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Carb Questions 1974 St12

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#1 smcelhe OFFLINE  

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Posted September 24, 2012 - 08:34 PM

I posted questions about the ignition on an ST12 a couple months ago. The SSI was bad...replaced it with a used one and it went bad.....so my son sold the tractor.

He's been searching for another tractor for weeks.....and finally found one. A 1974 ST12....in real nice shape. We put fuel in it and it fired up. It runs well.....however....it wants to stall under a load and it will stall if you have it on half throttle and push it to full throttle fast. If you slowly increase the throttle.....you can get full throttle.

What do you think???? Rebuild the carb??? Fuel pump????

I've been trying to read stuff about rebuilding it....some people are saying not to remove the main jet.

What do's and don'ts do I need to know. And....where can I get a carb kit?

Thanks Shawn



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Posted September 24, 2012 - 09:14 PM

Assuming a couple of things here. The unit has been sitting for several months without running before you bought it & when the choke is part way on, the engine respond better. If so, I would do two things first.
1. Check for air leaks around the throttle shaft and the gaskets where the carb bolts on. Many guys use a can of carb cleaner and give it a whiff around these areas. If the engine changes pitch, there's at least one of the leaks. Fix any leaks that you find. If throttle shaft area, you may want to investigate bushing the worn area.

2. Next, I would run a tank of gas heavy with Seafoam or gum out cleaner.

If the choke partially on helps, open the main jet 1/2 turn until you have run some of the treated gas thru, then adjust for best operation.

#3 JDBrian OFFLINE  


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Posted September 25, 2012 - 04:17 AM

As Alan is indicating above it sounds like a lean condition. I would check the air cleaner as well. Sometimes they get saturated with oil if your engine has a PCV that vents back into the air cleaner. Check the governor linkage as a gummed up or poorly adjusted governor can also lead to slow response to changes in load. I usually give the engine in a new to me tractor a good going over and tune up. There can be multiple minor problems that end up causing you grief down the road. A weak spark can cause poor performance under load as well. I'm not familiar with those tractors but carb kits should not be hard to find if you know the engine model and type numbers.

#4 smcelhe OFFLINE  

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Posted September 25, 2012 - 05:09 AM

The mower sat for two years......I'll check the things mentioned above....and run Seafoam thru it.


#5 Hugh OFFLINE  



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Posted October 03, 2012 - 06:19 PM

Run high octane fuel and SeaFoam or another additive thru it. Marvel Mystery Oil is a good one too. Set your carb tuning to 1.5 turns out from seat on both idle and high speed screws and tune from there.