Missing when fully warmed up

AA7TU · Jul 26, 2018

The old Ford 850 is now running with the Gyramor. The mower had never had anything but abuse in its life, so it took some fiddling, but now it's smooth and quiet.

The tractor has been running very well for the small jobs in cool air that I have been doing. I also mowed the field earlier this year in cool temperature without issue.

This afternoon was the first time I have run it with a load in warm air conditions. When I first start all is normal, but after about 15 minutes of heavy work in the heat, it starts to run rough and miss, and looses power. The first time it happened I turned the PTO off and started back to the shed, which ran through some cool shady air. About a minute of that and the engine suddenly started running normally again.

I let it sit for a bit, and returned for more mowing. The same thing repeated, normal for 15 or so minutes, then gradual loss of power and rough running. The temperature gauge is normal, about mid scale. Oil pressure OK.

I have not adjusted the carb at all yet, leaving well enough alone. Any suggestions as to what is going on?

I also made a counterweight for the hitch, from an old propane tank cut in half. Makes such a difference!

jaxs · Jul 26, 2018

What he ^^^said. To see if that's the problem,make sure the heat shield between valve cover and fuel tank hasn't been removed. Check to see if line from tank to carb is touching exhaust. Make sure that fan has a shroud around it. While engine is off you shouldn't be able to move fan blade with one finger (some insist belt be tight enough you can't move it with your hand). Make sure air filter is clean.

AA7TU · Jul 27, 2018

I was a bit amazed when I saw how the fuel line was threaded through the manifold, when I previously tore down and reassembled those components. I would have never imagined that was a good idea. The fuel line is not touching the exhaust manifold, but it certainly is close to it. I might be able to get a tiny bit more clearance by bending the line and re-tightening. But it does sound like an explanation.

The other aspects, heat shield, fan shroud, etc, are all OK. Plenty of air movement and radiator is clean. I'll tinker a bit more and report back.

jaxs · Jul 28, 2018

it's gravity feed with no pressure.you can splice the fuel line with fuel hose making adjustment much simpler.
Steel line on 75% of old tractors has been replaced with hose anyhow.

AA7TU · Jul 28, 2018

K7LN said:
I can't imagine that was OEM and I'd love to see a picture of it.

It is OEM, from the looks of it on the New Holland page. However the replacement part on Yesterdays Tractors is different and does not run that way. Here is how it looks at present.

AA7TU · Jul 28, 2018

jaxs said:
it's gravity feed with no pressure.you can splice the fuel line with fuel hose making adjustment much simpler.
Steel line on 75% of old tractors has been replaced with hose anyhow.

Probably the way to go eventually. I checked the pipe when I had it apart and it looked to be just fine.

I did the mowing this morning in cooler conditions and it ran much better, just some minor missing at times. I think I need to go over the carb and make sure it is adjusted properly while the engine is all the way warmed up.

jaxs · Jul 28, 2018

That look's oe. Something simple to try next time,loosen gas cap when it begin's running bad. That's to see if cap vent is stopped up. An easy test to see if something is causing fuel flow is slow down. With engine not running,set a fire extinguisher within arm's reach,hold a container below carburetor,remove drain plug from bottom of carburetor. 1 quart or more should run out in a steady stream. If stream slow's or stop's, something is stopped up or float is hanging.