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Hi guys, so I’m in a pickle with this mower and I’ve hit the top end of my expertise. Hoping you can shed some light.
I got an old buntin 36 walk behind for my very steep hilly property and it’s been running fine all summer. Usually starts second pull. During a recent mow it conked out on me. I narrowed it down to a fuel issue and because the carbs for these are cheap and have decent reviews I bought a new one for simplicity sake. After I mounted it it started up fine. Issue is the new carb had an engine breather input, but there was already one between the carb and air filter so i figured it wasn’t necessary and decided to just plug it since the old carb seemed to have a brass plug in that spot from the factory anyway. For some reason this created a vacuum or something and stopped the needle from seating. The carb flooded to the point the engine hydro locked when I went to start it. I gave it a couple of hard pulls as it was filling before it completely locked since I didn’t know what was going on and I think I messed up the timing or something. After fiddling with it a few times while sorting out the carb issue the engine will not kick over at all anymore.

Now that the background is covered here’s where I’m at. I have spark, air, and fuel. Starter fluid directly in the chamber does nothing. I’ve let it dry out for a few days with the spark plug out and still gotten nothing. For compression I’ve got 50 psi. I saw another post here where someone said 75 is the minimum for these. When I open the little window behind the carb the valves seem to be actuating ok. Should I pull the head? What should I look for with the valves and how would I adjust them? How can I check the timing and what do I do if it’s off? This is all very frustrating as this was the last now of the season and I’ve got a little strip of grass in the front of my lawn taunting me. Pretty close to just buying a new (used since I can’t find them new) motor, but I only paid 200 for the whole mower and that’s less than these motors go for haha.

Take the flywheel nut / bolt off and check the timing key
It is probably broken so the flywheel is out of sync with the piston

Those China carburetors are cheap for a reason. What is the model code of this engine? Also, minimum compression is 55 psi. So, you are too low.

Just checked and t

bertsmobile1 said:

Take the flywheel nut / bolt off and check the timing key
It is probably broken so the flywheel is out of sync

Click to expand...

Dang. I really thought this could be it. No luck though. The key was bent by about .25 mm, but I don’t think that would translate enough to cause an issue. Do you? Any other ideas?

Just checked and t

bertsmobile1 said:

Take the flywheel nut / bolt off and check the timing key
It is probably broken so the flywheel is out of sync

Click to expand...

Dang. I really thought this could be it. No luck though. The key was bent by about .25 mm, but I don’t think that would translate enough to cause an issue. Do you? Any other ideas?

You wrote that you had spark and fuel present. There is still the low compression issue. You would need to identify why. Do a leak down test is where I would begin. Or, replace this engine with an FS481VGS01S, 14hp two cylinder if you don’t want to buy a new mower. The FB460V was a great engine in its day but the old flat head is not worth putting much time or money into.

.25mm is more than enough to prevent the engine starting.
Mower engines run fixed timing so the timing is set for full speed running , 3000 to 3600 rpm.
Thus at cranking speeds of around 200 RPM the spark is already way too far advanced . add a little bit more and it will be no bangs
There have been many attempts to electronically vary the spark timing and while some worked really well all of them have been dropped .

bertsmobile1 said:

.25mm is more than enough to prevent the engine starting.
Mower engines run fixed timing so the timing is set for full speed running , 3000 to 3600 rpm.
Thus at cranking speeds of around 200 RPM the spark is already way too far advanced . add a little bit more and it will be no bangs
There have been many attempts to electronically vary the spark timing and while some worked really well all of them have been dropped .

Click to expand...

Makes sense. I’m sold. Waiting on a set of woodruff keys now. Been waiting on an excuse to pic one up. I’ll let you know if replacing it works.

Update. I replaced the timing key. No Luck. I also did a valve job, but I think I did a bad job with the clearance. The exhaust is like 11 thou and the intake is about 14 now. Whoops. Compression is up to 75PSI, but it coughs through the intake instead of starting. Might try to chase my screw up by re-cutting the seat for the intake to shave a few thou, but I'm going to do a leak down test before I tear it all down again. Waiting on the tested in the mail. It absolutely has to be a timing or valve issue because it ran fine with the new carb before it flooded.

Related question, would an FC420V bolt right up? Is the output shaft and bolt pattern the same for the two motors? I've heard they're more reliable and I honestly hate how finicky it is to mess with these valves. Looks really nice to simply adjust some bolts to set clearance. I see them a lot on similar mowers so it has me wondering.

The bolt pattern would be the same for both. As far as crankshaft, there is a good chance they would be the same (1” x 80mm) is standard. But of course you would have to confirm. OHV is better than the flat-head. Both models were very reliable.