Black Smoke When Engaging Blade

14 years ago

I have a Craftsman 17.5 Hp Tractor (model 944.605160) with a B&S motor (model 31C707-0230-E1). When I engage the mower blades I notice a puff of black smoke from the exhaust area. It doesn't smoke when I am mowing, only just as the blade engages.

Is this a problem? The mower does burn about an ounce of oil per hour (Mobil 1 Synthetic 10W30).

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Comments (4)

mownie
14 years ago
***"When I engage the mower blades I notice a puff of black smoke from the exhaust area. It doesn't smoke when I am mowing, only just as the blade engages.
Is this a problem?"***
Absolutely NOT A PROBLEM. What you are observing is simply a result of the engine governor "snapping" the throttle wide open, for a moment, as it reacts to the drop in RPM caused by "applying the load" of the cutting system to the engine crankshaft. The sudden loss of RPM causes the governor to try to increase the RPM to its former rate. The governor attempts to raise the RPM by opening the throttle plate of the carburetor. This action, in turn, will allow a greater volume of air to enter the engine, along with the greater volume of air, comes a lot more fuel drawn from the main jet. At first, the volume of fuel is great enough to result in a "rich mix", which is why you see the puff of black smoke. The engine quickly recovers its RPM and once it reaches its former RPM rate, the induction mixture has pretty much "leaned out" to a near optimum air/fuel ratio. If the load applied to the engine was greater than what the engine could handle, the governor would still open the throttle but instead of seeing a quick puff of black smoke........you would see a continuous plume of black smoke as the RPM got lower and lower til finally, the engine quit running.
optsyeagle
Original Author
14 years ago
Makes sense. Thanks mownie.
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The problem is usually in the control yes. If you can verify that there is a slight amount of slack in the cable, then it would be in the control I'd think too. When the mechanism has enough debris that prevents the clutch arm from returning all the way, then the dog inside the control doesn't grab the tab securely enough and can release with the slightest jolt. From the pic you've posted, I would guess you have looked up the part but gulped at the 40-50$ cost. You could pull the control, split it and have a look. Maybe a tab had broken inside that you can Macgyver up w/ some super glue or chewing gum!
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I put the mower on the lift and saw that the underneath of the frame had an even coating of old oil and dandelion fuzz. The muffler was clean in and out. The mower belt was outside one of the hold in tensions and when operating would tighten on the rod. Therefor the blue oil smoke and the hint smell of rubber. It seems to be working now.... T H A N K S
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As per your email- Hard to say for sure. Is the oil overfilled at all? Sounds symptomatic of a blown head gasket. Lugging the engine could create more crankcase pressure that could force oil out the breather, but I would think the oil would have to be quite overfilled to do that. A head gasket leaking could easily increase crankcase pressure and force oil out the breather. The fact you were working the engine harder than normal could cause the head gasket to blow sooner than it normally would. A lot of Kohlers have had trouble blowing head gaskets. Probably the simplest thing to do at this point is pull both spark plugs and compare them to each other. If one is noticeably darker/oilier than the other, that's a pretty good indicator the gasket is blown. If that's indicated, you can put the plugs back in and remove the valve cover on the suspected side. You should be able to see the compression leak "mist" out on the push rod side of the gasket when running. Have a rag handy. It might be a gigantic "mist"!
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Progress!!!!!........ I overhauled the carb and fuel pump, Changed the filter, changed the pulse line. She started up and now instead of an occasional dying down it was actively searching for RPM. I played around for a while and discovered that if I only brought her up to about 75% throttle she ran like a dream. I readjusted the throttle linkage so that i got maximum RPMs at 3/4 throttle and it ran fine. It then occurred to me that this tractor orginally had a 31d707 briggs, that engine had the choke built into the throttle linkage. It seems that by bringing the throttle lever all the way up I was causing the spring from the governor arm to end up in an unnatural position. (way to much travel from the throttle control). I installed a stop on the throttle control to prevent it from being put in the previous "choke" position and things are going great. Cut the yard tonight and only had an occasional shudder. I can engage the blades, cut up a hill satisfactorally. To put it in a nutshell, excessive throttle travel caused the spring from the governor arm to end up way passed where it should have been, AND the pin on the governor arm actually ended up bypassing the sheet metal cam on the throttle linkage, this allowed the governor to overshoot causing surging. Thanks for all the help, this is my very first lawn tractor, 2006 troy bilt, the mower was free and I have about $110.00 in the replacement engine.
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corvetteguy
14 years ago
My "new to me" Agway 16.5 hydro atarted to
run blue black smoke when blades engaged.
Thought I'd have to trash the $50 machine.

Turned out the deck belt was routed over the
belt pulley keeper rod. As the oily belt ran
over the keeper it smoked.

Re routed the belt and all is fine.
metal
14 years ago
You could try throttling down a bit when engaging the blades. That may help a bit (I take it you are engaging them at WOT).

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Black Smoke When Engaging Blade

Comments (4)

mownie

optsyeagle
Original Author

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Black Smoke When Engaging Blade

Comments (4)

mownie

optsyeagleOriginal Author

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corvetteguy

metal

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optsyeagle
Original Author