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Curtis B.
Curtis B., Technician
Category: Small Engine
Satisfied Customers: 13581
Experience:  have worked on and around most engine models for 35 yrs./Polaris ATV expert
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Mcculloch MS 1839 chainsaw. Walbro carb #671. I have taken

Customer Question

Mcculloch MS 1839 chainsaw. Walbro carb #671. I have taken carb apart without removing high / low speed screws. I dipped carb and components overnight. Then used ultrasonic jewrly cleaner with simple green degreaser and hot water. I did this for a couple cleaning cycles. I installed a rebuild kit for carb. Gaskets, diaphrams, fuel jet needle, lever, rod. And yes I double checked , gaskets first , then diaphragm. Now, fuel lines are intact. Fuel filter to carb, fuel tank to primer bulb, primer bulb to carb. The muffler is not clogged. I opened muffler and checked, cleaned spark arrestor screen. There was no blockage but I cleaned anyway. The gasket inbetween carb and engine is intact. I have run the saw with and without the fuel filter with no change. One thing I have noticed now is that the fuel tank cap hisses and spits some fuel every time I open it. I understand this to be a vacuum. This model chainsaw does not have a vented fuel cap. there is a vent on the tank itself with a check valve. I am unfamiliar with removing this type of vent. I have two of these saws. Same model, size, engine. After my carb cleanings / rebuilds and trouble shooting they both now wont run at high speed. They just start easy, idle fine and die when I hit the throttle.
Submitted: 2 years ago.
Category: Small Engine
Expert:  Vince O. replied 2 years ago.
Hello,did you move the carb adjustment screws? Did you remove the muffler to check the exhaust port? On the pump side of the carb where the screen is the diaphram goes on carb first. On the metering side the gasket goes on carb first.
Expert:  Vince O. replied 2 years ago.
Did you try opening the low speed adjustment screw?
Customer: replied 2 years ago.
I did not touch adjustment screws untill after reassembling the saw. Only when i could not adjust high speed at all. And i always turned each screw in all the way and started fromm 1 1/2
Turns back out from bottoming out the screw
Customer: replied 2 years ago.
I took muffler off. Exhaust port and spark arrestor screen clear
Expert:  Vince O. replied 2 years ago.
Is the dia and gasket on correctly on the pump side? Try opening the low screw a 1/4 turn
Expert:  Vince O. replied 2 years ago.
Start the saw and open the low speed screw(closest to the engine) so it will run without the choke on.Squeeze the trigger and if it hesitates or wants to stall open low speed a hair more.Run on high speed and fine tune adjustment,after it runs good open high speed screw a hair so it doesn't run too lean.
Customer: replied 2 years ago.
All these adjustments are starting from 1 1/2 turns out from bottoming. Correct? And your saying to basically adjust high speed with low screw, then fine tune with the high screw?
Expert:  Vince O. replied 2 years ago.
Open both screws 2 turns.the transition from low speed to high speed draws most of the fuel from the 3 low speed passages
Expert:  Vince O. replied 2 years ago.
Time for me to sign off.
Customer: replied 2 years ago.
Thank you.
Customer: replied 2 years ago.
Hi, this is Rob again. I just wanted to let you know i did try to adjust mixture screws like you said. But , no luck. The chainsaw will rev up to speed only by turning the low screw out so far that the engine will stall at idle. And when i try to fine tune with high screw , the engine will either bog down or race and overrun.
Customer: replied 2 years ago.
I did not remove mixture screws when i cleaned carb the first time. I have started all over again now. I removed carb again. This time i completely disassembled the carb. Butterflys, throttle shafts, mixture screws, welch plug, filter screen ect. Eill contact you again after reassembling and running the engine again.
Expert:  Vince O. replied 2 years ago.
Ok,You still have a fuel restriction if lo needs to be open that many turns