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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Thanks Walt, it's coming back to me now (I hope). I seem to recall something from another thread about a couple years ago to the effect of the two separate coils AND the little "bulge" in the wrapped harness where the diodes are. I hope the OP is keeping up with this so he can check out that possibility. I believe I can safely guess that if ONE of the coils were "going south" when hot, it could cause the symptoms wb3ffv is experiencing ( misfire, backfire, loss of power, seem to lose a cylinder)? But I see from your explanation of the two coils being connected by a pair of diodes, that if a diode were to "short" (continuity both ways), that would result in one coil "being influenced by the other coil". I am not sure what that scenario would create as far as symptoms, but it would seem that it could "kill" or weaken the spark on the "good diode" coil because the spark energy would dissipate into the coil served by the bad diode. The coil served by the bad diode would still throw a good spark because the other "good" diode would still be keeping the energy from the other coil from back tracking into its coil. Now, if the diode failed and became OPEN, the coil served by it would no longer have a "path to ground" (kill circuit conductor) and when the kill switch was closed, that coil only, would not stop sparking until the engine came to a complete stop. Sound about right??? Makes my head hurt some.
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You are correct about the valve guide causing the bent push rod. The most common cause of this is a cooling problem, plugged cooling shroud and or cooling fins. Some say ethanol fuel causes the valves to stick resulting in the valve guide coming out. I have no opinion on this. B&S says the only fix is a new head. I do have a fix at address below, put in proper format and remind me, model number and what you want. Simply pushing the guide back in place will not work. Walt Conner wconner5 at frontier dot com
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I'm not sure what the cross reference filter's part number was. It was just one that crossed and that was in stock. The outer diameter was 1/4" smaller than the B&S filter. Concerning the sequence of events - the clogged filter resulted in fouled plugs. I wasn't aware that the plugs were fouled but I did know that the filter was tremendously dirty so I changed it but didn't change the plugs until after I realized that the filter change had no effect. Once I noticed that the plugs were fouled I changed them and the mower seemed to be working properly but it then fouled the new plugs in after only running it for 1.5 hours. I went to the B&S website and submitted the information to them to see if they had any recommendations and their reply was to change the air filter. I wrote back and told them that I had already changed it but it didn't make a difference. The tech then wrote back and asked for the model number of my engine and when I provided that to him he responded by telling me that it was a documented problem with that engine and that there are several cross reference filters that will fit but the mower won't run well with. Twice he wrote to me and told me to change the filter to the B&S numbers and it would fix the problem. Obviously I had to change the plugs too since they were fouled and when I did that this weekend I could immediately tell that it was running much better. I used it for ~3.5 hours and it never missed a beat and I could tell too that it was no longer running rich. I'm sure it is fixed because even when it seemed to be running well when I had changed the plugs the first time it still ran rich. This time there were no signs of it running rich and it never once lacked power or backfired as it had before. The part number recommended by B&S is: Air filter 692519, pre filter 692520. BTW, the tech also stated that there are even hood stickers that call out the wrong air filter part number too. The printed material that I have for my engine even lists the wrong spark plug gap. I found that out when I had the tech verify all the numbers and specs for me before working on it this weekend. Lastly, the salesman at the parts store told me that they had experienced problems with that same engine when they had used other air filters. As he stated it, "that motor is very picky about the air filter that it uses". Strange but true! At least in this case. Thanks again. Scott
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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

B&S 21HP Intek OHV engine runs poorly

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walt2002

BlackPearl03GT
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B&S 21HP Intek OHV engine runs poorly

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walt2002

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