440 back firing

[JAJ]

About a month ago I started my 440 gas engine and it started right up as usual. While moving it around inside my storage shed it started to back fire, flames shooting up out of muffler about blew out my hearing aids!! Blew the flipper clean off the muffler!! OK, it’s time to do a tune up. JD dealer had all the parts > cap, rotor, points, condenser and AC spark plugs. I do the tune up per the operator manual. Started right up ran great, drove it out of the shed ran it about 20 minutes and parked it back in the shed. All was well!! Yesterday it started great ran about 10 minutes and started the back firing routine again!! Same ole carp! Checked the new cap for a crack > nothing visible. Looked at the points, still like new but I reset gap .022”, plugs look like new (they are new).
Rotor looked great. Reset timing, I do not know if it was off time or not I just set it per the manual again. In hind sight, I wish I knew if it had somehow jumped timing. It did start back up but was back firing. While it was still running I loosened the distributor hold done bolts and attempted to get it to stop back firing. No minute rotation of the distributor made any difference. It finally died and wouldn’t restart. I’m about ready to hop in the truck and head back to the dealer and get all new components again but thought maybe you guys had a trick up your sleeve. I don’t know much about the timing mechanisms in this engine but could there be a chain or gear issue causing my ignition to backfire? And how would I check that out?
Thank you for your help.

[Jim B]

How much do you run it? Possibly a valve(s) sticking from setting?

[JAJ]

Thank you for the quick reply. I start it and drive it to exercise the steering clutches every two months. Probably runs a total of 30 minutes during these exercise periods. It has not done any work in about two years. You may be on to something with the sticking valve. What is the best method to verify and rectify this problem?
Would a compression test be a good place to start?
Thank you

[pop pop]

i think a sticking valve would appear right away, not after running for 20 minutes.
what about condensation under the dist. cap.?

[Lavoy]

Timing can not jump on these barring a mechanical failure of some sort. It is likelly an ignition issue, possible faulty piece, etc. Check the timing with a trouble light, then you know for sure on timing. Check all connections, check polarity on coil and anything else you can think of.
Then for kicks and grins, pull the sediment bowl off, and check fuel flow there, check for water in fuel, etc. Drain the carb, check, for adequate fuel flow out the carb drain. Reset carb per specs, then run it again.
If it still acts up, it is ignition, did you replace the coil? Are you running it on 12 volts without a resistor? Keep looking, you will find it.
Lavoy

[JAJ]

Thank you guys for all the helpful suggestions. I will get back to you when I get this problem figured out. JAJ

[original possum]

Does it backfire at idle or low rpms or just when throttle is opened?

[JAJ]

The back firing takes place no matter where the throttle is set. Last time it ran (3 days ago) I tried to reset the timing while it was running and it didn’t matter where the throttle was set or what I did with the distributor rotation it just kept back firing. Then it stalled and won’t restart. I will get all new ignition components to include new spark plug and coil wires plus a new coil. Go through the tune up process “again”. This time I’ll use my timing light where as the last times I have set the timing per the owners manual only. I’ll initially still set it by the book and once it’s running check myself with the timing light. Can’t remember if my 50 year old timing light will work on 6 volts or not (been awhile) we’ll see. I pretty sure I use to time my old 1946 Dodge PU with light.
Speaking of 6 volts, what coil should I be getting 6 or 12 volt??
Currently the machine is stock 6 volt positive ground. Not sure what the existing coil is because there’s no ID on it.

[fundytides]

Coil should be 6 volt. Make sure that centrifugal advance in distributor is not stuck and will return ok if you turn the rotor against direction of rotation and then let it go. It should return with a definite click. Could be stuck or have broken/weak springs.

[JAJ]

Good plan. I’ll check out the advance mechanism while I’m at it.
Thank you