Bouncing RPMs

intrepid31 · May 25, 2015

Ok, I have a 1964 John Deere 1010U Tractor with the 540/1000 PTO. Here's my problem after a couple of hours of mowing the RPMs start bouncing from approx. 2200 to 1600. The tractor runs great before this and will run fine for about 30 to 45 minutes if I shut it down and let it sit 10 to 15 minutes after it starts acting up. If the tractor sits for a week then it will run fine for a couple of hours and then start acting up again. I have changed the cap, rotor, plugs and plug wires. The spark plugs are gapped correctly and are burning correctly (according to the plug burn page of my car manual). The temp gauge says that the tractor is only getting to about 170-175 degrees. I took the carb off last summer and cleaned and rebuilt it. I have also changed a the fluids and filters. I have not found any water in the fuel and as best as I can tell there does not seem to be any crud the fuel tank or system. BTW disengaging the PTO seems to have little or no effect on this issue.

thank you in advance for any ideas or help with this.

old · May 25, 2015

The moment it starts doing that check you spark and make sure it is a good blue/white in color and jumps a 1/4 inch gap or more. If you have that try removing the gas cap and see if maybe you have boiling gas or a suction in the tank either one will cause that

Steve@Advance · May 25, 2015

That it runs 2 hours before acting up sounds like it's heat related.

The RPM surge is commonly a symptom of too lean. Next time it does it, try partially choking it. If that
improves it, look for fuel delivery problems, like tank not vented, or vapor lock from the fuel line too
close to the exhaust manifold. If the line has been replaced with copper, go back to steel. Steel fuel line
is less likely to vapor lock than copper, less conductive.

If choking makes it worse or no improvement, possibly the coil is overheating or needs replacing. If the coil
has been changed, is it the correct coil or coil/resistor combo? Need 3 ohm resistance across the coil
(resistor included if so equipped).

intrepid31 · May 28, 2015

So ran the tractor yesterday and when it started acting up I partially choking it which did improves its
running, so it looks like it might be a fuel delivery problem. Also I checked the fuel line and it is steel
but it run really close to the exhaust manifold, I would say at that point there is less than a 1/2" of space
between the line and the manifold. Will be working on these today.

Bouncing RPMs

intrepid31

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old

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Steve@Advance

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intrepid31

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