Stihl MS 391 help needed.

[tomdcoker]

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

 

[a. palmer jr.]

I guess you've already checked the passage for the pulse to the carburetor, I don't remember if they have a hose or just a passage..

 

[Mad Professor]

Fuel filter and line, tank vent,screen on inlet of carb, diapraghms.

 

[Jasonrkba]

Sounds like an impulse problem or needle not opening.

 

[tomdcoker]

I have checked all of the above, but I agree that it could be any of the above, so I will check them again. Thanks Tom

 

[a. palmer jr.]

I usually just put a drop of oil on the impulse hose end, block off the carb opening in the intake and crank her over while looking at the impulse hose...

 

[tomdcoker]

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

 

[jltrent]

The problem is the diaphragm in the pump. Just get an AM kit and put in and it will run.

 

[a. palmer jr.]

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]

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]

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

 

[Edweiser]

Did you try the other carb? Also see if the carb will pass a pressure test. 7 psi. for a minute or so

 

[tomdcoker]

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.]

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]

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]

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.]

Do you think that heat might have warped it?

 

[tomdcoker]

Do you think that heat might have warped it?

I would not think that it got that hot. I had to replace the piston, but not the cylinder. Tom

 

[tomdcoker]

Ordered the manifold today will be in within a week I hope. Tom

 

[a. palmer jr.]

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...