Poulan Pro - How do I adjust mixture screws?

nepa · May 13, 2015

I bought a "spare parts" 42cc Poulan Pro that is only a couple of years old. I believe the only thing wrong with it is that it needs a new gas cap because the old one has swelled and was mutilated so air could not get in. Now that it is running, it idles way too fast even backing out the idle screw all the way. In addition, when I open the throttle fully, it dies. Looking at the mixture screws, they appear to be way backed out but they are not slotted and I see no way to turn them in or out. Does anyone know what tool I use to adjust the mix screws and any basic procedure?

Giles · May 13, 2015

I have removed them by tapping a thin wall copper or plastic over the heads. After removal You can grind or saw a screwdriver slot.
It sounds like you might have an air leak--intake boot or crank seals--- Screwing the air mixture screws out, richens the mixture.

CobyRupert · May 13, 2015

On most engines, I think the procedure is close to the following. Others commenters may want to refine some of this.

Low Speed screw: Turn clockwise (CW) until screw is seated/stops. Back off (turn CCW) screw about 1 turns. Start engine and let in warm. Adjust screw CW or CCW to where engine idles at fastest speed (about 1/2 way between where the engine starts to die because it's too lean when turning it CW, and too rich when turning CCW).

High speed screw: Turn clockwise (CW) until screw is seated/stops. Back off (turn CCW) screw about 1 to 1-1/2 turns. Run engine. Rev engine to full throttle for short durations. Adjust screw so that engine at high rev's has a sound referred to a "4 cycling". Hard to explain. If you have speakers: http://www.madsens1.com/sethi.wav
As the internet says: "After revving it up to max RPM and holding the trigger, we adjust the high speed rich (CCW), causing it to slow down and blubber. Then we adjust it lean (CW), causing it to sound smoother, but then it starts to starve for fuel. As we back it off from lean to rich again, the sound becomes slightly rougher. This slightly rough sound (or 4-cycle sound, as it's sometimes referred to) is where you want it to be. We like to say, as "close" to the smooth as you can get it, but still just slightly into the rough."
When adjusting using sound rather than a tach, I'd rather err on the rich side rather than the lean side or it will run hot and burn the piston.

Giles · May 13, 2015

CobyRupert said:
On most engines, I think the procedure is close to the following. Others commenters may want to refine some of this.

Low Speed screw: Turn clockwise (CW) until screw is seated/stops. Back off (turn CCW) screw about 1 turns. Start engine and let in warm. Adjust screw CW or CCW to where engine idles at fastest speed (about 1/2 way between where the engine starts to die because it's too lean when turning it CW, and too rich when turning CCW).

High speed screw: Turn clockwise (CW) until screw is seated/stops. Back off (turn CCW) screw about 1 to 1-1/2 turns. Run engine. Rev engine to full throttle for short durations. Adjust screw so that engine at high rev's has a sound referred to a "4 cycling". Hard to explain. If you have speakers: http://www.madsens1.com/sethi.wav
As the internet says: "After revving it up to max RPM and holding the trigger, we adjust the high speed rich (CCW), causing it to slow down and blubber. Then we adjust it lean (CW), causing it to sound smoother, but then it starts to starve for fuel. As we back it off from lean to rich again, the sound becomes slightly rougher. This slightly rough sound (or 4-cycle sound, as it's sometimes referred to) is where you want it to be. We like to say, as "close" to the smooth as you can get it, but still just slightly into the rough."
When adjusting using sound rather than a tach, I'd rather err on the rich side rather than the lean side or it will run hot and burn the piston.

Very good instructions--only thing I would add is that I don't like it to "four stroke" in a heavy cut.

JerryK · May 13, 2015

If you have access to an electrician, or someone that does electrical work... ask for a 14 gauge or 10 gauge barrel [ butt ] crimp connector. Can't remember which one fits, but you just crimp one on the end of a solid heavy wire you can put a loop into for a handle... The butt connector is just pushed onto the adjustment screw, sort of a jamb fit. Cheap, works great...

MossRoad · May 13, 2015

nepa said:
I bought a "spare parts" 42cc Poulan Pro that is only a couple of years old. I believe the only thing wrong with it is that it needs a new gas cap because the old one has swelled and was mutilated so air could not get in. Now that it is running, it idles way too fast even backing out the idle screw all the way. In addition, when I open the throttle fully, it dies. Looking at the mixture screws, they appear to be way backed out but they are not slotted and I see no way to turn them in or out. Does anyone know what tool I use to adjust the mix screws and any basic procedure?

I think the fuel cap is supposed to be sealed and not let air in. I also think there's a one-way tank vent in the top of the tank. You have to remove some things to find it, as I recall.

You can buy special bits to adjust those carbs on-line, but in reality, they should never need adjusting. Try remedying the fuel vent first before adjusting anything.

Gary Fowler · May 13, 2015

JerryK said:
If you have access to an electrician, or someone that does electrical work... ask for a 14 gauge or 10 gauge barrel [ butt ] crimp connector. Can't remember which one fits, but you just crimp one on the end of a solid heavy wire you can put a loop into for a handle... The butt connector is just pushed onto the adjustment screw, sort of a jamb fit. Cheap, works great...

OR you can just buy the tool from a repair shop. I wont mention the shop name because they said they weren't supposed to sell them. I think it was about $12. If I had thought about the butt splice thing, I could have saved some money.

JerryK · May 13, 2015

Gary Fowler said:
OR you can just buy the tool from a repair shop. I wont mention the shop name because they said they weren't supposed to sell them. I think it was about $12. If I had thought about the butt splice thing, I could have saved some money.

Life is not always fair...... It's the windshield/bug thing....

torqotrac · May 13, 2015

I have seen the tools on line. The EPA apparently warns shops about selling them. I guess your screws are different than the D shaped ones I have on my Ryobi and Crafstman. I used a tire valve brass tube. Heavy wire brush off the rubber. Then light tap end to create "D." If the unit is working properly it should be very sensitive to the adjustments mentioned above. When I bought my latest Craftsman it would not idle nor run top end. Both adjustment screws turned slightly and good.