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Harry Ferguson Tractors Discussion Forum
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Carb idle adjustment

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Don (Co.)

11-11-2004 08:05:58




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I can screw my idle adjustment all the way in and the engine instead of dying runs perfect. Now you would think intake manifold leak. Doesnt look or sound like one. Did that old trick of squirting oil around it and saw nothing nor heard a change in engine idle speed. Bought a new (not rebuilt but new) carb. and it does the same thing..Anybody have an idea of whats going on here ?




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Sodbuster

11-13-2004 05:08:13




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 Re: Carb idle adjustment in reply to Don (Co.), 11-11-2004 08:05:58  
If after checking linkages etc. and the tractor will still not shut down with the mixture screw screwed all the way in, do a vacuum check. It is a great tool. Should be a stud in the intake manifold where you can put a fitting for the check. You may also try disconnecting the air tube from the air filter system at the carburator. While the engine is running and the screw all the way in, plug the air inlet to the carb with a wadded up wrag or something similar. If engine does not shut down, you have a vacuum leak somewhere from the carb to the intake. I have a T030 and it will shut down if screw is all the way in. Don't know if this will help or not but, I have had problems with the same thing with my 55 and 57 chevy's. Can be aggravating. Good luck..... ..... ..

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John (UK)

11-12-2004 11:23:07




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 Re: Carb idle adjustment in reply to Don (Co.), 11-11-2004 08:05:58  
The settings for that carb according to the factory Service Manual are as follows:- Main Jet 1-1/8 turns open, the jet should be #130
Slow running Jet 2-1/2 turns open and jet should be # 115. The Choke tube should be 5/8"(16mm)dia.



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John (UK)

11-11-2004 13:15:22




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 Re: Carb idle adjustment in reply to Don (Co.), 11-11-2004 08:05:58  
As you have already been advised by gshadel you need to set the idle speed to 450rpm and see what happens, if it is running faster than this the screw wont make much difference. You dont say what your tractor and engine and carb is so cant advise you further unless you say.



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Gerald J.

11-11-2004 11:33:59




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 Re: Carb idle adjustment in reply to Don (Co.), 11-11-2004 08:05:58  
The governor can hide all effects of the idle screw adjustment. Just if the engine was to slow down the governor opens the throttle and you hear no difference.

There are two ways to overcome that. One is to observe the throttle shaft and adjust the idle mixture (and the high speed adjustment at a much higher engine speed) so that the governor closes the throttle the most.

The other way is to grab the throttle rod that comes from the governor an overpower the governor pulling the throttle closed against the idle speed screw. Then while holding the throttle closed, adjust the idle mixture for best running. You might want to back off on the idle speed screw to let the throttle close more to make the idle adjustment more of the mixture. As soon as the throttle plate opens, the carburetor starts taking fuel from the power circuit which obscures the idle mixture adjustment.

Sometimes you might find it easier to unhook the governor to throttle rod and hold just the throttle closed by hand. With no spring, vacuum will tend to pull it open.

Gerald J.

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gshadel

11-11-2004 09:22:02




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 Re: Carb idle adjustment in reply to Don (Co.), 11-11-2004 08:05:58  
That idle mix screw only comes in to play if the throttle is basically all the way closed, so your RPMs are ~450 RPMS or so. If your throttle is open at all, that idle mix won't affect much. Might want to check your linkage adjustments to the carb, and the idle speed screw to make sure they are letting the throttle close all the way.

Some needles vary gas, some air, I can't recall right now which one that idle mix adjusts, it might be bass-ackwards from normal, and screwing-in makes the mixture lean... don't remember on that one right now.
I think the manual just says idle ~450 RPM and adjust the screw until the engine runs the best.
I rebuilt my carb about a year ago. I recall that idle mix screw didn't seem to have a big impact, I could adjust it a full turn either way and didn't seem to hear any difference in the way the engine ran. You might not have a problem. That big power screw makes a difference how your tractor runs under load and a 1/4 turn on that guy makes a noticable difference.
Good Luck

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Don(co.)

11-12-2004 06:18:13




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 Re: Carb idle adjustment in reply to gshadel, 11-11-2004 09:22:02  
Oh I forgot to add. If I open the idle screw up say l/2 to a full turn the engine runs rough. As I said it runs its best closed completely.



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Don(Co.)

11-12-2004 06:12:27




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 Re: Carb idle adjustment in reply to gshadel, 11-11-2004 09:22:02  
First let me thank all of you who responded.
I have a Ferguson TO-30 with Marvel Shrebler (what ever) carb. The engine runs excellent its just that on any other carburated engine if you close the idle mixture screw it kills the engine.
I adjusted the rpm to 450.And even went lower just to see how low it would go so I could adjust the screw but their still was no difference. It ran its best at fully closed.
Thanks guys

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Ray,IN

11-11-2004 19:41:34




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 Re: Carb idle adjustment in reply to gshadel, 11-11-2004 09:22:02  
The idle screw adjusts the air mixture. The gas portion is preset. The main power gas mixture screw(large one) must be adjusted under load, such as brush cutting or plowing, to obtain the best engine performance. Never adjust the screw to less than one full turn or you risk engine damage. Usually 1 1/2 turns is close. Harry Ferguson tractors are designed to use the fuel mixture for additional engine cooling under load, this causes them to run rich at idle and putting around unattached to anything. It is not only a normal condition it is a designed condition.

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