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2 cycle with catalyst - carb settings? #5444632 06/02/20 01:22 PM
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cjolson140 Offline OP
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Working on a husqvarna 128c weed trimmer, which I understand has a catalyst in the muffler. It has probably less than an hour or two runtime, but sat for too long with fuel so the carb gummed up. I replaced the carb with an amazon Zama clone, and got it running decent, but it runs SO HOT. Like, the engine keeps running with the plug wire pulled hot. My understanding of tuning 2 strokes is to bias to the rich side. Traditionally with chainsaws, weed trimmers etc I will set the high speed mix by ear; with the throttle wide open I’ll play with the high speed screw to find the highest RPM, and then richen until it just barely starts “four stroking”, and leave the setting just barely leaner than that to make sure it has enough fuel to not melt down.

In this case, even running rich like that, the machine seems to overheat, which makes sense from the fact it has a catalyst that is burning up all the unburned fuel in the exhaust. How do I find the happy setting between piston melting lean, and catalyst/muffler melting rich? It seems to overheat, ping, and diesel instead of shutting off regardless of how I set it. Have done a few plug chops and the plug always has a nice chocolate color, but with shiny spots& nicks from pinging. I have a bad feeling this is just the nature of the machine...

Re: 2 cycle with catalyst - carb settings? [Re: cjolson140] #5444641 06/02/20 01:37 PM
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IndyIan Offline
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The cat getting hot shouldn't really effect the combustion chamber temps though? The cooling fins aren't full of dauber wasp mud or something? My little saw with a cat had a heat shield between the cat and cooling fins and that cat/muffler would get pretty hot but no pinging. I usually run 93 octane though. I would tune it so the motor doesn't overheat and you can shut it off. Don't worry about the muffler/cat, it will be fine at whatever temp it gets to, and eventually it will quit working anyways.


06 Focus ZXW, 5spd manual, 220km
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Re: 2 cycle with catalyst - carb settings? [Re: cjolson140] #5444642 06/02/20 01:40 PM
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Dave9 Offline
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It's nuts to replace a Zama carb with a couple hours on it, with a Chinese clone. That's where I'd start. Clean it and new gaskets. Next I'd try a 30:1 oil mix so it lasts a while. That alone may help, though it will clog up the muffler cat if it has one. Next I'd gut the muffler. The cat can't be doing much on a 2 cycle except another cause for repair.

Last edited by Dave9; 06/02/20 01:42 PM.
Re: 2 cycle with catalyst - carb settings? [Re: cjolson140] #5444650 06/02/20 01:49 PM
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tundraotto Offline
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Take the muffler off, drill it full of holes inside and triple the exhaust size, and retune. Way more power (30%+), runs cooler. Those cats are only functioning for the first 10-20 hours of use....

Last edited by tundraotto; 06/02/20 01:51 PM. Reason: detail
Re: 2 cycle with catalyst - carb settings? [Re: cjolson140] #5444789 06/02/20 05:05 PM
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Brian123 Offline
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Originally Posted by cjolson140
Working on a husqvarna 128c weed trimmer, which I understand has a catalyst in the muffler. It has probably less than an hour or two runtime, but sat for too long with fuel so the carb gummed up. I replaced the carb with an amazon Zama clone, and got it running decent, but it runs SO HOT. Like, the engine keeps running with the plug wire pulled hot. My understanding of tuning 2 strokes is to bias to the rich side. Traditionally with chainsaws, weed trimmers etc I will set the high speed mix by ear; with the throttle wide open I’ll play with the high speed screw to find the highest RPM, and then richen until it just barely starts “four stroking”, and leave the setting just barely leaner than that to make sure it has enough fuel to not melt down.

In this case, even running rich like that, the machine seems to overheat, which makes sense from the fact it has a catalyst that is burning up all the unburned fuel in the exhaust. How do I find the happy setting between piston melting lean, and catalyst/muffler melting rich? It seems to overheat, ping, and diesel instead of shutting off regardless of how I set it. Have done a few plug chops and the plug always has a nice chocolate color, but with shiny spots& nicks from pinging. I have a bad feeling this is just the nature of the machine...


If you have the old carburetor soak it in MMO for like 12 hours or so, this should work.

Re: 2 cycle with catalyst - carb settings? [Re: Brian123] #5444792 06/02/20 05:10 PM
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tundraotto Offline
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Originally Posted by Brian123
Originally Posted by cjolson140
Working on a husqvarna 128c weed trimmer, which I understand has a catalyst in the muffler. It has probably less than an hour or two runtime, but sat for too long with fuel so the carb gummed up. I replaced the carb with an amazon Zama clone, and got it running decent, but it runs SO HOT. Like, the engine keeps running with the plug wire pulled hot. My understanding of tuning 2 strokes is to bias to the rich side. Traditionally with chainsaws, weed trimmers etc I will set the high speed mix by ear; with the throttle wide open I’ll play with the high speed screw to find the highest RPM, and then richen until it just barely starts “four stroking”, and leave the setting just barely leaner than that to make sure it has enough fuel to not melt down.

In this case, even running rich like that, the machine seems to overheat, which makes sense from the fact it has a catalyst that is burning up all the unburned fuel in the exhaust. How do I find the happy setting between piston melting lean, and catalyst/muffler melting rich? It seems to overheat, ping, and diesel instead of shutting off regardless of how I set it. Have done a few plug chops and the plug always has a nice chocolate color, but with shiny spots& nicks from pinging. I have a bad feeling this is just the nature of the machine...


If you have the old carburetor soak it in MMO for like 12 hours or so, this should work.


might work to restore it to its origina,l barely running condition from being so choked up.

I run a 20" swath of .95 line on a 20 year old Husqvarna 325LX that could barely do the job with my tall grass - now it almost doubles as a brushcutter.

Re: 2 cycle with catalyst - carb settings? [Re: cjolson140] #5444814 06/02/20 05:41 PM
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super20dan Offline
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the 128 is a much cheaper engine desighn than the 323. the 128 is a throwaway desighn basically.


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Re: 2 cycle with catalyst - carb settings? [Re: cjolson140] #5444993 06/03/20 04:04 AM
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FowVay Offline
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On my Husqvarna 525LS trimmer the catalyst is simply a screen on the exhaust housing. On my previous trimmers they all had a removable/cleanable screen that served as the spark arrestor. On the newer model the screen is welded in place and not removable. The catalyst is used to reduce the high levels of NOx that 2-cycle engines emit. It's not nearly as sophisticated nor intended to be as efficient as a 3-way catalyst on a vehicle.

As for the poor running of your machine, make sure there are no mud dobber nests in the exhaust blocking the air flow. If you want to go deep into it then make sure the piston ring/rings aren't stuck causing low compression. These things are simple enough that you can completely disassemble it and clean things up and it should run like new.

Re: 2 cycle with catalyst - carb settings? [Re: cjolson140] #5446890 06/05/20 12:47 PM
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Cujet Offline
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The catalyst is responsible for the heat. There is no real way to reduce temperatures to "normal" via proper jetting.


People who count on their fingers should maintain a discreet silence.
Re: 2 cycle with catalyst - carb settings? [Re: cjolson140] #5447166 06/05/20 07:00 PM
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bchannell Online Content
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On all of my weedeaters and chainsaws, I've never really noticed them running any hotter with or without the cat in the muffler. On my saws, I always remove the cats completely and retune the carb, and get quite a bit more power. I did the same thing to the weedeater and it runs much better also, but you really dont notice it as much as on the saws, I guess because engaging wood gives you a much more noticeable power feedback, ie less forgiving than grass/weeds.I wish the government would learn to stay out of our daily lives and quit trying to destroy everything by "imporving" things they know nothing about.

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