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John Deere 318 B43G Surging and Other Very Strange Behavior

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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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Bored,

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...

Chuck
Hey Chuck, thank you for the response. I will do my best to answer all of your questions.

The surge occurs at all speeds, both idle and open throttle. Once the engine has been running for a little while and gotten sufficiently hot is when the surging begins. The surging isnt really like a racing to higher RPM, more like falling to much lower RPM and then "surging" back up to the RPM its supposed to be at. I dont remember for sure what the throttle was at when I did the compression test, which most likely means I had the throttle at the lowest it could go, I definitely had the choke wide open though as I rarely have to use choke to start the engine. Would it make much of a difference in compression if the throttle was set low?

As for points, you are right and I made a typo (more like I misread my feeler gauge lol). I just went out and set the points to 0.020 and the tractor ran great and mowed for about 40ish minutes before the surging started, it even stalled out and died, and had to be restarted before I could ride it back to the barn. I took the opportunity to check the ignition coil again and it looks like it may just barely be out of spec. When testing the spark plug connection I got 16500 ohms and when testing the -/+ terminals I got 5.1 ohms. According to this, that resistance is too high and out spec: https://www.justanswer.com/small-engine/4d18c-model-b43g-jd-318-test-ignition-coil-replaced.html but its really close to being in range so im not sure if that would cause this type of issue.

For a valve adjustment, will that require taking apart the engine? I am not super familiar with that process but I know there are plenty of videos out there so I can probably find some resources to make that a bit easier. Thanks again
Bored,

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.

View attachment 858997

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...

Chuck
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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Chuck
My 318 has a surge on occasion, I found if I blow carb cleaner into the hole you see looking down into the carb, it will blow small black chunks out the top vent, and then run great. I assume my fuel line is deteriorating.
surging, governor spring or the plastic flap which catches the air off of the flywheel gets plugged with grass.
set points correctly and REPLACE the capacitor with a new one, highly overlooked is a bad capacitor.
Yup replaced the points and condenser at the same time, seemed to have helped. When you say plastic flap which catches the air off the flywheel are you talking about the small 3inch diameter tube that sits next to the air filter? Or are you talking about something else?

Hopefully I will have some time to look into the valve adjustment this weekend. Would be great if its as simple as adjusting those to be in spec
surging, governor spring or the plastic flap which catches the air off of the flywheel gets plugged with grass.
set points correctly and REPLACE the capacitor with a new one, highly overlooked is a bad capacitor.
If you adjust the valves, you'll likely pull the intake off again for access. The points where the intake attaches to the head are a notorious trouble spot for air leaks causing a surge condition. New gaskets here are not enough in my experience. Clean the area thouroughly and put a generous bead of black high temp silicone all the way around both intake attachment points (along with new gaskets). Sometimes, after so may years those intake surfaces are no longer flat, and it's hard to resurface that. The silicone sealant overcomes this. Made a huge difference on my 318/B43G.
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