Author Topic: Stihl 048 AV - Runs Very Rich... Need carb rebuild?  (Read 385 times)

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Offline warreng5995

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Stihl 048 AV - Runs Very Rich... Need carb rebuild?
« on: April 02, 2016, 09:45:13 pm »
Hey All, I got a Stihl 048 AV from someone who wanted to sell it for a parts saw. It had no bar, but seemed complete. It was filthy and had obviously had not been cleaned in... ever ???... and been sitting for a long time. But glancing it over it seemed to be intact.

I gave it a closer look, and couldn't see anything obvious that would keep it from running. So I put some fresh fuel mix it, pulled the rope 5 or 6 times and she fired up and ran. But when I gave it the gas, it tried to bog down, found that the chain brake was on and it wouldn't release. Later found that one side of the chain brake handle was not attached to anything, which made it bend and flex instead of releasing brake.

Long story short, I thoroughly cleaned the entire saw, which was a job in itself. Replaced 3 of the vibration buffers, fixed multiple screws that were missing etc, and fixed other little issues and such.

So... after all that, tried to fire it up again.... pulled and pulled... and pulled some more. It just didn't' want to fire... So thought maybe the stop switch 'ground' wire had gotten pinched or something (as I had to install a new stop switch as the old one was broke and someone just cut the wire and apparently touched the wire to ground to stop the saw). All that looked good... So I checked for spark, which looked really good and strong on the new plug. Checked the air filter, which I had cleaned as I couldn't find a new one, didn't see any issues....

So I decided to reset the adjustments on the carb. I turned the L screw clockwise about 1-1/4 turns before it hit bottom. Then I turned the H screw, but it only went about 1/4 of a turn before it hit bottom  :o  wow, that seemed to be a way lean setting... Looked it up, and basic settings is like most Stihl saws, about 1 turn out on both H and L. So that what I set it at.

Fast forward another day or two, after the saw had been sitting for a bit, I could fire the saw up with the air filter removed. But even though it would run, when I tried to rev it, it would bog and smoke some. If I reinstalled the airfilter, and reved it, it would smoke a lot and quickly bogged until it would just die.

Thought maybe the spark arrester on the muffler... removed muffler but there was no spark arrester to be found  ???. So I took my air compressor and blew through the muffler... could feel the air flowing through, didn't see any debris or anything come out.

Reinstalled all again, tried to start it from cold... again pull and pull and pull....
Pulled the plug and it looked wet.. so its getting to much fuel. Let the cylinder air out for a while, Dried/cleaned the plug, also re-gapped this new plug to .20" (.51mm) as is listed in the specs for this saw, then reinstalled.
With the air filter removed, pulled and it fired up. It would rev to a point, a little smoke. Reinstalled the air filter, then rev and it bogged down significantly with a lot of smoke, and it would die if I keep trying to rev.

So.... Why is it running so Rich?? The previous owner obviously just leaned the carb out to only 1/4 of a turn out, to reduce the amount of fuel being sent, just to get it to run. But I really don't' want to do this. I would rather Fix what ever the issue is.

Is this a problem with the needle valve?? Does it sound like a carb rebuild kit would fix this issue?


Any other thoughts or something else I could check??


Any input much appreciated
"If you can't fix it, you just need a bigger hammer"

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Online jmester

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I would rebuild the carb for sure. Also check the fuel line and fuel filter. If you have good spark then fuel would be the next thing to get squared away to have a good running saw
FEAR NO TREE. FEAR GOD.

Offline warreng5995

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Hello jmester!

Yeah, I forgot to put in my first post, but I did check the fuel filter, looked clean.

I would expect if the fuel filter was clogged, or if it was a **** fuel line, it wouldn't be getting enough fuel though, right?
Instead its getting way to much fuel.
"If you can't fix it, you just need a bigger hammer"

Offline Al Smith

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This might sound silly but I actually did it from not paying attention .Keep in mind I've no doubt rebuilt at least a hundred chainsaw carbs in my life time .

The subject was an 042 Stihl,same as an 048 only smaller bore .Some how for whatever reason I had the diaphragm spacer gasket on the wrong side of the diaphragm.It did the same thing as the 048 in question .About drove me nuts.I overlooked it time and time again,had a brain fart .Made me so mad I banished the saw to the shelf until I got over my mad on which took about two years .

Finally after I regained my composure it only took me about 30 seconds to realize the error of my ways .In a nut shell it sounds to me like  that saw has a fuel delivery problem .

Offline warreng5995

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Hello Al Smith

Well, that sounds Very Interesting. I hadn't thought of something like that being the issue. I don't know the history of this saw, nor if the carb has been rebuilt before, but its likely.
I haven't taken the carb apart yet, as I didn't have a rebuild kit for it, so I didn't want to damage any gaskets or other if it wasn't the carb that was the problem. At this point though, a problem with the carb itself seems to be the only thing left.

I think I will order a rebuild kit for it today, found an OEM Walbro "Repair Kit" on ebay.  Which is basically a Rebuild kit + the Inlet Control Spring.

Going to try to find an assembly break down diagram for the carb. See if I can look at the diagram as I take the carb apart, and find any thing that looks to be out of place or in the wrong place/position.
I'll pay special attention to the diaphragm and the spacer. If someone has rebuilt this carb before, it could be the very problem just as you said.
I have a ultra sonic cleaner with a heater, so I might take all the carb parts and put them in there and let it run for a while.
Don't know if I can get to it today or not, but if not I will sometime this week.

Thanks for the Reply & Info
"If you can't fix it, you just need a bigger hammer"

Offline Al Smith

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More times than not a carb rebuild will cure the problem .Try as one might there's nothing you can do to prevent nor salvage a carb once the diaphragm and check valves stiffen up except rebuild it with new parts

A lot and lot of people with good intentions  have tried just cleaning them up but few have been satisfied with the results .

A rebuild kit is relatively inexpensive,often less than 10 dollars .I often order them 4-6 at a time .
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