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I bought an old Ariens riding mower recently, and it has a horizontal L-head 8 hp briggs and stratton mower. I cleaned the carb when I got it, and it started right up. After a minute of running, it blew a head gasket (1985 gasket was in poor shape). I took the valve cover off, replaced the head gasket, and while I was at it, cleaned up the top of the piston and the valves/seats. I put it back together, and got nothing.

Thinking that I might have opened up some leaks in the old gummed up valve/seat area, I took the valves off, and ground the seats and valves with coarse and then fine grit. Intake looked really good after this...exhaust still had deposits and imperfections on the valve, but the seat was good. Installed new springs for both valves, put it back together, and now all I get is a bunch of backfiring. The plug is pretty much soaking whenever I pull it out. I get no backfiring when the choke is on, and quite a bit of backfiring the first few times I pull the chord when the choke is off and the throttle is wide open. This eventually goes away too. I can't get it started at all - occassionally the backfiring will sort of sustain the engine for a few rotations.

Any ideas on what's causing this? I think the carb is clean enough - it seems to be getting an overly rich mixture. Could I have not cleaned the exhaust valve sufficiently? I replaced the flywheel key, and the coil, so I'm pretty sure its getting a spark, although I can't tell for sure when the plug is installed.

I don't think its the richness of the mixture that is bad, because even the first time I pull the chord after drying off the plug, I get nothing. I almost get the feeling that the timing is just a little bit off....spraying carb cleaner in the plug hole does not help start the engine.

I don't have a compression tester yet, so I can't tell you if I have good rings/piston. But judging by the fact that it ran well when I first started it, I think the piston rings are OK.

Any thoughts would be appreciated.

One thing that I did notice, when looking at the valves carefully as I rotate the engine manually, is that the intake valve closes at what looks like the correct time (BDC), but after rotating the engine almost back up to top dead center, the intake valve tightens just a little bit (visible). I can't imagine what would be causing this, unless there was a serious problem with the crank....

-James
 

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i've got a 1982 briggs 8 horse. it use to back fire. the carb needed a rebuild and the card high speed screw wasn't set right. the screw at top needs to be 2.5 turns out from screwing it in. the screw on the fuel bowl needs to be the same then start it and fine tune it. the valves on these things are old and never really sealed good but the plug is wet so its the carb. see when on choke it doesn't get a big amount of fuel. when you take it off it will act up
 

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one way to check compression. take the plug out, put your finger over the hole tight then crank it. if your finger blows off then it had sufficient compression
 

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Discussion Starter #6
wait, so are you thinking its running rich or lean? my thought was that its running rich...I'll do the thumb check, although I read on another post that someone thought that something as low as 25 psi could blow your thumb off the plug hole...

for reference, I have the idle screw and main screw (under the float) both set at 2 turns out. Adjusting to 2.5 or 3 didn't change anything.
 

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Sorry to resurrect this after so many years but... Thanks for the tips. Cleaned carb and it still backfired like mad. Took out "new" magneto, cleaned up original one (was a bit rusty) and set the gap to .020" and put it back... no more backfires. Was something wrong with the magneto. Also found that the wire was a bit cracked. I think maybe some of the sparks were grounding to the block either partially or entirely. Put on some liquid electrical tape.

Working now!
 
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