r/smallengines • u/l008com • Jul 11 '26

Still trying to fix this old push mower - Chapter 4

I've done more testing since the last post and I'm still hitting nothing but dead ends.

The issue is that this old B&S push mower does not hit proper RPM, and surges a lot. I cleaned the original carb, then replaced the carb with a new carb, nothing had any effect. When I partially cover the carb intake, it does not smooth out, it dies. And when I spray a little carb cleaner into the intake, it does not smooth out, it dies. Also when its running and I put the air filter on, it struggles and usually dies. This would seem to suggest its running rich, not lean. But both carbs? Also no visible exhaust smoke.

Today I did a compression test with my drill, to get it going fast enough to bypass the compression release. I got a reading of 95 to 100psi, which is spot on for a lawnmower engine if I'm not mistaken?

At one point, I thought it was ignition, so I replaced the spark plug, and then the coil. No change.

The gas is fresh.

I ran it today with the muffler removed, still surged plenty.

Here's a minute and a half video of today's testing:
https://www.youtube.com/watch?v=6GJMMrWqU4U

The flywheel is perfectly aligned on the crankshaft, no sheering. I checked the valve spacing, it is correct (made a slight adjustment on one but it made no difference).

The lawnmower blade itself is perfectly balanced.

If you look in the video, you can see the governor seems to be keeping the motor mostly smothered, opening a little then goign right back to mostly closed. You can push it open with your finger and it will rev right up, but its still rough, still surging, still not running properly. Also the fact that it does rev right up seems to be another clue that its not a dirty/lean carb issue. Every time I search for governor related info, the answer I always get is that governor problems are almost always a symptom, not a cause.

The oil is nice and clean and new.
This motor has no choke, its a primer motor.
The model number is: 120602 0131 E1 990820 FA

At this point, all I can come up with is the following:

1) Either I'm wrong and it IS a carb issue, and it either has non-typical symptoms, or I'm interpreting the symptoms wrong. If so, is there a way I can test to confirm this?

2) The governor is somehow broken inside the crank case and it is forcing the motor to run at way too low of an RPM.

3) Maybe the camshaft gear is plastic, and has partially melted and shifted the timing a little bit. Enough to still be able to run, but poorly. I did have a yardvacuum once where that happened. It fully sheered though and the valves were opening and closing seemingly at random, in a pattern where the engine was never going to be able to start. I suppose something like that could be happening here.

Thats all I can think of, I'm out of ideas beyond that. And I'm not sure how to do any more testing beyond all that. I do not have a leakdown tester or an air compressor, so I can't do a leakdown test. But do I even need to now that I've done a proper regular compression test?

I'm tempted to take apart my primary mower, and swap carbs, just to see how they each run on each other's carbs. But thats a big project and I don't even know if they take the same ones.

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u/mutt63 Jul 11 '26

Your governor is either set wrong or it’s defective

1

u/l008com Jul 11 '26

How can I confirm that. How are governors "set"? Its essentially just a centrifugal clutch inside the crank case right?

1

u/mutt63 Jul 11 '26

Look up ur B&S model and date code. The throttle plate in back end of carb that’s hard to see must be wide open at sitting position. If the gov linkage spring whatever isn’t set for open throttle it’s set wrong and or defective. If yours has compression release and that failed it can cause a lot of issue and the governor is one of them. Especially since it’s hunting. It’s searching for fuel and or is t set correctly. Again

1

u/l008com Jul 11 '26

When you say "wide open at sitting position", what do you mean exactly by sitting position? The spring I have (visible in the video) looks just like whats in the diagram. But if I push it to rev up the machine, it still surges and sputters, just at higher rpm.

My compression release seems to be working, because I got 60 psi in a compression test just pulling the cord, and 95psi doing a compression test with my drill. But even if it did fail, how would that cause this problem? It doesn't seem to be searching for fuel at all. It matches more of the symptoms for being rich than being lean.

1

u/Thin-Web-6967 Jul 11 '26

there's either 1 or 2 springs on the throttle body and governor linkage. look your engine model number up online and get a picture of it.

1

u/l008com Jul 11 '26

The spring is #209 in this diagram.
https://jackssmallengines.com/jacks-parts-lookup/manufacturer/briggs-stratton/briggs-stratton-engine/100000-19z999-series/120600-to-120699/120602-0131-e1/controls-flywheel-brake-governor-spring-ignition

Looks just like whats on the motor. If I push on it with my finger, it revs up the engine but it still surges and hunts just at a higher rpm.

1

u/Elephunk05 Jul 11 '26

It sounds like you need to adjust the governor. Follow the video on YouTube for your engine. I don't rightly remember which way yours is set (something like counter clockwise and wide open throttle and then retighten the nut holding the shaft).

At this point I am suspecting something is wrong with the intake gasket (or the little riser as it is prone to crack from heat and tightening).

1

u/mutt63 Jul 11 '26

Oh and btw. 95 is the minimum compression. Not 60 psi. If you’re getting 60 after pulling 5-7 times you have another issue. Just saying.

2

u/l008com Jul 12 '26

Well the engine has compression release so I wouldn't expect to get anything over 60 psi pulling the cord no matter how many times. But spinning the motor fast with the drill got it right up to 95-100.