Dry Purge Bulb on Stihl 192T

[Wood Doctor]

This one has me stumped. I installed a new purge bulb and pickup body on a rather new 192T that would start and then die after running awhile, unable to hold any low speed. The purge bulb remains dry. Rather amazing that the saw even starts. I checked to be sure the fuel line and the purge line back to the pump were installed correctly. The short end of the purge goes into the top entrance hole and connects to the fuel line. Pressing the bulb only draws in air. I even checked two or three new bulbs and they all so the same thing -- air only. In short, the carb cannot be primed.

I disconnected the fuel hose from at the elbow and at the fuel pump for blockage. It's not blocked, so I have no idea why the bulb refuses to fill up. Seems to me that the carb simply cannot continue to pull fuel from the tank either, and that's why the engine dies. I have also tried two carbs and both do about the same thing -- stall out after a short run. I even tried a dummy line to a removed purge bulb and it will fill up with fuel extracted from a jar. But, connected to the saw it's helpless and cannot move any fuel from the tank.

Here are the parts I've been working with:

Any ideas on what I could do next? TIA

 

[Old2stroke]

The purge bulb has the symptoms of being hooked up backwards. Where is #23, on the carb or fuel tank? The short tube on the bulb is the suck one and should go to the carb nipple that comes from the fuel chamber. Any chance the impulse nipple and the purge nipple on the carb are mixed up? When an engine just runs for a short while on the fuel sucked up by the choke, it usually means the fuel pump isn't working due to missing impulse or defective pump. The impulse nipple on the carb is always located near the pump side cover.

 

[pioneerguy600]

I had this problem earlier this year, the fuel pickup line had a crack in the tubing wall, it was difficult to find but after removing it and flexing/bending the line the crack showed itself.

 

[Wood Doctor]

The purge bulb has the symptoms of being hooked up backwards. Where is #23, on the carb or fuel tank? The short tube on the bulb is the suck one and should go to the carb nipple that comes from the fuel chamber. Any chance the impulse nipple and the purge nipple on the carb are mixed up? When an engine just runs for a short while on the fuel sucked up by the choke, it usually means the fuel pump isn't working due to missing impulse or defective pump. The impulse nipple on the carb is always located near the pump side cover.

I checked the two impulse nipples. I believe they are OK. The longer one is underneath the shorter one that connects to the hose leading to the carb. Stihl's diagram shown in post #1 (and in both the service manual and the IPL) is not clear to say the least. I even tried reversing the nipple orientation and the purge pump still would not collect any fuel from the tank or fill itself. It made no difference how the bulb was plugged in. It pumped air only in either case.

So, I started from scratch and rebuilt the entire assembly, including the fuel line and filter inside the the tank, making sure that all connections were secure. Then I filled the tank and pressed the primer bulb. It took about 20 presses for fuel to finally appear in the bulb. I then started the saw, tuned the carb, and it's running fine now. Incredible that the complete fuel assembly rebuild was needed, but that's what seems to have fixed it. During the tune-up I opened the low-speed set screw a half turn more and that cured the fuel starvation problem at idle.

Cheers!

 

[pioneerguy600]

Did you happen to check the fuel lines from the suction side the ones that bring fuel to the carb then the bulb? Just asking because I could not see the crack in the line I had on the JD56 and even using the vac tool it would pull fuel ,just not when pressing the purge bulb. When I removed the lines and bent/flexed them the crack then showed up. The lines look like Tygon but was likely generic lines of the same color, same lines that was on the saw when it came to me. Worked fine for me for over 5 years but early this spring I could not get the bulb to pull fuel, on close inspection there was a fine crack in the bulb itself, replaced that and still no fuel.

 

[Wood Doctor]

Did you happen to check the fuel lines from the suction side the ones that bring fuel to the carb then the bulb? Just asking because I could not see the crack in the line I had on the JD56 and even using the vac tool it would pull fuel ,just not when pressing the purge bulb. When I removed the lines and bent/flexed them the crack then showed up. The lines look like Tygon but was likely generic lines of the same color, same lines that was on the saw when it came to me. Worked fine for me for over 5 years but early this spring I could not get the bulb to pull fuel, on close inspection there was a fine crack in the bulb itself, replaced that and still no fuel.

Yes, I checked those out carefully. Fuel flows from the tank through the hose part #16 to part #18 that then connects to the carb. Part #18 is primed and the carb's pump diaphragm takes over from there to deliver the fuel. When the long hose is dead dry, the primer/purge bulb has to work its butt off to help pull that fuel from the tank. Any crack in the bulb disables it completely, lets air in, and likely works against the carb's pump diaphragm.

It's amazing the system works at all and one of the reasons why Stihl designed the M-tronic saws to eliminate the primer bulb.

 

[pioneerguy600]

Yes, I checked those out carefully. Fuel flows from the tank through the hose part #16 to part #18 that then connects to the carb. Part #18 is primed and the carb's pump diaphragm takes over from there to deliver the fuel. When the long hose is dead dry, the primer/purge bulb has to work its butt off to help pull that fuel from the tank. Any crack in the bulb disables it completely, lets air in, and likely works against the carb's pump diaphragm.

It's amazing the system works at all and one of the reasons why Stihl designed the M-tronic saws to eliminate the primer bulb.

The 56 resides at the summer place on the lake,it sits often for long periods of time and often it has to pull fuel up from the tank through the carb and any outside source of air entering that fuel system no fuel will move up the lines. The carb gaskets and covers must be absolutely air tight also as well as the fuel lines,even a slightly loose fitting line would allow air to enter countering any vacuum created by the purge bulb. Airtight is mandatory. The vac tool can pull so much more air at a greater rate than the little purge bulb so it can overcome a very slight air leak like a tiny crack in a fuel line or slightly leaking carb gasket so the tool may give one a false feeling that the system is tight.