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

[David]

If a governor spring were made of two springs with one fitted inside the other so that they lightly touched-sort of like dual valve springs on a car-would the damping effect settle down some of the harmonics and make for a steadier governor performance?

[bgsengine]

If a governor spring were made of two springs with one fitted inside the other so that they lightly touched-sort of like dual valve springs on a car-would the damping effect settle down some of the harmonics and make for a steadier governor performance?

Correct. There have actually been such springs on O.E. Equipment and engines. Also used on cars for the same purpose.

basically any change in weight, diameter, tension, or composition can affect the harmonics of the spring. Engineers designing engines do have to consider these things - Even the orientation or position of a spring in relation to the engine can affect harmonics. Likewise, so can the linkages - even though they are not springs they will have a harmonic frequency - just like a train rocking down the tracks or a big truck going down the road, or a car with bad shocks.. when you hit the harmonic frequency of 2 or more parts, it often doubles or triples the effect of vibrations. - To the point that trucks can roll over , trains derail, cars go bouncing uncontrollably on seemingly smooth roads, and more... It's the same effect wherein you shatter wine glasses with high pitched sound waves.

[pylek]

Hello bgsengine. If I hold the throttle lever on the carb steady, the engine will run smoothly. I have tried holding the throttle lever on the engine to various rpm's, engine runs smooth. Let it sit on its own, surging. I have bent the governor bracket attaching point for the governor spring in both directions, speeding up and slowing down the engine speed. The effect is the same, the engine surges at any and all rpm.

Once again, I feel like it is something simple and stupid I am missing. I am going out to take it apart again and look everything over.

Seth K. Pyle

[pylek]

I have three other 12cid Quantum engines in my posession right now. I am going to compare IPL's, then start swapping governor spings and linkages. We shall see.

Seth K. Pyle

[bgsengine]

have you tried pulling carb off and while off, inspect for *any* binding or wobble in its range of motion on the throttle lever and shaft? inspect all linkage attachment points for any high spots or out of round spots - linkage only has to move just a tiny bit to kick the throttle plate open a little -

Also, last resort: as an experiment, make sure you have the *correct* intake manifold gasket - air velocity when air flows around that corner can be affected by the gasket dimensions, slight puddling of gas can occur until it drips back into the air stream , inspect carb for any potential cracks, perhaps (highly unlikely) or flexing of the carb and o-ring around the intake pipe - you DID replace the o-ring , right?

[mikenj]

I've read every single post on your problem. If the engine runs fine holding the throttle plate in all positions, your problem is not with the carb. At one point, though, you said it would still surge but in a later post it seems like it's consistent now. Try this - try running the engine holding the throttle plate in various positions, but wiggle it from side to side. Does it surge or mis then? My apologies if you already tried that.

The one thing that caught my attention was your leak down test (think you said it was > 30%). That's high and in a later post you already discovered the exhaust valve and cylinder gouge. What is your compression on a hot engine right now? The engine just might be tired and a lot of times it doesn't pay to fix in this case (but there's something to be said for fixing something regardless of cost just for the achievement )

Also do you have all of the shrouds on? It's important that vane has the right air flow over it and make sure there's no obstructions. The engine does sound like it's running way too high (think you said it runs up to 4100rpms?). Governed speed shouldn't be over 3600rpms, at least when I was fixing these things.

[pylek]

Grrrr...

bgsengine, I have replaced numerous gaskets numerous times. So far, this engine has eaten a carb kit(STENS), a complete engine gasket kit(PRIMELINE/NAPA Small Engine Line), an additional head gasket from another complete engine gasket kit, an additional intake pipe O-ring, 3 or 4 breather gaskets, 4 intake pipe mounting gaskets, and 4 air cleaner mounting gaskets, all Rotary.

Here is my procedure for installing the intake pipe: Ensure mounting surface on the pipe is clean, ensure mounting surface on the block is clean with no gasket residue remaining. Install gasket in proper orientation to intake pipe, securing with screws. Mount intake pipe to block, leaving screws slightly loose. Install carb mounting/governor/throttle bracket to block, tighten. Lightly lube intake pipe O-ring with silicone grease, install on intake pipe. Mount carburetor to bracket, tighten bolts. Now, tighten intake pipe to block mounting screws to proper torque. In my mind, this procedure aligns the intake pipe to the carburetor, as there is a good amount of play in the holes of the intake pipe for the screws.

Today, I swapped in the governor spring (699056) from an identical B&S Quantum on another Murray self-propelled pushmower (12H882 2312-E1 98011057). Initially, I had hope, as the engine didn't seem to surge. But, within a few seconds, it fell into the familiar surging. However, the frequency of the surging seemed a little slower. Again, holding the throttle lever on the carb caused the engine to run smooth. I removed the carb and entire governor linkage. Under 12X magnification, I saw no burrs, no out of round places, nothing. I saw some wear marks. For giggles, I pulled the governor rod link from the other engine for comparison. The two rods were identical in every way. The same bends, the same wear marks. Just because, I cleaned the wear marks on the original rod on my wire wheel.

I reassembled all, installing the original governor spring. Restarted the engine, same old same old. This time, the frequency of the surging was a little faster. I believe from the original governor spring.

I have come in to eat dinner now and regroup.

mikenj, that also confused me. Way back when I started working on this engine, if I held the throttle lever, the engine would slowly die. Now, it just runs. All I can figure is the lapping of the valves helped that. I have not yet performed a compression test, as I prefer to do a leak down. However, I will try anything at this point. According to the owner, this pushmower doesn't have all that much use to it. He bought it new, and used it sporadically, then took it to his beach house. It was again used sporadically, then left for a couple of a years under a tarp.

I do all my testing now with the various shrouds and air filters in place, unless conducting a specific test for something. Though this engine doesn't have an air vane governor, I have learned my lesson dealing with the 9D902/10A902 engines. I tested one with the blower housing off, and the engine raced away. No shroud equals no air flow equals no pressure against the vane to close the throttle. Oops.


Thanks!

Seth K. Pyle

[pylek]

For giggles, and lack of a better idea, I installed my test bowl nut. It is a 'D' bowl nut, part #494870, in which the main jet has been opened up to .026". I know that I have tried different carburetors, and that the smooth running when holding the throttle lever points away from the carb as being the problem. I am just running out of ideas.

Anyway, it didn't help much. The surging changed slightly, but was still there.

Hot compression test was 90psi. I checked the MCR on the camshaft when I had the sump off. All parts were present and in working order.

Seth K. Pyle

[David]

Another experiment would be to install a high drag blade such as one with a lot of curve on the ends or a "mulching" one. More load on engine even when not on grass>governor would have a bit more throttle angle. (If it doesn't already have such a blade).

[pylek]

Good idea David, but it already has a mulching blade. However, load does make a difference...more on that in a moment.

Last installment for a little while. I am off to Charlotte in the morning for a 4 day repair school for Fanuc automation robots for work. I will get back to this pushmower this coming weekend.

Today, I started off by installing at known good intake pipe(known good in that it came from a good running engine.) New intake pipe gasket. No change, still surged. On a whim, I went out mowing with this pushmower. Got it good and warm. Under no load or light load, engine surged. Under load, such as mowing heavy grass, the engine smoothed out, throttle lever on carb did not cycle. Remove load, back to surging. I noticed that a few times, the engine would run smooth no matter the load. However, shortly later it would go back to surging under light/no load. Hmmm... Remember, this is with the 'test' bowl nut with the main jet opened up to .026". I pulled the air filter, and saw a little fuel in the paper air filter. I am not sure if that is spitback from surging or from being too rich.

For giggles, I ran the carb(identical and identical part #) from the other 12H882 through my ultrasonic cleaner, then put a kit in it. I installed this carb with the correct original bowl nut and a new intake pipe O-ring. Restarted engine, same surge, though a little sharper. I believe that is due to having the correct bowl nut/main jet. While mowing, it was more of the same. No or light load, surging. Heavy load, ran smooth. However, at no time under light/no load did the engine run smooth. When I was done mowing, I noticed the rpms were a little rough, but not surging. The carb throttle lever was almost 3/4 way open, but the rpms weren't racing away. I manually moved the throttle lever closed, and it went back to surging. Out of curiousity, I 'blipped' the throttle lever. The engine stumbled and bogged. Hmm...when I did that with the test bowl nut, the engine would race away.

This tells me that the engine is running lean. If it has the correct bowl nut and jet, that means it is sucking air somewhere...

I feel like I am chasing my tail. Un metered air can come from intake pipe O-ring(replaced a few times), intake pipe(inspected, no cracks, no ridges worn in at O-ring area, gasket replaced repeatedly), intake pipe mounting surface on block( cleaned with razor blade, no indication of hi or low spots), head gasket(replaced twice, head and block surfaces carefully razor bladed and dressed with scotchbrite pad.)

So... normally, when a head gasket leaks, it leaks pressure out, right? I had heard some small, staccato noises coming from the muffler area, I thought they were just a result of the surge. Could a leaking head gasket(around the valves) cause these noises and cause the engine to run lean?

Seth K. Pyle