Stihl 192T primer bulb

[purdyite]

Yes or no question.
Replaced the primer bulb on my 192T. Starts and runs ok. But the primer bulb doesn't fill with fuel--looking at the tubes, it has an odd fuel routing from the tank, to the carb, to the primer bulb and back to the tank. It's as if the bulb only provides suction and the fuel doesn't have to go through it. Should it fill with fuel or not? Should I even worry about it? On other saws, if I don't see fuel in the bulb then I don't have a sealed fuel hose somewhere.

 

[Wood Doctor]

Yes or no question.
Replaced the primer bulb on my 192T. Starts and runs ok. But the primer bulb doesn't fill with fuel--looking at the tubes, it has an odd fuel routing from the tank, to the carb, to the primer bulb and back to the tank. It's as if the bulb only provides suction and the fuel doesn't have to go through it. Should it fill with fuel or not? Should I even worry about it? On other saws, if I don't see fuel in the bulb then I don't have a sealed fuel hose somewhere.

Yes, it should fill with fuel. Be sure the fuel lines are connected up right. And, something could be blocked up or a fuel line could be crimped.

 

[Old2stroke]

As noted, it SHOULD fill with fuel as it sucks fuel out of the carb and pumps it back into the tank, there are two built in check valves in the primer to keep fuel moving in the right direction. If it doesn't fill with fuel, could be a faulty primer, a leak in the fuel supply line or one of the two carb check valves not sealing. If the main nozzle check valve isn't sealing it will cause serious problems with idle fuel. If the check valve in the floor of the fuel chamber is leaking, it's not serious as it's only purpose is to allow the primer to function. Any problems with the primer can be resolved by blocking the fuel line from the carb to the primer to disable it. If the saw has a good choke, you don't need the primer, you will just have to pull the starter a few more times to fill the carb when cold starting.

 

[purdyite]

Trying to determine a more pesky problem--the saw stalls when I hit the throttle, and if the bulb should fill with fuel, then I must be sucking air somewhere in the fuel circuit and causing it to stall and die on lean condition. I know the crankcase is sealed--put on a new P&C and tested it. Going through the fuel supply piece by piece to fix the problem(hopefully).

 

[Wood Doctor]

Try opening up the Hi-speed setscrew a half turn and see if it still stalls when you hit the throttle. It might be fuel starved.

 

[Old2stroke]

Actually it's the "L" screw you should back out a little, the idle fuel has to be set a bit on the rich side to provide enough fuel from the idle and transfer ports to allow a smooth transition to the high speed circuit as rpm increases. The "H" screw is adjusted to get the tuning right for high speed, full load conditions and should not be fooled with to compensate for incorrect idle tuning.

 

[Wood Doctor]

Actually it's the "L" screw you should back out a little, the idle fuel has to be set a bit on the rich side to provide enough fuel from the idle and transfer ports to allow a smooth transition to the high speed circuit as rpm increases. The "H" screw is adjusted to get the tuning right for high speed, full load conditions and should not be fooled with to compensate for incorrect idle tuning.

He may have to adjust both to get it to run right. That's been my experience.

 

[stihltech]

Get a carburetor, primer and fuel hoses. You are working on a saw from 3 generations ago. Fuel has been eating on the whole fuel system. Time for a refresh.

 

[purdyite]

I have the fuel circuit working, filling the primer bulb. I have adjusted the carb as described here, and here is the problem now: it starts, idles well, still hesitates on the throttle, but it will sit and idle and then begin to surge and stall, and it continues until it dies. About the only thing I haven't done is replace the diaphragms in the carb. Is the surging and stalling a symptom of old diaphragms?

 

[stihltech]

Sounds like a sticking high speed nozzle in the carburetor.
You have vacuum and pressure tested the motor, correct? Checked piston? If not, you could be wasting your time.
Put some more parts on it and let us know.

 

[purdyite]

I put new diaphragms in, didn't help. Even removed the welsh plug, ran the bare carb through the ultrasonic cleaner, rebuilt and still no luck. Did a vacuum/pressure test on the crankcase, it passed the vacuum but failed the pressure test. What gives? This is beginning to drive me nucking futs!

 

[stihltech]

Follow the bubbles and find the leak! It may even solve the issue.
Spray on soap/water or put it in a bucket of water while pressurized.

 

[purdyite]

Apparently, the new carburetor fixed the problem. Haven't had it in wood yet, but sounds fine--throttles up and runs full out.
Now--I want to do a forensic analysis of why the old carb was the problem, when it had new gaskets, diaphragms, and was clean as a whistle. What, in your experience, would be the failure point? Abrasive fuel that has worn the orifices/jets? Do carbs simply wear out like other components?

 

[cscltd]

I can’t recall if ms192 had accelerator pump on carb. It could be source of problem as they were out and suck air

 

[ZeroJunk]

Most of the time if it won't prime it is a carb issue, whether the primer is on the carb or connected with fuel lines.
Bad check valve I assume.

 

[Wood Doctor]

I replaced the carb today on a Stihl MS440. The old one looked fine and I also tried a similar rebuild that Purdyite performed, but mine also failed. So, the old carb went to the boneyard.

The new Walbro carb required an increase in the idle setting, but now it holds it and runs perfectly at WOT. Adjusting the idle upwards from the factory setting has been a rather common experience for me over the years. Otherwise the engine will die at the low end. Closing the lo-speed setting also raises it up.

These adjustments to a new carb vary from one saw to the next and yes, carbs do wear out, even though some seem to last forever.

 

[purdyite]

Thanks for following this thread and offering your insights and anecdotes. My goal is to learn it all and become "excellent" at maintaining small engine equipment. This site is certainly worth it. Appreciate all the help, now and to all my other pleas for help!

 

[Old2stroke]

Apparently, the new carburetor fixed the problem. Haven't had it in wood yet, but sounds fine--throttles up and runs full out.
Now--I want to do a forensic analysis of why the old carb was the problem, when it had new gaskets, diaphragms, and was clean as a whistle. What, in your experience, would be the failure point? Abrasive fuel that has worn the orifices/jets? Do carbs simply wear out like other components?

The two check valves in the carb becoming sticky and intermittent is a common problem and no amount of cleaning will cure them and they are never included as part of a repair kit. The nozzle check valve can often be repaired (do a search) but the smaller body one may have to be replaced. They can be tested by plugging the fuel inlet port to the carb and applying a vacuum to the primer port (for carbs that have a remote primer). If they won't hold a vacuum, one or both are leaking.
The only part of a carb that actually wears out is the throttle shaft, on really long life saws the wear can be significant enough to leak air and upset the idle. A drop of oil on the shaft bushings regularly helps. Accelerator pumps can go bad but can be replaced. The ends of the control lever can show wear but can be replaced. The only "wear" item that ends the life of the carb is usually the throttle shaft.