Idles fine but dies at WOT

[CountryBoy19]

While cutting last night I encountered a problem with my MS170. It happened right after I filled up the tank. It ran fine for a little bit (maybe a minute of cutting) then I shut it off, and it didn't want to start again. I had a heck of a time getting it going, had to feather it between half/full choke to get it started, and it wouldn't run. I finally got it to run fine at idle but when you engage the throttle it bogs and dies.

My first thought was a clogged main jet. That would seem plausible seeing that the problem occurred after a small amount of use just after a fill-up. It's always possible that spec of dirt found it's way into the carb.

Would the group agree? Am I in for a carb disassembly, or just try to blow down into the jet and put the carb back on?


ETA, I have been dealing with vapor lock for a while now. I attributed that to running in the heat and not having an adjustable carb. But this was different, no matter how long I messed with it it was doing the same thing; it definitely isn't vapor lock.

 

[the westspartan]

sounds like bad gas. If the fuel has gotten hot and has been sitting longer than a month or so it may be bad. Try some fresh mix first, it's an easy fix.

 

[chopperfreak2k1]

i had a poulan pro 33cc weed wacker do that to me. it was flooded, i held the throttle wide open, gave it a yank and it was cured. i'm not saying that's the problem you have but it's worth 10 seconds to give it a try.

 

[CountryBoy19]

sounds like bad gas. If the fuel has gotten hot and has been sitting longer than a month or so it may be bad. Try some fresh mix first, it's an easy fix.

Definitely not bad gas. It's only a couple weeks old and I've been cutting on it every day for the last couple weeks. It wouldn't just turn bad from one tank to the next.

Something definitely happened during or after the fillup. The saw was running top notch on the first 2 tanks yesterday, then after the fillup it just choked. It will run on half-choke, just not well. I was able to use it on half choke to finish up the last little bit of the noodle job I was working on so I could get the wood loaded up and hauled home.

 

[longbar]

i had a poulan pro 33cc weed wacker do that to me. it was flooded, i held the throttle wide open, gave it a yank and it was cured. i'm not saying that's the problem you have but it's worth 10 seconds to give it a try.

X2 had this happen before also!

Definitely not bad gas. It's only a couple weeks old and I've been cutting on it every day for the last couple weeks. It wouldn't just turn bad from one tank to the next.

Something definitely happened during or after the fillup. The saw was running top notch on the first 2 tanks yesterday, then after the fillup it just choked. It will run on half-choke, just not well. I was able to use it on half choke to finish up the last little bit of the noodle job I was working on so I could get the wood loaded up and hauled home.

Anything is possible. Gas is cheaper than a new saw might wanna get new gas anyways.

 

[Stihl-Pioneer]

Kind of a long shot but, have you checked the clutch drum for anything hanging it up. My f.i.l. had one that acted kind of like that and I found a wad of fishing line wrapped in the sprocket, it had melted together from the frictional heat.

 

[CountryBoy19]

X2 had this happen before also!



Anything is possible. Gas is cheaper than a new saw might wanna get new gas anyways.

I can see both of these being possible if it wouldn't run well at all, but it will idle just fine (no choke), but as soon as you touch the throttle it dies unless it's on half choke and sometimes it needs more than half choke.

That's why I thought of a plugged jet. As soon as you touch the throttle taking away the vacuum from the idle jet it dies out.

I think I'll go home and try it after work just to see if it still does it. I'll report back here tomorrow with the results.

 

[CountryBoy19]

Kind of a long shot but, have you checked the clutch drum for anything hanging it up. My f.i.l. had one that acted kind of like that and I found a wad of fishing line wrapped in the sprocket, it had melted together from the frictional heat.

Good thought, but definitely not the clutch. Once you get it started it idles fine (chain not turning), but dies when you touch the throttle unless it's choked. Which means that it's not getting fuel when the throttle is pressed. Chain isn't hung up either because I was able to finish off my cut last night with it on half choke and going nice and slow (very little power).

 

[chopperfreak2k1]

I can see both of these being possible if it wouldn't run well at all, but it will idle just fine (no choke), but as soon as you touch the throttle it dies

i know it is just a weed wacker but it's still a 2 stroke. and i could start it very easily and it would idle very well UNTIL i touched the throttle. as soon as i hit the throttle it died almost instantly. and it continued to do this even after i had let it sit for an hour or so. hold the throttle wide open and give it a shot, what do you have to lose, right?

 

[rmh3481]

If the engine is running lean open the high jet a half a turn. If the tank vent is plugged this could also be the reason because the carb cannot draw fuel against a vacuum.

Also many just dont consider how moisture in the air can effect your fuel especially now that most gas is laced with alcohol. If your down low in the storage tank that could be the problem. I see alot of oldtimers that just leave their storage tanks open all the time. This is a no-no anymore. Just like dew forms on the grass when the sun goes down, the same moisture will accumulate in your storage tank if its un-capped. Gas will not hold water, it was developed not to. Unfortunately, alcohol will hold water. What you end up with at the bottom of an uncapped storage tank is a slurry of water, alcohol, gas and oil that wont mix. Ive seen alot of two stroke engines ruined from that slurry especially when you cant see it while pouring.

 

[CountryBoy19]

If the engine is running lean open the high jet a half a turn. If the tank vent is plugged this could also be the reason because the carb cannot draw fuel against a vacuum.

Also many just dont consider how moisture in the air can effect your fuel especially now that most gas is laced with alcohol. If your down low in the storage tank that could be the problem. I see alot of oldtimers that just leave their storage tanks open all the time. This is a no-no anymore. Just like dew forms on the grass when the sun goes down, the same moisture will accumulate in your storage tank if its un-capped. Gas will not hold water, it was developed not to. Unfortunately, alcohol will hold water. What you end up with at the bottom of an uncapped storage tank is a slurry of water, alcohol, gas and oil that wont mix. Ive seen alot of two stroke engines ruined from that slurry especially when you cant see it while pouring.

No high jet to adjust, this is a 170 with fixed adjustments. I've been looking for an adjustable carb so I can do a muffler mod but I haven't found one yet.

The can is getting lower, but it's a newer, carb-compliant Blitz can with the tight sealing spigot. It can't really draw a whole lot of moisture, when it cools off it just sucks the sides of the can in (collapses it).

 

[the westspartan]

No high jet to adjust, this is a 170 with fixed adjustments. I've been looking for an adjustable carb so I can do a muffler mod but I haven't found one yet.

The can is getting lower, but it's a newer, carb-compliant Blitz can with the tight sealing spigot. It can't really draw a whole lot of moisture, when it cools off it just sucks the sides of the can in (collapses it).

I run a ported 441, a muff modded 346, a stock Jonny 2095 and an MS 180 and have had them all act just the way you say at the end of a can of gas that has been around for a month or so, especially when it's hot in the day and cool at night. My can is a new style CARB compliant can.

It may not fix it, but it wouldn't hurt to try new gas. Oh, the 180 was fried with this old gas. Needless to say, I learned my lesson. At least it was the cheapest of the saws effected at the time LOL.

 

[CountryBoy19]

Well, I went home, and started it up. Same thing, ran really doggy and no power, but at least it didn't die at WOT like last night. After a couple minutes of pumping the throttle and goofing around with things it wound up and ran great again. I have no idea what the cause is/was but it runs now (at least it did).

I don't really have much more cutting to do for a few months so I probably won't get a chance to truly test it out for a while. Maybe that will give me a chance to find an adjustable carb for it so I can finally muffler mod this thing.

It's been a really good saw so far. Not too bad for $75. I'm at the "only one saw" point right now so I've been cutting all my firewood with this beast; so far it has about 8-9 cords cut in the last 3 weeks. I'll be set on firewood for the next couple years but I still thing I might cut some more later this fall once I get the fireplace installed and get some other projects done.

 

[CTSawTech]

Most saws I've seen that needed the choke on partway usually had an air leak at some point after the carb.

I would check the intake boot, impulse hose, fuel lines and seals.

Make sure the muffler screen isn't plugged too, if it is throw it away. You don't need it anyway.

In short...check everything and anything.

 

[Kenskip1]

How about draining the gas and looking inside the gas tank. From the sounds of this a kinked gas line could be the culprit.Sometimes the gas filter can get wedged cutting off the gas supply, Ken

 

[ZeroJunk]

I think your first thought was likely correct, a bugger in the high speed jet. Don't you hate those carbs. Pain to get that jet clean when a simple squirt with carb cleaner down the needle hole often works on the old ones. Sounds like it cleared out on it's own.