B&S 650 series won't start

[illinicj]

Hi guys,
I've searched for an answer but cannot find one. Hoping one of you can help me out.
I have a Craftsman 917.376241 mower with the B&S 650 series 190cc engine, family 7BSXS.1901VH. This weekend I mowed half the yard, stopping and starting the mower a few times without trouble. I stopped the mower, moved it around to the other part of the yard and it would not start. I tried the pull start over and over, and it sounds like it wants to run, but doesn't fire. About 1 of every 20 pulls I get a single "pop." I had to finish with the neighbor's mower, and have been screwing around with this one for the past few days to no avail.

There is oil and gas in the engine. The gas was a couple months old (snowblower season) but had been stored with stabil so should be ok. The air filter is pretty new.
I confirmed I have spark by pulling the plug, grounding the plug to the block, cranking the engine, and visibly seeing the spark.
I removed and disassembled the carb, ran a thin wire through the jets, cleaned it with carb cleaner, soaked the parts in parts cleaner solvent.
Drained the gas from the tank, bought fresh gas, and refilled tank. Reassembled carb. Same symptoms.
Sprayed starting fluid into the carb - no change.
Inspected linkage to "dead man" handle and switch on engine. It is operating properly.
Pulled upper shrouds off and inspected flywheel to see if pin sheared and lost timing. Appears in good condition, shear pin is fine.
Removed the plug, felt for compression with my thumb as I turned it over. Not sure how strong its supposed to feel, but I feel compression and hear air rushing in/out.
Pulled the head off. Confirmed both valves and piston are moving on proper time. Piston bore is smooth.
I'm probably forgetting a few things I checked.

I'm frustrated and don't know what else to look at. Grass is growing fast... Help?

Thanks

 

[Mowerman99]

Did you wait for it to cool down and try it? Its an auto choke model yes? WHat is the Engine model #? After you pulled the head you need to replace the head gasket. (but thats just secondary to the initial problem)

 

[Rivets]

The first thing I would do is remove as much of the old gas as I could and put in fresh. Have had at least a dozen units this spring where the gas (even with Stabil) has gone bad.

 

[illinicj]

Did you wait for it to cool down and try it? Its an auto choke model yes? WHat is the Engine model #? After you pulled the head you need to replace the head gasket. (but thats just secondary to the initial problem)

Thanks mowerman99.

Yes, it has cooled down and still won't start.

Auto choke. I noticed the choke is always open, which i thought was strange. Held the linkage so the choke was closed and still won't start.

Model 124T05-0947-B1

 

[illinicj]

The first thing I would do is remove as much of the old gas as I could and put in fresh. Have had at least a dozen units this spring where the gas (even with Stabil) has gone bad.

Thanks Rivets. That was one of the first things i tried and didn't help. I removed the tank and drained it thoroughly. No luck.

 

[Rivets]

If the choke is open when the engine is cold, then the auto choke is not working proper. Take the plastic cover off and look to see if you may be missing a spring from the auto choke lever to a metal tab where the carb is mounted to the block.

 

[illinicj]

Ok, I have figured out the cause. The exhaust valve seat has come partially out of the block. The exhaust valve cannot fully close, which I couldn't tell from the angle I had earlier. Now I have the engine off and on the bench, and I can see the valve doesn't seat all the way around. Now what???

 

[Mowerman99]

If you purchased this unit less than 2 years ago, take it to a Briggs dealer. Tell them you want a shorblock. If not your going to have to press the seat back in and ping around the seat with a punch. You have to punch the aluminum over a bit to hold the seat in place. Its not pretty but it will work. Like I said, be sure to replace the head gasket. The one thing that bothers me is that you said you had good compression... you wouldn't if the seat was popped up...

 

[Mowerman99]

oh i see you just used your thumb... haha thats not the best compression test. yeah that makes sense now... also the choke should be closed when the engine is cold. if it isnt, the sping could be missing, or the thermostat needs adjusted, or replaced.

 

[illinicj]

oh i see you just used your thumb... haha thats not the best compression test. yeah that makes sense now... also the choke should be closed when the engine is cold. if it isnt, the sping could be missing, or the thermostat needs adjusted, or replaced.

Yeah, i didn't know what it was supposed to feel like, so i went over and did the same thumb test with my neighbor's mower and could feel a big difference.

I bet the choke has never worked right. There is a spring on the linkage by the carb only. Should there be another? I need to look into adjusting the thermostat.