There is the possibility that you need a valve adjustment. This could be due to the exhaust valve on one cylinder having "too little" clearance, or "lash". Exhaust valves tend to "grow longer" or "stretch" because of them becoming much hotter in operation than the intake valves. In addition to their tendency to lengthen, they also erode the valve seat and valve margins more, all of which result in a loss of (or decrease in) the neccessary gap or clearance between the valve stem end and the rocker arm. If an exhaust valve has too little clearance at that point, the exhaust valve might not seat properly when it gets hot. When that happens, the effect worsens rapidly (because it makes the valve even hotter and so on) and may cause enough loss of compression on the affected cylinder to cause a "dead misfire". Continued operation with insufficient valve lash will burn and/or warp the valve and valve seat. With around 70 hours on the clock, you can justify a valve check and adjustment, you will be just a little early (Briggs recommends every 100 hours). I recommend it because you have covered nearly every one of the usual things that might (or might not) cause the engine to run like yours is running.

Yep, I think I will get the tools out, and see what it takes to lash the valves. I am very used to working on the large auto engines, and granted same overall principals but there are differences between a nice 427, and this lil Vanguard.

Hopefully no biggie to get the covers off and get to the adjustments.

I did find some other chatter about this motor, and people having a similar issue, but most talk about getting black smoke, and that is not the case here, just a loss of power and worsening misfire as it gets hot from running.

So guess I will look at this as the next step, and I'll be sure to follow up with how my luck goes. As always I am open to any ideas or suggestions..

Before you go off and adjust the valves.......post a request into this thread asking Walt Conner for his e-mail address. Then contact him for instructions and "tips" on the correct procedure. He can make sure you don't mistake this "lil Vangaurd" for "a nice 427". :^)

Thanks for the plug, Mownie, BUT the detailed instructions I wrote are for 28 and 31 series engines. I do have pages copied from the B&S Service Manual I would gladly send.

Walt Conner
wocnner5 at verizon dot net

OK Walt, wb3ffv stated he "just picked up the Briggs service manual". I guess I missed that in my first read of the opening post. MY general "rule of thumb" is that if a person obtains the OEM factory manual "voluntarily", THAT PERSON is "probably capable" of using it to his advantage. I've only known of a few folks that obtained the needed manual "of their own volition" that could not "make hay" with it. On the other hand, I've known of more cases where a person was "instructed to get a manual", and then they still couldn't do anything right. Those manuals usually end up on ebay or in used book stores.

"what do you think about the possibility of the "Kill Wire" harness? (diodes)"****
Walt, the OP has not identified the make/model of tractor so we don't know how the OEM might have "configured" the kill circuitry. Since the Vanguard utilizes Magneto ignition, wouldn't it "kill" by grounding the coil (as is the norm for Briggs)? I don't KNOW of any configurations that use an elaborate set up where the kill wire would be grounded via a relay or other situation requiring a diode. But then, I don't KNOW EVERYTHING either. It should be easy enough to disconnect the kill wire for testing the hunch though.

"Since the Vanguard utilizes Magneto ignition, wouldn't it "kill" by grounding the coil (as is the norm for Briggs)?"

Both the B&S Intek "V" engine and the B&S Vanguard "V" engine utilize a separate coil for each cylinder. These are connected with a wiring harness with some diodes in it to prevent the coils from shorting each other out during operation but still be able to "Kill" both from a single contact switch. When one of these diodes fail, the "Kill Wire" system does some weird things.

Your are right, he hasn't posted a model number but I am fairly sure it is a "V" twin Vanguard.

Walt Conner

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.

Creates "weird" symptoms, not consistent, depending on kind of failure and whether constant or intermittent. Way to check is disconnect the "Kill" wires from BOTH coils and see how engine runs.

Walt Conner

Thanks, my head feels better now.

I am having the same problem. With the kill wires disconnected, the engine runs perfect. With them connected, backfires, black smoke,......not firing every stroke. Should the tab on the coils be grounded without the wire connected? I would think not.

Have the same mower, it cranks but it won't turn over. No spark when I test the plug. Bad coil?