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Rwj strange tuning

Dorlumb

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Hello all, this is my first post here. I bought a few 372xp’s parts boxes. So far I have built 2 saws. First one runs great with zero issues. I only had one carb so I scrounged up a rwj off an XT. I’m having a strange problem with the low end. Saw runs great on the top end and returns to idle just fine. Let it idle for 10 seconds and it leans out like it is on high idle. I get a momentary lean bog when reving off this idle and then quickly jumps to max rpm. I can fully tune the high side. Low is a bit more touchy but I can richen or lean it out to a certain extent. After a few turns out i can’t get any richer. No matter what low setting I still get the lean idle after 10 seconds. Saw runs normal and revs just fine until it hits the lean idle. It will continue run with the lean idle, it doesn’t idle up and die as if it were completely running out of fuel. I have tried spraying carb cleaner around the intake and base gasket with no change in the idle. I have gone up one size in all the jets which has calmed the high side but not changed much with the low. I removed the baffle from the carb and filter elbow. New fuel and impulse lines.
I grew up working in my parents small engine shop. Been tuning 2 strokes for almost 30 years and have never come across anything like this. I am determined to get this carb to work but I just can’t wrap my head around it and what I need to change.
Thanks all.
 
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huskyboy

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Maybe a bad nozzle? I’d press it out and try a new one.
 

Dorlumb

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Correct me if I’m wrong but doesn’t the nozzle only effect the high side?
 

Deets066

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An RWJ has a large Venturi, even more so with divider out. You may have to turn the low out much farther than normal.

Might not hurt to vac test the saw as well
 

Dorlumb

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After some research I think you may be onto something with the nozzle. If the check valve is bad it will allow air to bleed under the diaphragm. It will idle ok until the fuel is burned up then the extra air will cause a lean condition on the low side. Reving the engine will reverse the flow in the nozzle, fill the carb with fuel and run good until it starts sucking air at idle again. I am having a hard time finding a nozzle. I found the husqvarna number but I can’t find a walbro parts breakdown to find the walbro number. I would like to get some extra Welch plugs while I’m at it. Thanks
 

Dorlumb

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I pulled the zama off the other saw and put it on this one this morning. I just got back from cutting firewood for the last 7 hours with it. This is the first time it has been in wood since built. It has a 272 piston and a 52mm jug with the squish band cut and I have done quit a bit of port work on it. I am pleasantly impressed with the way it runs. I’m curious now if it will improve with a larger carb.
 
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