394XP surging at WOT

[aquan8tor]

I tried to run my saw today, and ended up getting pI$$#d off instead. Worked great last time I used it, but not today.
When you hold the throttle down, the engine will rev up, then surge up too high, then sputter like its getting way too much gas, then surge up again; over and over; I thought maybe it was just working a bubble or something through the fuel line, but it does the same thing over and over again. I checked the fuel line; no obvious holes or cracks, no leaks around the carb that I can see, and I checked the diaphragm & didn't find a hole. Could be a tiny one in there; I don't have the tools to do a pressure test on the carb, but this is a new problem. I just don't know. I thought maybe it was water or something. I don't know how to change it, as it isn't listed in the shop manual or the parts list on the 394XP, but I KNOW there is a breather for the tank, behind the right hand anti-vibe spring, which doesn't look too clogged; I honestly don't know how to tell.

Other info; I always run premium gas, running stihl "performance blend" 50:1whatever the dealer has..., new plug just last month, washable filter element, so I know its clean,I rebuilt the carb over the winter & have used it several times since with no problems. ?????

 

[begleytree]

imo, the carb needs adjusted, they come out of a box and are put on at the factory, seen then where they wont hold steady at max rpm, its either 500 rpm under max or shoot the moon.
sounds like yours is running lean, then dies out, then fills up again and repeats. check the filter and fuel/impulse lines too.

 

[cbfarmall]

check the filter and fuel/impulse lines too.

Have a Homelite 650 that surged at WOT and would die in the cut. I couldn't adjust it out. So, a little investigating and I saw that the new impulse hose I installed was kinked making a turn to the carb. Fixed that (by shortening the hose) and the saw runs perfect.

Chris B.

 

[aquan8tor]

Anything else that could make it surge?? I've tried adjusting the carb a bunch. No amount of changing the carb settings will keep it from surging. The impulse hose is alright.-no kinks or pinholes. Fuel filter was changed at the same time as the sparkplug.

 

[Lakeside53]

Usually, this is either something to do with varying impulse pressure (inconsistent fuel) or the metering diaphragm and metering spring/lever. Could be something wacky like a piece of sealant from a welch plug floating around in the metering chamber. I assume you have the metering diaphragm and gasket in the right order?

tank vent -just loosen the gas cap (make sure your tank isn't full) when it does the surging and see if it stops.

 

[aquan8tor]

gasket/diaphragm def. in correct order. It was working GREAT for a while, then suddenly stopped. I'm going to try the gas cap vacuum trick later. Thanks. Never would've thought of that. If it does draw air, (and I think its going to, as the tank vent is about the only wear-part that I haven't replaced.) how do I go about replacement?? I know you're a stihl guy, lakeside, but have you done this on a 394?? Thanks.

 

[aquan8tor]

Meant to note; also, just ordered a new fuel hose & another carb kit; just in case. I thought about it, and I didn't really examine the diaphragm under a magnifying glass or anything; I figure its worth $8 for a new carb kit and a little time at my desk to be sure. Otherwise, if before the part gets here and I figure its the tank vent, I still have a kit for next season; I have a feeling its going to need it as dusty and grueling as milling is on a saw.

 

[aquan8tor]

The tank definitely is sucking in when I open it, and stops surging when I crack it. How the heck do I replace the tank vent?? It isn't listed on the parts list I have. ??????? I know where the vent is, I just don't know how to remove it without damaging the tank, and how do I get a new one, or can I just clean it?

 

[andrethegiant70]

Kinda sounds like an intake leak, especially if you've just taken it all apart recently. I'd be checking over what I had just done. I find I am usually the culprit in such instances.

 

[aquan8tor]

Its not leaking; I removed the carb again & resealed when the problem started. It did run great for a few weeks, then just decided to stop working right. I milled a bunch of hickory in the meantime, which coated the ENTIRE saw in fine powder. I run a 5* ripping chain, which makes a smoother cut, but makes really fine dust. Back to the tank breather, how the heck do I replace it?? As was suggested, I cracked the tank when it was running, and it runs fine. I ordered a fuel line just in case there is a pinhole leak that its sucking air through, but I don't even see the part number listed for the breather valve on the gas tank. Somebody has to have done this on a 394 or 395....???

 

[spacemule]

I don't know how you change it. Have you looked at the ipl?

 

[aquan8tor]

thanks, spacemule. All I had was the 395 IPL, and its not the same configuration.

 

[aquan8tor]

I replaced the fuel line, and took the tank off today & took out the VERY CLOGGED breather & stuffed some cotton balls in place. Saw runs smooth like butta. The fuel line seemed ok, but I figured it was worth the few dollars I paid for it. I'll order a replacement breather from my dealer next time I"m in. Thanks again for the IPL, spacemule.