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Ford Tractors Discussion Forum

641 runs great but won't idle. Linkage and adjustment ?'s

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Author  [Modern View]

02-23-2018 06:42:07

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Hey guys. I just brought an old 641D back to life with a nice gas engine conversion. The engine seems to have had a very recent rebuild, I was shocked to see how clean the inside of it was, just a film of carbon in the exhaust ports, and shiny gray iron in the intakes, no tar on the valve stems, clean piston domes. New gaskets throughout, new ignition setup, unburned never seize on manifold studs. All pretty big surprises for an engine that was full of acorns, exterior surface rusted like it was parked when I was a little kid. So it runs great!

I was surprised to read a few things while trying to tune my Schebler Marvel carb though (I just rebuilt that, and have experience doing carbs). Before reading, I set the idle and main mixture screws to 2.5 rotations out, a good baseline for many carbs. It started and ran great like that, but low end was 1200 rpm. I adjusted the throttle linkage (alot), and got it to drop down to about 900 rpm. The lowest I've been able to drop it is 600, and it runs awful, threatening to die, but I guess my goal is 4-500, from what I've read. It also likes to run rich. The idle mixture screw is best set between bottomed out, and one turn out (bottom is rich on these carbs), and the main mixture likes to be 2-2.5 turns out, when I read it should be near 1. I guess that all screams vacuum leak, right? But with such a simple intake system, that's hard for me to believe. It all looks like it's making good contact.
The throttle linkage also required dramatic adjustment just to allow the idle speed screw on the throttle arm to make contact with the pin, and I'm not confident that I have the spring tension where it should be related to the (governor??) arm lever at the front of the engine. Is there a baseline for setting all of that?

Maybe my ignition timing is off, messing up everything else? That's a matter of twisting the distributor I assume?

Thanks for any help, Alex

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Dollar Bill

02-24-2018 04:02:12

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 Re: 641 runs great but won't idle. Linkage and adjustment ?' in reply to the_workmaster, 02-23-2018 06:42:07  
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Governor and throttle linkages are different between diesel and gas. If you did not make the conversion, it would be worth your time to ensure you have all of the gas linkages. Also, you can disconnect the governor while sorting out carb idle problems.
To verify governor rod compensator linkage: Bring engine up to operating temperature and adjust engine idle speed to 450 RPM. At idle speed, the tip of throttle rod end assembly should be against governor arm with throttle lever fully forward. If there is a gap, adjust compensator spring assembly as necessary.

Engine speed range for a 4 speed tractor is 450-2250 RPM.

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02-23-2018 20:14:14

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 Re: 641 runs great but won't idle. Linkage and adjustment ?' in reply to the_workmaster, 02-23-2018 06:42:07  
You can squirt a little starting fluid on the throttle shaft and choke shaft to check for leaks.

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02-23-2018 19:05:24

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 Re: 641 runs great but won't idle. Linkage and adjustment ?' in reply to the_workmaster, 02-23-2018 06:42:07  
Yes, I removed the throttle plate, spun in the screws and then wiggled it while closed before snugging them down, so the position is right. Replaced the shaft seals when I was there.
What do you mean the governor shuts the throttle?

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02-24-2018 21:02:29

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 Re: 641 runs great but won't idle. Linkage and adjustment ?' in reply to the_workmaster, 02-23-2018 19:05:24  
Spring tension opens the throttle the governor pushes it shut. I most cases with a governor the carb is wide open until the engine starts.

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02-23-2018 07:01:47

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 Re: 641 runs great but won't idle. Linkage and adjustment ?'s in reply to the_workmaster, 02-23-2018 06:42:07  
Look for a vacuum leak. Did you remove the throttle plate in the rebuild? If you did you need to make sure the plate was sealing good before tighting the screws to the throttle shaft. If you didn't remove the shaft check the shaft seals look for vacuum leaks there. Remember the governor shuts the throttle.

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