or springs to go in on the governor arm or carb? Are they in the correct holes?  Do you have a manual for this engine?  Please post a picture of the engine tag.  Also, what is your compression pressure on each cylinder?  Even if the compression release kicks in you should see even numbers on each cylinder.  If you had say, 120 on one cylinder, but only 75 on the other that could explain your issues.  I also agree that the flywheel key could be partially sheared.  Muffler could be partially blocked as well.  Also, change the fuel pump and lines.   You said you had good fuel flow, but do you have enough fuel pressure?  That can make the difference when refilling the fuel bowl properly under full load.  If someone else that didn't know what they were doing was in this engine, you could be fighting multiple battles.

Edited July 29, 2022 by Bill D

[Rick3478 434]

How about increased vibration under heavy load causing float bowl to foam or overflow?  Trying to think a little outside the box...

[RED-Z06 2,491]

1 minute ago, Bill D said:

Is it possible the governor arm is slipping or has slipped on the shaft?  If the governor is improperly adjusted or can't respond properly when the engine is under load that could be causing your issues.   Are there multiple holes for the linkage or springs to go in on the governor arm or carb? Are they in the correct holes?  Do you have a manual for this engine?  Please post a picture of the engine tag.  Also, what is your compression pressure on each cylinder?  Even if the compression release kicks in you should see even numbers on each cylinder.  If you had say, 120 on one cylinder, but only 75 on the other that could explain your issues.  I also agree that the flywheel key could be partially sheared.  If someone else that didn't know what they were doing was in this engine, you could be fighting multiple battles.

Its a 405777 0140-e1, compression showed even, dont recall the numbers but i can retest.  Leakdown was just under 20% on both cylinders, cold.  The governor response is instant, as soon as a load comes on it pulls it wide open,  once open it rapidly compensates for the load it just runs like crap while it does it.

[Bill D 2,130]

1 minute ago, RED-Z06 said:

Its a 405777 0140-e1, compression showed even, dont recall the numbers but i can retest.  Leakdown was just under 20% on both cylinders, cold.  The governor response is instant, as soon as a load comes on it pulls it wide open,  once open it rapidly compensates for the load it just runs like crap while it does it.

I edited my post after you quoted me.  Do you have another fuel pump you can swap on it?  Just throwing ideas out there.

[Bill D 2,130]

Any gas smell in the oil?

[Tuneup 1,441]

If I over strain the 16 Onan - 48" deck and climbing a 30° grade with the hydro on full, it'll drain the bowl and want to stall. I'm thinking I shouldn't be climbing a 30° grade but maybe a weak pump or low float set. That said, putting it against a wall and trying forward like you did would allow a controlled squirt of Gumout into the throat to check on leaness, right? Other than the replacement carb might be Chinese and not a perfect fit but you noted the problem existed before the carb replacement.

[gwest_ca-(File Mod) 11,284]

This is the 100 page service manual

 

[RED-Z06 2,491]

8 hours ago, Bill D said:

I edited my post after you quoted me.  Do you have another fuel pump you can swap on it?  Just throwing ideas out there.

 I temp swapped on a new pump and it pumped at the same rate, ran the same.