Briggs 20HP V-Twin Over-Heating Impacts

scgrant176September 12, 2013

My White lawn tractor with Briggs 20HP model 351777 type 1036-A1 ran way too hot and chugged for 2 hours before I figured out the blower housing was clogged with grass. After clearing everything out it runs, but with less power and at what sounds like a lower RPM, so I have to cut at a much slower speed. Its been a great machine for almost 30 years and I'd like to get it back to full power. While I cleaned out the grass I adjusted the valves, replaced the air filter and replaced the plugs none of which restored the lost power. Strangely enough the engine RPM increases and runs smoother when I turn hard left, but coughs and slows down even more when I turn hard right.

Also, just after it starts there is a short "slapping" sound and excessive engine vibration, then the same thing again just after it is shut off. Starts and stops easily, but abnormal noise and shaking. No noise or shaking once it gets to full speed, just less power.

Just wondering if anyone has experienced anything similar and what I should focus on when I tear it down this winter and go through piece by piece to see if I warped a valve seat, scored the rings or burned up something else during the overheating.

Would appreciate any suggestions on where to start the diagnostic process.

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mownie(7)

As to the "speed up on left turn, slow down on right turn" aspect of all this..............check your parking brake caliper to make sure it is not sticking.
I have seen this occur before. The reason has to do with differential speeds between the left and right axle during turns.
Turning one way decreases the effect of an applied brake while turning the opposite way can dang near lock up a wheel.
I'm sure the grass pack did not help matters but it might be a dragging brake causing much of your power loss.

    Bookmark   September 13, 2013 at 12:07AM
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wheely_boy

Is the break caliper hooked to the flywheel on this thing?

    Bookmark   September 13, 2013 at 4:55AM
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mownie(7)

"Is the brake caliper hooked to the flywheel on this thing?"
Yes it is. By means of the drive belt.
The OP stated the the engine seems to run at slower RPM than before the heat treatment and that now executing turns has an effect on RPM/power, depending on which way you turn.
I can only correlate that to something that would affect how much effort is required to move the machine across terrain. It IS NOT that the engine really makes more power during turns,m it is because the loading of the engine changes during turns (due to a sticking brake caliper).
After I posted last night I also realized that it is entirely possible that this V-Twin Vanguard engine might only have 1 cylinder making good power.
With only 1 cylinder putting out good power, the other cylinder constitutes an added load, thereby lowering RPM.......and making the engine run at full open throttle but never really reaching governed RPM.
Hence the engine is able to speed up some when conditions occur that lessen the total loading.......or to "bog down" easier when loading increases. And a sticking brake caliper is capable of doing that.
Vanguard twins can lose a cylinder entirely and the exhaust note will not sound much different, so it is not as apparent that you only have 1 good cylinder remaining.
Heck, it's easier to detect an intermittent misfire than a completely absent cylinder on a Vanguard Twin just using the ears.

This post was edited by mownie on Fri, Sep 13, 13 at 11:33

    Bookmark   September 13, 2013 at 11:12AM
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scgrant176

Thank you for the quick replies, I will check the brake this weekend. I think the brake on this mower is connected to the side of the hydro transmission.

    Bookmark   September 13, 2013 at 11:14AM
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mownie(7)

***" I think the brake on this mower is connected to the side of the hydro transmission."***
That is correct.

    Bookmark   September 13, 2013 at 11:35AM
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scgrant176

Mownie has uncanny insight or prescience. I checked the brake over the weekend and it does not appear to be sticking. However, I also pulled the plug wires while the engine was running and the left cylinder (viewed from the driver's seat) is not running. It may have been the over-heating or it may have been coincidence, but one cylinder stopped working during the over-heating incident. I'll have to check tomorrow night and see if that cylinder is not getting gas or spark--at this point I suspect no spark as I can smell raw gas in the exhaust.

    Bookmark   September 18, 2013 at 12:15AM
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mownie(7)

At this point, you want to "read" each spark plug that you remove, paying particular attention to the one from the dead cylinder.
Let us know if the plug has any trace of oily residue on it, or if it seems to be wet from gasoline.
And let us know if it has spark as you said you would check.

    Bookmark   September 18, 2013 at 12:23AM
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