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Antique Farm Tractors Vintage farm tractors on rubber tires with various implements. Ford, John Deere, Oliver, McCormick and more.

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JD 720 Cranking engine will not start..Help


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  #1  
Old 03-29-2008, 08:54:28 PM
Steamcar Steamcar is offline
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Default JD 720 Cranking engine will not start..Help

I have an early 1957 720 diesel. the cranking engine ate 2 throttle plate screws. I put in one new piston, new rings and did a valve job. It has a rebuilt carburetor by Roberts, new distributer, and plugs. Compression is 125# per cylinder. I rechecked valve settings and they are OK. The crankshaft and camshaft were never separated. it is Timed according to the JD shop manual. There is spark and the plugs are wet. The engine fires, but will not start. I have put a separate 6V power source at the distributer. There is also spark while the engine is turning over after the starter button is released. I have poured a small amount of gasoline in the carburetor throat, tried ether, all with the same results.
I am at my wits end and do not know what to do next. Any ideas. I went so far as to get the parts to convert it to 24V electric start. I have a buyer for the tractor that wants the cranking engine to run before he will buy it but doesn't want the electric start parts. I am even considering trying to sell it as is. It has the square wide front, 3pt hitch , live pto. I'v owned the tractor 12 years and the previous owner had it 17, and in all that time it has never sat outside.
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Steamcar
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Old 03-30-2008, 01:36:44 PM
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Default Re: JD 720 Cranking engine will not start..Help

Steamcar, the #1 cause of JD ponies not starting is burned-out coils. The coils are really sensitive to too much voltage, i.e. jump-starting w/12 volts, or even leaving the switch on for a long time. The original switch has a resistor to limit the voltage, even a toggle switch w/no resistor may burn them out. You say there is spark, but the plugs are wet, the coils may be weak enough that the plugs won't fire under compression. Did your new dist. include new coils, or are they the old ones? If there is in fact a good hot spark, try a new set of plugs, this fine gasoline we have these days will kill a plug w/ one good flooding, esp. a Champion plug. Try the Autolite equivalent if you have Champions. As I remember, the Champion # is a J8, I don't know the Autolite number...
Just don't junk it or give up & convert to 24v, the ponies are the best-when they run...
HTH-Ron in CO
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Old 03-30-2008, 07:44:51 PM
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Default Re: JD 720 Cranking engine will not start..Help

Just re-read your post & you didn't mention points. The points have to be set at .015, & they are sort of fussy to set. They wouldn't be set on the new dist., as the cam isn't part of the dist. assembly. Just wild guesses...
Ron
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Old 03-30-2008, 10:26:34 PM
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Ken Majeski Ken Majeski is offline
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Default Re: JD 720 Cranking engine will not start..Help

I Agree with Ron... If you have One bad coil it will kill the spark on 2 cylinders.... And remaining 2 cylinders that small just isn't enough to get that little engine in Motion...

John Deere coils used to be real Pricey... But the aftermarket ones were more reasonable...
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Old 03-31-2008, 12:00:20 AM
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Default Re: JD 720 Cranking engine will not start..Help

Yes, Ken, the complete aftermarket dist. w/new coils, points & condensers is less $$ than the coils alone from JD. (Last I checked, may be different now)...
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Old 03-31-2008, 11:54:05 PM
Steamcar Steamcar is offline
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Default Re: JD 720 Cranking engine will not start..Help

Thanks guys,
The new distributer had new points, coils, condensers and wires. I also replaced the resistor that mounts on the switch. I put a a new set of Autolite plugs in it. I have put the old distributer and plugs back on it, as the engine ran when it ate the throttle plate screws. It acts the same. It does have spark on both sides of the engine. I have tinkered with the throttle linkage adjustment.
I am certain that whatever the problem, it will be very simple. Now finding the problem, thats a different story.
Steamcar
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Old 04-01-2008, 08:00:25 AM
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Default Re: JD 720 Cranking engine will not start..Help

Go back and startover with the engine. Review the basics , Compression,gas and spark at the correct time. There is something simple that is the problem. Merton
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Old 04-01-2008, 10:26:11 AM
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Default Re: JD 720 Cranking engine will not start..Help

Well.... One thing I remember about my 70 was the Gas tank wasn't much higher than the carb... And I would end up sticking my mouth over the tank neck once in a while to prime the system... (Yuk...) Seemed it would air lock quite easily more so if the tank wasn't completly full... That Little Rubber hose didn't help as it was a low spot...

Some of them are a Quite Fussy little engine.... I would think by priming or ether you could flood them quite easy.

And once a set of plugs are Flooded with Todays Gas it isn't a Good thing...

Both of the ones I had (70 & 830) seemed to be more user friendly than Most...
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Old 04-01-2008, 02:05:44 PM
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Default Re: JD 720 Cranking engine will not start..Help

Are your valves closing? If they close but the lash is just a little tight it will drastically change you manifold vacuum and compression numbers. I think of the three things needed for an engine to run compression is the least important. A good example of this is an idling engine. The least amount of air gets to the cylinder and there is actually a slight vacuum when the intake valve closes in idle mode (when the intake is the most closed off by the throttle plate). That being said I would think a timing problem is more the problem.

Double check valve/crank timing... I have seen a number of these motors that have had timing marks added over the years that do not help with reassembly.
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Old 04-01-2008, 06:54:50 PM
Steamcar Steamcar is offline
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Default Re: JD 720 Cranking engine will not start..Help

I've rechecked the valves, which is no easy task with the engine installed. The timing marks on the flywheel and housing are the JD ones. On my next day off, I will pull the carburetor and intake manifold and check the gaskets for air leaks. the main engine of the tractor is really strong and a pleasure to operate. I wish I could say the same for the cranking engine.
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Steamcar
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