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Farmall & IHC Tractors Discussion Forum

300 Carb settings

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01-07-2018 16:12:08

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I rebuilt the carb for my Farmall 300 and finally got the float to not leak. The problem I am having now is I have to set the idle screw in all the way. If I get the tractor down to 425 RPM I have to screw the load needle in to get the tractor to quit smoking and to run like a sewing machine. At this setting though, the tractor is too lean for acceleration and idle. For now I am dealing with the smoke at low idle by not idling the tractor that low. The throttle shaft had a few thousandths of play but I would think if there was a vacuum leak from the shaft I would need the idle screw out further to get more fuel in, right? It was cold here (10-15 degrees) when I tried this, could the oil in the oil bath be too thick causing an increased vacuum and pulling more fuel through the jets? I will be getting warmer this week so I will try again.

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01-08-2018 20:09:01

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 Re: 300 Carb settings in reply to Bkpigs, 01-07-2018 16:12:08  
Finally got a chance to play with it for a few minutes. I removed the oil bath and it made no difference. But when I went to shut the tractor off, instead of turning the fuel off first I just shut it down. About four or five drops of fuel came out of the drain. That tells me the float is set too high or there is another restriction in the air flow. It quit dripping after that so the float needle should be seating. Think I am on the right track?

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01-08-2018 13:22:53

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 Re: 300 Carb settings in reply to Bkpigs, 01-07-2018 16:12:08  
I looked up the idle jet on McDonald Carb and that is the one that I cleaned and put back in. The adjustment on this style carb is in for lean, if I adjust it out it really smokes.

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01-08-2018 12:44:28

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 Re: 300 Carb settings in reply to Bkpigs, 01-07-2018 16:12:08  
I adjusted the float per instructions from McDonald Carb. Could someone post a picture of the idle jet? The kit came with a new main jet (long tube with 6 holes) and I replaced another jet that had a slot in the top of it that screwed into the bowl assembly.

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pete 23

01-09-2018 16:50:14

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 Re: 300 Carb settings in reply to Bkpigs, 01-08-2018 12:44:28  
The jet with a slot in it that screws into lower body is the idle jet. It should have a hole the size of .072 or .075 drill bit. That is right about .022 so is very small. So, it sounds like you have the proper jet unless it was boogered up . Like I said before, you need to look inside of the carb air inlet when running to determine if fuel is coming out of the long nozzle with several hole you replaced. If it is, then the float side of carburetor is the problem. If no fuel coming from that nozzle, it is in the idle circuit. All fuel to idle circuit comes through that little hole in that slotted jet unless you have passage in bowl gasket or damaged threads where jet screws into. If that hole in idle jet is oversized for any reason, you will experience the problem you have. A high float setting allowing fuel to be drawn out of the main nozzle , or fuel leaking under the nozzle gasket will also add fuel to idle circuit.

If you pin it down to being in idle circuit, you will need to double check all those small hole in carb throat above the butter fly as that is where the air is coming in to idle circuit.

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pete 23

01-08-2018 11:05:57

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 Re: 300 Carb settings in reply to Bkpigs, 01-07-2018 16:12:08  
Yes, a 300 idle mixture adjustment is up high on the throat and any that are made that way, you screw them in to lean the mixture. I would suspect the idle jet, that long one, is MISSING, OR BROKEN OFF. If you are not familiar with those carburetors you may very well not know if it is missing. Other wise, I would look in the throat of the carburetor when running and see if any gasoline is coming out of the main nozzle. If not, idle circuit is the problem. If gasoline is coming out of nozzle, other problems like float too high or gasket problem.

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01-08-2018 04:28:54

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 Re: 300 Carb settings in reply to Bkpigs, 01-07-2018 16:12:08  
If it's the idle air adjustment, you're turning it the wrong way. Turn it OUT to lean the idle mixture.

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D Slater

01-08-2018 07:13:31

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 Re: 300 Carb settings in reply to BarnyardEngineering, 01-08-2018 04:28:54  
third party image

My memory is getting so that kind of afraid to disagree with posts. But going to stick my neck out and say Leroy and you are thinking of previous carburetors like a SH. Original 300 carburetor for a gas with idle mixture screw in the flange like picture adjusts leaner turned in. 361525R92 carburetor looks the same as the 400, 450 in picture.

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D Slater

01-07-2018 19:03:21

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 Re: 300 Carb settings in reply to Bkpigs, 01-07-2018 16:12:08  
Carburetors can be switched so it would help to know if it's a number 361525R92 carburetor. If so you still have a fuel problem, float to high, not holding, wrong jets or drilled or a leakage somewhere. That Carburetor should lean the fuel mixture way down with idle mixture screw when screwed in or quit if idling slow enough. Is the idle jet intact? Carburetors can ice up in the cold also or thick oil or stopped up air cleaner. Ignition needs to be good enough shape to fire the fuel mixture also.

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leroy in ne

01-07-2018 17:01:47

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 Re: 300 Carb settings in reply to Bkpigs, 01-07-2018 16:12:08  
On most carbs the idle screw is also known as idle air adjustment. It allows more air in as it is adjusted out, it does not adjust fuel, as it is adjusted out it allows more air to bypass the throttle plate.

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