Briggs 12H802 Surge, don't think it is carb...

[pylek]

Yeah, David, that is confusing me as well.
Today, after I wake up, I am going to put on yet another carb and see what happens.

The foam seal is just to keep dust out. I have plenty more, if necessary.

Seth K. Pyle

[David]

I have been known to put a drop of SAE 20 electric motor oil or of graphite lock fluid at such locations on my menagerie of old mowers.. I don't know if that is supposed to be done,might encourage dirtiness but then again it might act like a miniature Oil Foam filter or discourage wear a little bit? (Dirt on the outside better than dirt going in past the shaft?)
I don't think those junctions are very tight anyway. Perhaps a drop of thick oil would temporarily stop some of the air from leaking in and smooth out the engine a little bit if enough air is getting in there to upset the air/fuel ratio.
If the engine had a lot of carbon under the head would that make it more sensitive to lean mixture?

[pylek]

That makes sense David, I had thought of something along those lines. If this were mine, I'd do that and call it good. However, this is for a customer(friend, but still a customer). This has to continue to work. I would be concerned that the heavy oil would eventually all be drawn past the throttle shaft.

A few weeks ago, I pulled the head and valves for cleaning, lapping, carbon removal, and setting of valve clearances. I cleaned most of the carbon from the piston crown and combustion chamber in the head.

I put a different carburetor on today. All same. Still surged, still bogged when brake cleaner was sprayed directly at the throttle shaft opening in the top of the carb.

Hmmm...

Seth K. Pyle

[bgsengine]

That makes sense David, I had thought of something along those lines. If this were mine, I'd do that and call it good. However, this is for a customer(friend, but still a customer). This has to continue to work. I would be concerned that the heavy oil would eventually all be drawn past the throttle shaft.

A few weeks ago, I pulled the head and valves for cleaning, lapping, carbon removal, and setting of valve clearances. I cleaned most of the carbon from the piston crown and combustion chamber in the head.

I put a different carburetor on today. All same. Still surged, still bogged when brake cleaner was sprayed directly at the throttle shaft opening in the top of the carb.

Hmmm...

Seth K. Pyle

what happens if you hold throttle lever steady and keep it from moving?

could be a governor problem. If the intake pipe is plastic, it could have a crack in it, if it is steel, they do tend to separate from the "elbow" - remove carburetor and see if intake pipe will wiggle.

[pylek]

If I hold the throttle lever steady, the engine runs smoothly. I have to hold it just right for it to run smoothly, otherwise it still surges a little.

When I pulled the head and valves, I inspected the intake pipe. It is plastic. I found no cracks or such, even under 12x magnification. When I reinstalled the intake pipe, I used a new gasket, and torqued the screws to the general specification. While running, I have hosed the intake pipe with brake cleaner on the exposed part, at the carb to intake pipe to carb mounting bracket junction. I have sprayed brake cleaner along the intake pipe along its length, up under the blower housing. I have even sprayed brake cleaner into the starte recoil guard openings to see if it would get into the airflow and sucked in at any leak of the intake pipe to cylinder block.

At no time did the engine respond to the brake cleaner. The only time it did, was when I sprayed brake cleaner directly at the throttle shaft where it meets the carb body, on the original carb, and a different one today.

I have an old 92358 3.5hp that has the cast body/steel tube intake pipe, and had to replace the pipe due to the wiggling you mentioned.


Thanks!

Seth K. Pyle

[pylek]

I think my next step is to pull the engine and remove the sump. I want to see what the oil slinger/governor weight set looks like.

Seth K. Pyle

[David]

I wonder if most of those carburetors have sloppy clearance/sealing around their throttle shafts. Around the neighborhood I seem to hear various mowers (some of them Quantums?) that surge a little bit especially when they have just been started up but just barely.
Actually it is kind of neat that those things can run as well as they do if the carburetors have no idle circuits or fuel feed except for at the main nozzle. An older Quantum (the 12 cubic inch type) that was on a Snapper that one of my old coworkers had did have an idle system and adjustment on it as I remember.
As I remember with the old Tecumseh fully adjustable carburetor engines if I completely seated the idle needle those would hunt like all getout even at maximum (no load) governed speed.
I guess it wouldn't be legal for a professional to take dental floss and baking soda toothpaste and floss the metering hole in the bowl nut.

I guess there is no possibility at this point that there is any question as to valve seating or wobble in the guide or anything like that. I wonder if a plastic cam would be more apt to wear down and not have enough lift than a steel one. If I remember someone on the forum was curious about a specification for valve lift for a Tecumseh and the specification didn't seem to be in the service book. I would think that changing the valve lift would change the sensitivity to lean mixture. I would think that if the surging could not be made to stop by feeding just a little propane through the air intake from an unlit torch with the engine running then that would practically eliminate lean running as contributing to the surging leaving mainly the governor as suspect.

outside unlikely guess--warpage of head or cylinder deck? no,leakdown test you did would have caught that?

[bgsengine]

@ Seth: On the right track, methinks - before tearing down the engine, closely inspect the linkages for worn "flats" and out of round linkage holes where the linkages will slip into place and "stick", also try a static governor adjustment - just a millimeter of difference can make a change.

@david: The Briggs mower engines like the Quantum, the newer pulsa-prime Classic series, etc, with no idle circuit per se are designed to run at one speed - as you may have likely noticed, a lot of lawnmowers in recent years have no throttle control levers - just start and go at whatever speed they pre-set - Problems can be had when the governor spring gets weak or someone "plays" with the linkages or tangs to increase speed.. - they have no idle circuit because they don't have an idle speed - they *do* have transition passages to feed extra fuel under acceleration and load changes, just no specific idle adjustments or circuits.

[pylek]

Ugh...more work, same outcome. @#$%^&*

I have been working every day straight since Memorial Day, so progress has been slow on this engine. Tonight, I wrapped it up again and restarted it. Same as before, immediately starts surging.

This time, I:

-inspected all parts of governor linkage, found no flats or sticking spots
-removed sump, removed rod cap, removed crank. Inspected governor/oil slinger, all bearing surfaces, and camshaft. All good, no issues. Had to replace a seeping lower crank seal anyway, also replaced PTO side shaft seal.

-I did find something bad. There are very noticeable scratches, even call one a gouge in the cylinder, could see them from the bottom of the bore, looking in the crankcase. This could mean broken rings. Leakdown showed leakage out the exhaust valve, not the breather tube...

-removed head, removed and relapped exhaust valve, reset clearance to .008" Cleaned head and deck surface, installed new head gasket. I noticed an oil sheen on the piston, combustion chamber, and tops of the valves. Could be oil ring is broken. However, spark plug was not oil fouled.

-reinspected intake pipe again, reinstalled with new gasket, used tiny inch pound torque wrench to general specification for #8 hardware.

-performed static governor adjustment


Running out of ideas...I am considering putting on a different intake pipe, just because I have one. I am also considering installing a 'test' bowl nut that I have. The main jet in the bowl nut has been opened up slightly with pin drills.

We shall see...


Seth K. 'lousy witchdoctor' Pyle

[bgsengine]

try changing out your governor spring, or, simply bend one of the attaching points of governor spring to change its operating length , or install it with the loop ends in opposite direction from where they are now , or bend the link to the throttle a tiny bit to change the linear direction of travel on it , reset governor, see if any of those simple changes makes a difference. if they do then you've got a harmonics issue - changing governor spring to new one, or perhaps adding a linkage spring to the governor link will make a difference

It is not carburetor if it will run fine while holding throttle steady (assuming you can move throttle manually to different engine speeds and it still runs smooth)