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View Full Version : Stihl carb blowback


BZB_Helpers
02-18-2008, 03:21 PM
I bought a Stihl 340 blower on 5/20/05, then replaced the air filter on 4/16/07, then again on 12/14/07. The filter arent getting dirty from dirt, they are getting clogged with fuel. Is this a carb problem or a cylinder problem? I use the Stihl oil @50:1 with 89 fuel like the book says.

MowerMedic77
02-18-2008, 07:31 PM
Do you still have the spark arrester installed? Is it clogged, or is the exhaust port clogged with carbon?

topsites
02-19-2008, 12:33 AM
Yeah Stihl's are famous for soaking their filters in fuel, at least more than a few of mine have always done this crap...
Now it might need a new carb but at $80 each here are some things I have learned:

Keep your old filters, then when one gets soaked just throw it in your next washing machine load (thou to be safe I would make sure it goes in with your work clothes). Let it air dry, perfectly good for another round, I've washed mine via this method 3-4 times then I think it's about time for a replacement but now I've gotten my money's worth, plus I always have at least one that is clean and ready when another gets messed up.

When I first start the machines I let them idle for 30-40 seconds or so, especially once they get older it doesn't help to rev them as soon as they sputter to life... Maybe 15-20 seconds is good enough but give it that little putt-putt happy time before putting it to work, because I'm a firm believer our own attitudes towards the machine allow it to run better or worse.
Not saying you should kiss the machine's tail, but it wouldn't hurt its feelings if you did, and don't kiss mufflers or spark plugs.
As they say, a little TLC goes a long ways.

Once the engines are going, steady or constant throttle... Doesn't have to be Wot but keep the finger steady and don't constantly pump that throttle, instead ease off of it then ease back onto it, this also helps, kind of like a car, take it nice and easy.

Oh and don't run them completely out of fuel, once it runs low you need to stop and refill... This may not be related to this problem but I do know letting them run dry isn't good for it either.
Well that's all I know, make sure the spark arrestor is clean or out of there.

BZB_Helpers
02-19-2008, 11:50 AM
There is no spark arrestor screen, been gone since the first day. I thought these air filters were paper? DO you wash the filter with detergent? Stihl filters are $15 here so I might go aftermarket now. I only work 1 day a week so it doesnt have that many hours compared to you guys. I do idle it for about 15-30 seconds before I strap it on and walk to the account then slowly increase to WOT, usually stays at WOT, half, or idle.

MowerMedic77
02-19-2008, 12:55 PM
I would still check for carbon build up in the exhaust port, easy enough to check and eliminate as an issue.

BZB_Helpers
02-19-2008, 01:24 PM
I'll do that when it gets alittle warmer outside. Still wondering about washing the filter? The new filter has 4hrs on it so far but ill check it when I check the exhaust.

topsites
02-20-2008, 12:16 PM
There is no spark arrestor screen, been gone since the first day. I thought these air filters were paper? DO you wash the filter with detergent? Stihl filters are $15 here so I might go aftermarket now. I only work 1 day a week so it doesnt have that many hours compared to you guys. I do idle it for about 15-30 seconds before I strap it on and walk to the account then slowly increase to WOT, usually stays at WOT, half, or idle.

Oh crap that's right the backpack one might be a paper filter, yeah, sorry about that.
I was thinking of those foam ones like the trimmers and some hand helds have, no, don't wash the paper filters.
You might take it to the dealer and see what they say, in my case a $75 carb rebuild fixed my trimmer but it was also running crappy.

But for now you can also try, give it that 15-20 seconds putt-putt time after it starts, then once you put it on your back slowly ease into it.