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Engine 125K05-0186-E1 Hunting & Surging: What am I missing?

#1

tjdunlap

tjdunlap

Mower hunts & surges while idle as well as while mowing. I've replaced the following with genuine B&S parts (where appropriate):

  • new spark plug
  • fresh gas & oil
  • base primer #496116
  • filter pre-cleaner #493537S
  • filter #491588S
  • replaced carb #790845 (cleaned old carb first even used carb bushes; no change)
  • manifold intake #497465 (wasn't cracked, but figured a new one couldn't hurt)

My next step is to remove the head cylinder (and replace the head cylinder gasket #692249) to clean carbon deposits with a stiff wire brush. If that doesn't work, my next adventure would have me examine the values and valve springs.

What else should I be looking at? What am I missing?


#2

M

mechanic mark

Results

www.briggsandstratton.com www.briggsandstratton.com
Your engine manuals posted above


#3

tjdunlap

tjdunlap

mechanic mark said:

Results

www.briggsandstratton.com www.briggsandstratton.com
Your engine manuals posted above
Click to expand...
Thanks!


#4

cpurvis

cpurvis

Only the last two things on your bullet list are items which cause surging. That includes the 'next' steps you intend to take.

Surging is a fuel/air mixture or governor issue. If the engine surges under high idle conditions (full rpm with no load), the problem is a lean mixture, usually caused by improper adjustment or an air leak. If it has adjustment screws, try opening the high speed adjustment in small increments. If it doesn't have adjustment screws, try slowly closing the choke in increments to see if the surging goes away.

If the surging continues with load applied to engine, it can still be a carburetor problem but could also be a governor problem.

Did you replace the OEM carburetor with a cheap Chinese replacement? Those are a crap shoot. You might get lucky and you might not.


#5

B

Born2Mow

Agreed. Surging caused by a lean condition. Suggestions...
  • Adjust the main jet screw (if present)
  • Check for air leaks. (Loose carb mounting ? Aging manifold gasket ?)
  • Clean the carb. Leaving Ethanol fuels in the carb can coat the fixed size jets, making them run leaner.
  • Wallowed out butterfly pivots allowing air to enter in places other than air filter.
  • Check the flywheel key. Slight change in ignition timing could have same effect.
  • Faulty fuel pump not pumping enough fuel. (Aging diaphragm.)
  • Low float bowl fuel level due to float or float needle issues. Lower level = Leaner mixture.
  • If fuel pump is external... then replace the vacuum hose with SAE J30 spec fuel line. (Loss of vacuum = Loss of pumping = Lean condition)


#6

tjdunlap

tjdunlap

cpurvis said:
Only the last two things on your bullet list are items which cause surging. That includes the 'next' steps you intend to take.

Surging is a fuel/air mixture or governor issue. If the engine surges under high idle conditions (full rpm with no load), the problem is a lean mixture, usually caused by improper adjustment or an air leak. If it has adjustment screws, try opening the high speed adjustment in small increments. If it doesn't have adjustment screws, try slowly closing the choke in increments to see if the surging goes away.

If the surging continues with load applied to engine, it can still be a carburetor problem but could also be a governor problem.

Did you replace the OEM carburetor with a cheap Chinese replacement? Those are a crap shoot. You might get lucky and you might not.
Click to expand...
Thanks!

There is no adjustment on this specific engine. It's this carburetor: Briggs & Stratton 799868 (https://www.amazon.com/gp/product/B007JV701U/). And I replaced the manifold intake with: Briggs & Stratton 794305 (https://www.amazon.com/gp/product/B00524MSDQ). Both replaced with actual Briggs & Stratton parts.

I was able to hold the throttle shaft in one spot. The surging stopped. So I assumed it could be the governor causing the issue. I adjusted the governor control lever (in both directions mind you!). In one position, the engine raced at high speed like it was ready for flight! No surging by the way. Adjusting the governor control level to the opposite extreme brought it back to a normal RPM, but surging was still present.

I then attempted to make small adjustments but even the smallest adjustment made the engine race at an extreme RPM.

Does all that tell you anything?


#7

tjdunlap

tjdunlap

Born2Mow said:
Agreed. Surging caused by a lean condition. Suggestions...
  • Adjust the main jet screw (if present)
  • Check for air leaks. (Loose carb mounting ? Aging manifold gasket ?)
  • Clean the carb. Leaving Ethanol fuels in the carb can coat the fixed size jets, making them run leaner.
  • Wallowed out butterfly pivots allowing air to enter in places other than air filter.
  • Check the flywheel key. Slight change in ignition timing could have same effect.
  • Faulty fuel pump not pumping enough fuel. (Aging diaphragm.)
  • Low float bowl fuel level due to float or float needle issues. Lower level = Leaner mixture.
  • If fuel pump is external... then replace the vacuum hose with SAE J30 spec fuel line. (Loss of vacuum = Loss of pumping = Lean condition)
Click to expand...
Thanks! It's this engine: 125K05-0186-E1 (https://www.ereplacementparts.com/b...-engine-parts-c-16758_17347_17495_227482.html)

There is no main jet screw (unless I'm clueless and can't find it!)
Installed new OEM carb & gaskets
Installed new OEM manifold & gaskets
Installed new breather assembly gasket
Cleaned piston & values (removed carbon deposits), replaced gaskets
No fuel pump (again, unless I'm clueless and can't find it!)

The only thing I have not looked into is the flywheel key you mentioned.


#8

B

Born2Mow

Thanks for the parts visual.
• No screw on your model, that was a 60's & 70's thing.
• No fuel pump, your model is gravity-fed. That makes it very important to check flow, because there is NO pressure.

When you unplug the fuel line from the carb, do you get a "drip, drip", or a solid stream of fuel ? Modern fuels will eat your fuel lines out from the inside, causing a blockage inside a perfectly good looking fuel line.


#9

tjdunlap

tjdunlap

Born2Mow said:
Thanks for the parts visual.
• No screw on your model, that was a 60's & 70's thing.
• No fuel pump, your model is gravity-fed. That makes it very important to check flow, because there is NO pressure.

When you unplug the fuel line from the carb, do you get a "drip, drip", or a solid stream of fuel ? Modern fuels will eat your fuel lines out from the inside, causing a blockage inside a perfectly good looking fuel line.
Click to expand...
I tested that today. I release the clamp on the line and it shot out a solid stream. I think it's good.

Could the governor spring be the issue? How do I know I have a bad spring? That's one component I have NOT replaced.... yet! :)


#10

B

Born2Mow

I'd believe the governor simply came out of adjustment before I'd believe a spring went "bad".


#11

B

Born2Mow

Just saw this. Watch and see how a mass of grass clippings could jam the governor....



#12

S

slomo

tjdunlap said:
I was able to hold the throttle shaft in one spot. The surging stopped. So I assumed it could be the governor causing the issue.
Click to expand...
Surging is either a rich or lean condition. Read your plug for more clues.

Most of the time a dirty carb is the culprit.

Might want to clean the fuel tank out and verify fuel flow AT the carb.

Clean your cylinder cooling fins yearly. It's in your engine manual. Neglect this and you are looking at future engine damage.

slomo


#13

S

slomo

Spray carb cleaner around where the carb and intake manifold mount to the engine. If it smooths out you found your vacuum leak.

Don't over torque those new parts you put on. All you need is a gentle snug. You might of warped something causing vacuum leaks.

NO impact guns are needed. Leave those for blade removals.

slomo


#14

B

bertsmobile1

slomo said:
Spray carb cleaner around where the carb and intake manifold mount to the engine. If it smooths out you found your vacuum leak.

Don't over those new parts you put on. All you need is a gentle snug. You might of warped something causing vacuum leaks.

NO impact guns are needed. Leave those for blade removals.

slomo
Click to expand...
NEver liked the idea of trying to find an induction leak with a flammiable gas / liquid.
Too much of a chance of it igniting while your nose is in there looking closely at what you are doing.
The USA is the only place on the planet whee mechanics get taught to use propane to check for manifold leaks.

Usually for safety reason I would use WD 40 or similar from trigger pack ( because spray can propellants also burn ) as you can see exactly where it is going and if sucked in generates a lot of white smoke but no engine damage.


#15

S

slomo

bertsmobile1 said:
NEver liked the idea of trying to find an induction leak with a flammiable gas / liquid.
Too much of a chance of it igniting while your nose is in there looking closely at what you are doing.
The USA is the only place on the planet whee mechanics get taught to use propane to check for manifold leaks.

Usually for safety reason I would use WD 40 or similar from trigger pack ( because spray can propellants also burn ) as you can see exactly where it is going and if sucked in generates a lot of white smoke but no engine damage.
Click to expand...
Okay, great tip as usual Bert. Call me a converted WD-40 vacuum leak hunter LOL.

slomo


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