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I have a Troy built 17.5 Briggs & Stratton lawn tractor. I suspect vapor lock because when I turn the key the engine will not spin over almost as if there was a 2 x 4 in the driveshaft or something. I took the spark plug out cranked it over several times a lot of fuel shut out and then eventually just puffs of air and and vapor. I put the spark plug back in and try to turn it over it cranked about 1/4 turn and backfired through the exhaust pipe.
Any suggestions on what to do? Thank you very much.
Fred

Either the decompressor on the cam shaft is buggered or the valve lash is too large which is defeating the automatic decompression.

You should not be getting raw fuel coming out of the spark plug hole. Check the needle and seat in the carburetor.

bertsmobile1 said:

Either the decompressor on the cam shaft is buggered or the valve lash is too large which is defeating the automatic decompression.

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Listen to @bertsmobile1. ^^^^^^^

Definitely not "Vapor Lock".

Fmkehoe said:

I have a Troy built 17.5 Briggs & Stratton lawn tractor. I suspect vapor lock because when I turn the key the engine will not spin over almost as if there was a 2 x 4 in the driveshaft or something. I took the spark plug out cranked it over several times a lot of fuel shut out and then eventually just puffs of air and and vapor. I put the spark plug back in and try to turn it over it cranked about 1/4 turn and backfired through the exhaust pipe.
Any suggestions on what to do? Thank you very much.
Fred

Click to expand...

Not vapor lock but is hydro locking. First you must resolve the carburetor problem of leaking fuel.

With these engines using wasted spark it might have ignited excess fuel in the exhaust once it had clear enough the excess fuel.

As for cranking problem first the fuel load must be removed since you have a leaking carburetor. Note this may have and probably have diluted the crankcase oil which would need changing. Either remove and plug the fuel line or clamp it off. Again remove the spark plug and clear the cylinder of excess fuel load. Reinstall and continue cranking. Now if it stop at the top of the compression stroke then either valves are out adjustment or the camshaft ACR is broken. IF the latter then the camshaft would need replacing.

If a broken camshaft ACR is suspected open the engine up before ordering parts to remove broken parts and to inspect for other damages. The broken ACR likes to take out the governor/oil slinger so it maybe damaged too. The minimum parts need to change is camshaft, crankcase gasket, and lower oil seal. Of course the governor might need replacing too which why you do the inspection before ordering parts. Please don't fall for the after market camshaft kits that replaces the 793880 (superseded to 84005207) camshaft are sub par with many not even fitting the camshaft journals in the engine block an oil pan.

Even with me knowing what has to be filed down they are still not worth the headaches. I am only using them currently because Briggs cant supply them for some reason. They have been on long term back order. Spending an hour or so reworking the camshaft is a pain that I don't wish to do regularly. Installed four so far and all had this problem along with an oil pump drive shaft is over sized too. I haven't even tried the tappets that came with the junk camshafts.

Thank you all for your informed information. That sounds like something that is beyond my mechanical ability. Is this something that a lawnmower repair person is going to have to do. And is it going be so expensive that it’s not worth it?

Fmkehoe said:

I took the spark plug out cranked it over several times a lot of fuel shut out

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Carb needle and seat need replaced.

Smell the oil dipstick for fuel. If so fix the carb first then change the oil

If your problem is not the carburetor and it is a mechanical problem with the engine, you probably should consider replacing the engine block and reusing the cylinder head, valve pushrods, and other engine peripherals on the new block. I bought mine on Amazon for the best price and it worked great after I had the engine reassembled. My cost was less than $600 doing it myself which is a lot less than the cost of a new lawn tractor and I have essentially a new mower even though the original one is 14 years old.

bertsmobile1 said:

Either the decompressor on the cam shaft is buggered or the valve lash is too large which is defeating the automatic decompression.

Click to expand...

Just a real late update because I forgot all about this forum, sorry lol. Yes it was the decompressor on the camshaft which is bad. I haven’t changed it yet, but I have learned how to spend the engine backwards until it stops, then cover the air intake, then crank the engine till it spins freely let go of the air intake and it starts right up.

Put an in-line shut-off valve on it, and use it. Doesn't fix your needle but solves your problem.

Come folks it is a Briggs that probably has a single Nikki carburetor and the fuel bowl gasket set is leaking. Now if it has a Walbro or Ruixing then can be the needle and seat not sealing.