Stihl MS 650 Running Too Lean...

[Wood Doctor]

... with a new carburetor, new fuel line, and new fuel filter on board. All fuel lines to the carb have been replaced and checked, even the short little puppy inside the carb housing. No adjustments to the Hi/Low carb settings save the situation. The engine runs for a short while after starting and is running too fast at full throttle. Then it cannot draw in fuel at low RPM and shuts down. Worse yet, it can only be restarted after a choke.

I do notice that the connection between the breather tube and the carb's sheave seems loose and the tube seems too short so that the connection there is poor. I'm not sure if that would cause all this, but something seems to be allowing too much air into the fuel mixture and in a very short time, the engine cannot run because it is starved for fuel.

Any ideas? TIA.

 

[tlandrum]

sounds like youve got an air leak,do a vac test

 

[Wood Doctor]

sounds like youve got an air leak,do a vac test

Yes, I agree there is an air leak. Rather obvious. Now, without doing a vac test, do you have any clues or hints as to where it is? Can the weak breather tube connection be causing it?

 

[funky sawman]

Are the head bolts tight???

 

[Beaver eager]

Crank bearings may be shot. Could just be something leaking around the carb though. Leak down check is in order.

 

[Wood Doctor]

Are the head bolts tight???

Yes, and compression is huge.

 

[tlandrum]

i generally change seals and check all bolts when i have one acting that way.

 

[Jacob J.]

Bad crank seals probably along with the possibility of one or both of the crank bearings being worn out.

 

[Wood Doctor]

Crank bearings may be shot. Could just be something leaking around the carb though. Leak down check is in order.

After sitting awhile, the engine starts and tries to act normally. Then after 20 seconds or so of running time it leans out, will not run at lower RPM, dies, and then cannot be started without a choke. That's completely unacceptable. Again, Hi/Low screw settings are meaningless. Compression is excellent.

 

[funky sawman]

As in new carb, is it actually a New out of the box carb or a new carb for that saw from another saw? Also check for impulse blow by pulling carb off and pulling engine over, you should feel a puff of air coming out of the impulse hose. What happens if you try to choke it while running??

 

[Wood Doctor]

As in new carb, is it actually a New out of the box carb or a new carb for that saw from another saw? Also check for impulse blow by pulling carb off and pulling engine over, you should feel a puff of air coming out of the impulse hose. What happens if you try to choke it while running??

This is a brand new carb that has never been used before. Took a month to get it from the Stihl dealer, (back ordered). That impulse hose (breather tube as I called it) is about an inch long, has a very loose connection to the carb sheave and looks to be about 1/4" too short. Are we on the same page? In other words, the rubber impulse hose connection only makes it to the first half of the carb's sheave. What I don't know is how tight that connection is supposed to be. It's intact and not split but seems loose as a goose and half-way attached.

I haven't tried choking it while running. Good suggestion. This machine cannot be partially choked. It's either all the way or no choke at all. I do know that starting unchoked after warm is hopeless. It has to be choked again after it dies.

 

[funky sawman]

check to make sure the carb diaphragms are installed correctily, sometimes the get them wrong..... If you are getting fuel, getting impulse, and the intake tube doesn't leak, Main seals, case half gasket, head gasket sound like the cause of the problem. Without a pressure and vac test a guy is shooten in the dark

 

[Jacob J.]

That impulse hose (breather tube as I called it) is about an inch long, has a very loose connection to the carb sheave and looks to be about 1/4" too short. Are we on the same page? In other words, the rubber impulse hose connection only makes it to the first half of the carb's sheave. What I don't know is how tight that connection is supposed to be. It's intact and not split but seems loose as a goose and half-way attached.

The impulse hose doesn't normally go all the way onto the pump cover nipple, so it sounds like it's installed correctly.

From the symptoms you're describing, I'm pretty sure you have a bad crank seal or seals. Even when a saw sits for
a period of time without being used, the seals can harden and lose their ability to hold pressure and vacuum in the crankcase.

 

[Wood Doctor]

The impulse hose doesn't normally go all the way onto the pump cover nipple, so it sounds like it's installed correctly.

From the symptoms you're describing, I'm pretty sure you have a bad crank seal or seals. Even when a saw sits for a period of time without being used, the seals can harden and lose their ability to hold pressure and vacuum in the crankcase.

I agree that the crankcase oil seals are a potential problem but really hard for me to fix. I wonder if it might be just the intake manifold aseembly betweeen the carb and the cylinder. I think that seal is mainly a simple flat sleeve that the carb queezes against when bolted into place. So, air could be leaking in there by getting past this sleeve, even with the two bolts tight as a drum. This original connection was disturbed, of course, when the new carb was installed.

Is there any simple goop I could try (such as silicone) to seal that manifold assembly up? Is it worth the attempt?

 

[tlandrum]

ive never had a leak where carb seats agains intake. the seals are going to be the most likely leak

 

[Fish]

My first take on this is a ripped intake boot, happens usually when av mounts are bad.

I will try and reread this again.

 

[Wood Doctor]

My first take on this is a ripped intake boot, happens usually when av mounts are bad.

I will try and reread this again.

Let me help:
(1) The "loose" impulse connection was not the problem. That's normal.
(2) The carb intake manifold connection is sealed tight. Sealing that one way or the other (goop or no goop) solves nothing.
(3) All of the head bolts are tight.
(4) The fuel lines and filter have been checked. All are OK.
(5) The engine fires when choked but will not continue to run when at half throttle or lower. Choking while running also kills it.
(6) No carb settings solve anything; the carb is brand new, out of the box.
(7) All anti-vibe mounts seem normal and functional.

Fish, what "intake boot" are you talking about? Please explain.

 

[aadoublea]

Let me help:
(1) The "loose" impulse connection was not the problem. That's normal.
(2) The carb intake manifold connection is sealed tight. Sealing that one way or the other (goop or no goop) solves nothing.
(3) All of the head bolts are tight.
(4) The fuel lines and filter have been checked. All are OK.
(5) The engine fires when choked but will not continue to run when at half throttle or lower. Choking while running also kills it.
(6) No carb settings solve anything; the carb is brand new, out of the box.
(7) All anti-vibe mounts seem normal and functional.

Fish, what "intake boot" are you talking about? Please explain.

I am having the exact same problem on a 064 I just picked up. I am going to vac test mine tommorow and see if its seals. I read that if seals are bad bearings could be as well? They seem very smooth and tight to me. So hopefully its just seals.

 

[lentil]

Intake boot is the black rubber tube that runs from the rear of the carby to the cylinder. Sounds to me like this is damaged, would only take a small hole to cause an issue. Also check the clamp on this as ive seen a few recently that have snapped and then caused leaks. Failing this, sounds like crank/cylinder gasket or oil seals. the only sure way of finding the exact place of leak is with a vac/pressure test.

Also, have you checked that the Sparkplug is tight?

 

[atvguns]

You can check for air leaks with carb leaner when the saw is running