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Stihl MS 391 help needed.

tomdcoker

tomdcoker

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I have the above saw that I have rebuilt. I got it a couple years back with a scored P & C. It came from a construction company and I was told that it was straight gassed.
I cleaned up the cylinder and replaced the piston and rings. I have pressure and vacuum tested it twice and it passed. When I try to crank it I have to prime it. It will crank and run WOT until it runs out of gas (one second maybe then die). It does not pull any gas into the carb. I know that I am missing something, but I have been completely through it twice. T he carb. and manifold are a pain,, but I think everything is right. Notice I said " I think". This is not my first rodeo. but I am at a loss on where to go next. Any help would be appreciated. Tom
 
tomdcoker

tomdcoker

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I worked on he saw today, rechecked and rethought every thing to do with carb. to no avail. Probably going to buy a new carb. Tom
 
a. palmer jr.

a. palmer jr.

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I don't know what the results would be but I purchased a Walbro clone carburetor for a friend of mine but his problem was just the opposite, too much fuel. The Walbro had been rebuilt but it didn't help so we're trying the knockoff to see if it makes it any better..
 
Edweiser

Edweiser

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Did you try taking your fuel line off the carb and blow air through the tank vent line to see if fuel pushes out to eliminate tank vent or fuel filter obstruction?
 
tomdcoker

tomdcoker

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It has a new carb. kit and when I removed the carb. the pressure in the tank forced probably two oz. fuel out before I could loosen the cap and release the pressure. Tom
 
tomdcoker

tomdcoker

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I do not have another carb. and a new one is $75. I did not pressure test the carb. When I get upset because something is not working as it should I sometimes forget things. A sign of old age I guess. Tom
 
a. palmer jr.

a. palmer jr.

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The $75-$80 cost is why we're trying the clone @ around $30+shipping. Of course Indiana has to stick their hand out with the dreaded sales tax on online purchases now so that brings the price up a bit...
 
tomdcoker

tomdcoker

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OK. Today I got the saw to crank and run without poring gas mix in the carb. and found the problem.. It has a massive air leak. The plate that mounts to the cylinder is warped and will not make a good seal. when you lay a straight edge across it from top to bottom there is almost an eight of an inch clearance. It has three screws that hold it on , one at the bottom and two at the top. Don,t ask me how it got warped. I don't know, probably something that I did. I will try to straighten it tomorrow. If that does not work I will call Four Seasons. Tom
 
HarleyT

HarleyT

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OK. Today I got the saw to crank and run without poring gas mix in the carb. and found the problem.. It has a massive air leak. The plate that mounts to the cylinder is warped and will not make a good seal. when you lay a straight edge across it from top to bottom there is almost an eight of an inch clearance. It has three screws that hold it on , one at the bottom and two at the top. Don,t ask me how it got warped. I don't know, probably something that I did. I will try to straighten it tomorrow. If that does not work I will call Four Seasons. Tom
There are two versions of the manifold. 3 screw, and 4 screw....
Now we know why.
 
a. palmer jr.

a. palmer jr.

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I hope that takes care of the problem...I've not had much luck on that series of saws because of little aftermarket and most of the customers around here don't want to spend too much money on repairs, they just go buy a new saw instead...
 
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