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Forum Home > Deere LP-Gas Tractors > Its a test

Allen
Administrator
Posts: 1

OK guys and gals here is something to think about. Lets say you are servicing the oil bath aircleaner on your John Deere 630 LP tractor. You have the oil bath cup off the tractor and for some reason decide to start the tractor.

Question is: Will it start? And Why?

April 29, 2011 at 9:18 AM Flag Quote & Reply

DaveKamp
Member
Posts: 1
So I'll jump in and test. With the oil-bath air cleaner removed, it may, or may not start, and if it does, it may not start as easily. Gaseous fuel systems utilize a 'demand port' in the intake flow. Sometimes, this is a port in the venturi dedicated to only ACTUATING a fuel controller (aka 'zero governor' or 'negative pressure regulator'... and sometimes, the fuel controller not only uses it as a fuel-demand signal point, it also uses it to introduce fuel into the venturi. The fuel controller senses demand by the venturi vacuum signal. When there's vacuum IN the venturi, this applies a LOW pressure signal to the control diaphram in the fuel controller, lifting it's fuel needle valve off seat, which introduces fuel flow to the intake tract. When there's no demand, there's no vacuum, hence, the fuel controller goes into a state of fuel 'cutoff'. When you change the intake pathway restriction, the vacuum signal that occurs at ANY point in the intake tract (not just manifold, but the venturi) affects the intensity of the demand signal that the fuel controller detects. IF the system was previously adjusted to provide proper fuel flow and mixture WITH a restrictive element in place, the resultant vacuum signal at the fuel controller would have been somewhat higher, hence, more flow. The cutoff bias adjustment of the fuel controller, therefore, would be set with higher tension, meaning, more vacuum is required IN ORDER to lift the fuel metering valve and flow fuel. Under that circumstance, the removed restriction causes the vacuum signal to be slightly lower, which might be insufficient to lift the fuel metering valve enough to provide sufficient fuel flow to start the engine. Now... if there's just-enough-signal to get it to flow a LITTLE fuel, it may be feeding, but too lean to fire. If the mag is hot enough, it just might light it off and go... or if you spin it up fast enough, it might be able to draw enough fuel to fire. And of course, if you hit the PRIME button (basically just forces the diaghram to lift the needle off-seat for a moment), you'd get enough flow to fire it off, and then it would run, albeit a little lean, but still... running. More often, someone making a fuel mixture adjustment with the oil bath cleaner OFF, and then reinstalling it (perhaps still dirty, or with gloppy oil), the result would be too much enrichment, and thus, a flooded engine. The gaseous fuel, displacing oxygen, yields an incombustable combination... no O2, no fire.
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The more I work on cars, the more I love my old tractors.

October 1, 2013 at 12:51 AM Flag Quote & Reply

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