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John Deere 318 B43G Surging and Other Very Strange Behavior

882 Views 7 Replies 5 Participants Last post by  Yooper3039
Hi everyone, this is my first post here as I have just recently purchased an '85 JD 318. When it's running the way it should, it really runs great. But the problem is that it seems to want to surge after a short while of running. The carb is clean, the air intake manifold was very recently resealed, the carb gets plenty of fuel (maybe too much?) and there is spark. I've run into a couple issues with the points and had to replace the unit. I've tested the ignition coil with a multimeter and it stays within spec hot and cold. Fuel lines are all brand new and tank is freshly cleaned. All new filters as well.

Here are a couple of the really weird behaviors I've noticed: the engine runs rich even with the carb set to run as lean as possible. Also, I spent some time messing with the point gap and found that if the engine was surging set to the standard .16, it would stop surging completely if I set the point gap way too high like .3 for example. Audibly you can tell the engine is running rough when the points are at .3 but it doesnt surge at all. Maybe there is an obvious reason for that, but I'm very new to this and not much of an engine guy.

It's worth mentioning that compression is at 60-65 PSI for both sides (I know this is on the low end according to official specs but is it enough to cause this weird behavior?). Not sure what else to add off the top of my head but if you need/want any more info I'm happy to answer.

Thanks in advance for any help in diagnosing this weird behavior
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Does this "surge" occur only at idle speeds or is it present over the full range of engine operation? Is the surge a variation of engine speed and recurring/cycling, or is it a racing to a very high speed?

Yes the low compression could be a factor. When you did the compression test, did you use the procedure shown in the TM1590 manual? Since you have points, your engine is a B43G where the minimum compression would be 100 PSI per cylinder... After warming the engine to normal temp, remove both plugs and crank to read compression on the test gauge, ensuring the choke and throttle are both wide open while cranking.

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Setting the points too wide (0.030) does change the timing a bit -- and I hope the numbers you quoted above are typos...

Because the symptom seems to be a bit related to coming up to temperature, you might want to ensure the valve adjustment is correct -- if the lash is too tight, it could both cause low compression and the lack of smooth running.

Let us know what you find...

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Thanks for the added detail -- very helpful!

I don't think the minor variation you see in the coil resistance is meaningful...remember the spec is at room temperature, so if it warms up substantially expect some change to be evident.

The valves can be adjusted without taking the engine apart or even out of the tractor -- but you will need some knowledge of how the valves should be adjusted with illustrations as found in the engine manual CTM2. The below illustration shows the head off the cylinder, but the adjustment can be done without removing the heads as long as you verify the proper TDC on compression stroke. Having the engine out of the tractor is optional but removing the carburetor and intake manifold will be required to have access.
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