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M670 shuts down under load

B

BennettMachine

New User
Hi Everyone,
I have an M670 that starts to misfire and backfire while I'm pulling a large mower. The tractor will sometimes run great under the load and then other times it acts as though it is running out of fuel. I have made sure the fuel tank is clean, I have a new fuel filter, I have new fuel, I have new spark plugs, the air cleaner is clean, I installed a new ignition coil, I adjusted the carburetor according to the manual for the tractor. The tractor runs fantastic when I am not pulling something. Any ideas or thoughts would be much appreciated.
 
Like the others said, you may have a fuel problem.

I do recall a U once that would spit and pop under load and it had an eroded valve seat. For some reason it sounded o.k. at idle. I guess it just didn't miss the bad cylinder until it had a load on it.

If you haven't checked the valve adjustment, you might want to try that first.
 
There might be a piece of dirt floating around in the carb. If I was positive that I was always getting a good stream of fuel to the carb, I'd take that carb apart and give it a good cleaning and make sure all the little holes are open. something seems to be restricting the flow of fuel when it needs more under load.
 
(quoted from post at 19:47:20 04/22/16) I will go first to bat dirt in the carb somewhere or a bad condencer
Click to expand...

I took the carburetor off and cleaned it thoroughly, I don't think it is dirt in the carb. It could be the condenser, I will try that. Thanks
 
(quoted from post at 21:40:02 04/22/16) Like the others said, you may have a fuel problem.

I do recall a U once that would spit and pop under load and it had an eroded valve seat. For some reason it sounded o.k. at idle. I guess it just didn't miss the bad cylinder until it had a load on it.

If you haven't checked the valve adjustment, you might want to try that first.
Click to expand...

That's a good thought. I never had a problem with this tractor until recently. Do you think the valve adjustment could be something that would change after it ran so well for years?
 
(quoted from post at 22:37:50 04/22/16) could be either fuel problem or weak spark. Have you checked your points and gap?
Click to expand...

I will check the points, that a good idea. Right now I'm leaning to a spark issue. I feel like I have exhausted the fuel issue. I even went as far to install a water separator on the fuel line.
 
We did have an Oliver 770 that had a bad ignition switch. It would pop and backfire
something terrible. Im guessing it ended up having "extra" spark being the reason. Replaced
the switch and it worked fine. Had the same thing a few years later when I worked in town at
a shop. Had a 4020 that had the same problems as our 770. It had been to the IH shop and
then to our shop (independent). The IH place had replaced the common parts, points, cap,
plugs, wires etc. The boss was checking the distributor and a couple of the other parts.
Still had the same problem. I told him it might be the ignition switch. He looked at me
funny. Turned out it was.
 
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