McCulloch Sp125c Carb Question

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Homelitesuperxl

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Just bought a Sp125 and it has a McCulloch SDC carb, it will only run while the Choke is Partially on. Any ideas of what I need to do to get it run right. I think it's one of these Pressure Pulse carbs and I'm not too familiar with them. Does it just need a good cleaning or should I fool with the High and low adjustments.
 
Just bought a Sp125 and it has a McCulloch SDC carb, it will only run while the Choke is Partially on. Any ideas of what I need to do to get it run right. I think it's one of these Pressure Pulse carbs and I'm not too familiar with them. Does it just need a good cleaning or should I fool with the High and low adjustments.

Turn the low adjust counter clockwise until it runs good. This will probably be somewhere between 1/4-1/2 turn out. There is also an adjustment to raise and lower the idle. All it does is change the stop on the throttle to allow more or less air to enter the saw at idle. The H will change your top end. Don't run it too tight, you want to hear a "4 stroking" sound at wide open throttle.

If the low screw has not effect, the carb needs to be rebuilt.

These carbs are highly desirable on that saw, because most SP125's come with a Tillotson HS carb with a fixed H adjust.
 
I think Ambull really means 1-1/4 to 1-1/2 turns open. Good starting place for both H and L if you have the fully adjustable carburetor. Many saws were produced with a fixed H jet to keep the operators from running them too lean. There are both Walbro SDC and Tillotson HS carburetors that are fully adjustable, I have never seen a Tillotson with a fixed H but certainly have a few with the fixed jet SDC.

If you haven't done it yet, remove the carburetor, tear it completely down and give it a good cleaning, make sure the diaphragms are soft and pliable, and insure the metering lever is correctly adjusted. The Walbro link above (thanks Eccentric) will provide assistance if you are not familiar with these small carburetors.

If you still can't get it to run properly, pressure test the fuel line and impulse line, check the boot, and if that still doesn't get it pressure/vacuum test the crank case.

You don't want to make a mess of that saw from a too lean condition.

Of course you already did verify that you have good quality clean, fresh fuel in the tank and have replace the fuel filter to rule out the most obvious faults...

Mark
 
So both are a 2 jet carb? Is there more to the SDC model like a number after the letters?
 
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