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Hi!

I have a 17.5 hp B&S model number 31C707 that won't start. It turns over fine and I have nice spark at the plug. After cranking for a while, there's a loud backfire through the carb (loud enough that the neighbors mistook it for a gunshot). Here's what I've done so far:

1) Checked the valve clearance. I was able to get it to run (although very poorly) by increasing the clearance a bit. It made it very difficult to crank, though. Obviously not a way to run it long term.
2) Pulled the head and the valves look OK (picture attached). Some carbon buildup, but they seem to close fine.
3) Checked the piston and cylinder lining. Looks fine from what I can see without removing the piston (which is a pain). No scoring or scuffs.
4) Checked the ignition timing. I've heard of engines where the key for the flywheel breaks and knocks the out of alignment. I didn't have the correct wrench to remove the main bolt, so I wasn't able to check the key, but the magnets line up with the ignition coil a few degrees before TDC, which seems reasonable.
5) Checked the operation of the camshaft the best I could without pulling the engine apart. The valves seem to open wide and the action is equal across both intake and exhaust.

I'm now out of ideas? Anything else I can do without pulling the engine apart completely?

Even a few degrees can throw of the ignition timing if the key is sheared.
The nut size is more than likely a 15/16 or 3/4, not too familiar with this particular model.

Scrubcadet10 said:

Even a few degrees can throw of the ignition timing if the key is sheared.
The nut size is more than likely a 15/16 or 3/4, not too familiar with this particular model.

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It's probably a 15/16. I may have one of those lying around, but I guess I need a proper torque wrench to put it back. Oh well, may need to make a trip to Harbor Freight to pick up something cheap...

Put an old plug back in the plug hole then fill the head with fuel see if it leaks out the inlet.
Pull the inlet valve out and see if you can lever the valve seat up with a screwdriver.
If so then you can sometimes get away with stakeing it in with a center pumch , but with the price of heads being so cheap it makes replacing the head a no brainer.

Nope. The key looks fine to me (see photo).



The only other thing I could think of is that there's some leak around the intake valve I can't see. I guess I could go get a compression tester to see how I'm doing.

Full disclosure: A couple of years ago, it overheated (mice had built nests in the cooling air duct) and stopped running. I pulled the head and noticed the valve seat liner (is that what it's called?) had popped out jamming the valve. I put some freeze spray on it and was able to gently tap it back into place. After that it ran flawlessly for two years. I mean, it could be that my temporary fix finally failed, but I would have thought I would be able to see visible evidence.

Of course, I could just get a new head and pop it on to see if it makes it run. But those things are $150, so I'd rather not buy one unless I'm sure I need it.

Ideas?

The other thing worth noticing is that I've replaced the ignition coil. It looked a bit different than the original but it fits and the yoke lines up perfectly with the magnets. I don't think that should change the timing. Besides, it was having the same issue before I replaced the coil. I originally though I had poor ignition (which I probably did).

bertsmobile1 said:

If so then you can sometimes get away with stakeing it in with a center pumch , but with the price of heads being so cheap it makes replacing the head a no brainer.

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Yeah, that's kind of what I did the last time it stopped working. I guess the seat deteriorated to the point of not sealing properly anymore. May be time to drop $150 on a new head then.

796026 is the part number for the head from the model you gave, be sure to get the right Type and code off the engine too.

"but with the price of heads being so cheap it makes replacing the head a no brainer."

"Of course, I could just get a new head and pop it on to see if it makes it run. But those things are $150, so I'd rather not buy one unless I'm sure I need it."

??

I would look for a good used head, $20 here. There are plenty of them as most of these engine failures is from thrown rods. You can buy a whole good used engine for $150.

Walt Conner

Tinkerer200 said:

"but with the price of heads being so cheap it makes replacing the head a no brainer."

"Of course, I could just get a new head and pop it on to see if it makes it run. But those things are $150, so I'd rather not buy one unless I'm sure I need it."

??

I would look for a good used head, $20 here. There are plenty of them as most of these engine failures is from thrown rods. You can buy a whole good used engine for $150.

Walt Conner

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Where can you find that? I've looked and I haven't found much. The only thing I've found on eBay is a really ugly engine without carb or starter for $200 + 50$ shipping and a used head assembly for $100.

Search ebay "Briggs 31C707 head" - some $25-35 ones. Search ebay "Briggs 796026". Look at the seller feedback # and read some reviews to find your trustworthy quotient.

East Central Illinois, I have a used head myself but shipping would be more than the head. I have traded, rebuilt and sold engines all over this area until I "retired" about 5 years ago as I am 86. May be higher on east coast but likely you are not looking in the right places, small one man shops in out lying towns.

Walt Conner

I can get a new one from Amazon Prime for $142 shipped and that includes all screws and gaskets. Almost the same as a used one of you include shipping...

njswede said:

I can get a new one from Amazon Prime for $142 shipped and that includes all screws and gaskets. Almost the same as a used one of you include shipping...

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Which is why I did not suggest getting one from me but shop your area. Actually, the best deal is often buying a "junk" mower with running engine. Deck gone bad, bad hydro, etc. And frankly I don't want to mess with boxing up, taking to PO, etc.

Walt Conner

I can't believe I didn't think of this simple test before: I put the head back on but didn't attach the pushrods, leaving the valves closed. Then I spun the flywheel by hand and could clearly hear air rushing backwards through the intake.

OK. So it's the head/valves. I'm thinking it got overheated and warped either the valve or seat. Either way, I'm getting a whole new head/valve assembly. Done messing with this now.

Tinkerer200 said:

Which is why I did not suggest getting one from me but shop your area. Actually, the best deal is often buying a "junk" mower with running engine. Deck gone bad, bad hydro, etc. And frankly I don't want to mess with boxing up, taking to PO, etc.

Walt Conner

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Yeah... I'm done messing with this for now. Just getting the original part with all the accessories shipped seems like the least amount of pain.

Thanks everyone who's helped with ideas!

I finally got a chance to work on it. It runs better than it’s done in years with the new head.

https://youtu.be/1Bixf11-DBk