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

[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?

 

[mechanic mark]

Results

www.briggsandstratton.com

Your engine manuals posted above

https://www.google.com/search?q=B%26S+ENGINE+TROUBLESHOOTING&oq=B%26S+ENGINE+TROUBLESHOOTING&aqs=chrome..69i57j0i390l4.27146j0j4&sourceid=chrome&ie=UTF-8

 

[tjdunlap]

Results

www.briggsandstratton.com

Your engine manuals posted above

https://www.google.com/search?q=B%26S+ENGINE+TROUBLESHOOTING&oq=B%26S+ENGINE+TROUBLESHOOTING&aqs=chrome..69i57j0i390l4.27146j0j4&sourceid=chrome&ie=UTF-8

Thanks!

 

[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.

 

[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)

 

[tjdunlap]

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.

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?

 

[tjdunlap]

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)

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.

 

[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.

 

[tjdunlap]

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.

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!

 

[Born2Mow]

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