B&S 21HP Intek OHV engine runs poorly

10 years ago

Briggs & Stratton Intek 21 HP 331877-1371-B1 engine mounted on a Craftsman 91728944 46Ã¢ÂÂ lawn tractor.

I had the hard-starting issue, which was resolved after adjusting the valve lash. It now rotates freely though the compression stroke. Although it starts, it takes a few attempts with what appears occasional backfiring through the carb, but it could be fuel vapor IÃ¢ÂÂm seeing. Once started the engine runs slowly with reduced power and a rough idle. Attempts to change the engine speed result in a rough idle (hunting?) and stalling out, unless changes are input slowly where it finds idle a little better but may still stall. Upon stalling it also appears to occasionally backfire through carb, again likely fuel vapor. While running the exhaust pipe glows a dull but visible red, and an infrared thermometer shows the muffler at over 500 degrees. The spark plug has black carbon on it after running, but no sign of oil residue.

For reference, the fuel is fresh, the fuel line, fuel filter, and spark plug were recently changed. The carburetor is new (as a result of misdiagnosis of the hard start issue). I checked the flywheel key to make sure it is intact and aligned.

Any suggestions are appreciated.

Regards,

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Comments (7)

walt2002
10 years ago
Are you sure you adjusted the valves correctly? That a rocker arm stud is not loose and that a rocker arm is not cracked?

I can send you a Service Manual IF you like, address below, put in proper format and remind me, engine model number and what you want.

Walt Conner

wconner at frontier dot com
BlackPearl03GT
Original Author
10 years ago
Walt,

Thanks for the quick reply. A service manual would be great; I'll email you.

It was the exhaust valve that was way out of spec, so much that the valve cap came off! This tractor has less than 100 hours on it. When I saw that, I checked the rockers, pushrods, studs, etc. I've re-checked to lash several times after running the mower to ensure it stayed within spec. I have the intake at .004" and the exhaust at .006". The only thing I haven't done while testing is the gasket around the valve cover.

If this were a car I'd check the carb further, but there is nothing to adjust on these B&S engines. Furthermore, I notice the throttle linkage does not go all the way to the idle speed set screw at the lowest throttle setting. Could I have a governor issue? Just seems unlikely it happens to pop up at the same time, right?

Best,

Michael.
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bill_kapaun
10 years ago
I'd "suspect" a blown head gasket.
Remove valve cover
Have a rag handy
Start engine and see if you observe a "mist" blowing out on the push rod side of the head gasket.
BlackPearl03GT
Original Author
10 years ago
Bill,

Thank you, too, for the quick reply.

I did a compression test before and after the valve adjustment, 110psi and 140psi, respectively. I don't know what spec is yet, I did them for future troubleshooting, and I haven't done a leak down test. But I did perform yours. I can't get it to start, but while cranking I think I see a faint, whitish mist from the intake (top) pushrod. I shouldn't see anything but oil spray, should I.

Still thinking head gasket? Any other things I should eliminate/test?

Thanks,

Michael
walt2002
10 years ago
The top push rod of a 33 series engine is NOT the Intake. You need to properly identify the valves before you under take to adjust the valves.

I don't see where I sent you a Service Manual unless I don't recognize the name and you need one along with My valve Adjustment Instructions.

Address below IF you want them, put in proper format and remind me engine model number and what you want.

Walt Conner
wconner5 at frontier dot com
BlackPearl03GT
Original Author
10 years ago
Walt,

Thanks for that info; disappointing since a local dealer told me the order of the valves - twice. I'll emailed you for the service manual, but I guess it disappeared into the ether since your prior post didn't have the "5" in the address.

Thanks again,

Michael.
optsyeagle
10 years ago
May be nothing but this guy has a leaking exhaust valve that shoots flames out the exhaust. Might be a similar problem. Perhaps your exhaust clearance is too low, especially if you thought the exhaust valve was the intake valve, and vice versa.

http://www.youtube.com/watch?v=pAAAJPeJzzQ