Carb leak

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""If that doesn’t work, my next step is to (as first quoth by the venerable Ellen Ripley) “…take off and nuke the entire site from orbit. It’s the only way to be sure.”"

I'll counter your quote with:
"...Gotta nuke something." - Nelson Muntz
 
Old2stroke said:
So we are confused. Does this carb get the impulse signal from a separate line directly from the crankcase or does it get it from a hole in the flange where the carb mounts to the engine. If you are actually testing an impulse port on the carb, it just operates the pump chamber diaphragm and should hold both vacuum and pressure, has nothing to do with the fuel inlet from the tank and cannot be used to test the fuel inlet valve in the carb, this valve has to be tested by applying pressure to the fuel inlet port and applying vacuum to it is a meaningless test. The nozzle check valve should be easy to blow through going from the fuel chamber into the carb throat and should hold vacuum going in the other direction. These devices have a little rubbery disc that is often distorted and becomes intermittent so "bench testing" it doesn't mean it will continue to work in a running saw.
Click to expand...
I see that now. Forgive me, I’m still learning this. Yes, the impulse has nothing to do with fuel directly, it’s not really even a fuel line, just an air pressure line that “heartbeats” the pump side. This carb gets impulse signal through the flange inlet pipe it mounts to, it doesn’t have it’s own hose nipple.

So the test for the metering needle is on the fuel inlet (should hold min 7psi pressure and vacuum). The other inlet is for the purge. That should hold pressure, but not vacuum?
 
Backstage said:
I see that now. Forgive me, I’m still learning this. Yes, the impulse has nothing to do with fuel directly, it’s not really even a fuel line, just an air pressure line that “heartbeats” the pump side. This carb gets impulse signal through the flange inlet pipe it mounts to, it doesn’t have it’s own hose nipple.

So the test for the metering needle is on the fuel inlet (should hold min 7psi pressure and vacuum). The other inlet is for the purge. That should hold pressure, but not vacuum?
Click to expand...
You are right, the fuel inlet should hold a minimum or 7 or 8 psi for normal operation, however if the pressure in the fuel tank builds up (hot day, saw sitting in the sun), the inlet valve should not be forced open until the pressure exceeds anything the tank can produce, this is called the "pop-off" pressure and usually 15psi is enough. A vacuum test has no meaning on the fuel inlet.
If you are applying a p/v test to the purge port on the carb, it should not hold pressure, the air being forced in would just dump out through the main jet and the idle ports. If you apply a vacuum to the port, it probably won't hold, the main diaphragm will just be sucked down and open the inlet valve, however, if you block the fuel inlet port and apply a vacuum, it SHOULD hold a vacuum if the check valves in the carb are working properly (main jet check valve and may be another body check valve, depending on carb)
The purge bulb contains 2 check valves that keep the fuel moving in one direction only, from the carb to the tank, so the suction port of the purge should hold vacuum but not pressure. Generally, the purge is hooked up with clear lines and if you can see fuel being pumped through in the right direction, it's fine.
 
Now
Old2stroke said:
You are right, the fuel inlet should hold a minimum or 7 or 8 psi for normal operation, however if the pressure in the fuel tank builds up (hot day, saw sitting in the sun), the inlet valve should not be forced open until the pressure exceeds anything the tank can produce, this is called the "pop-off" pressure and usually 15psi is enough. A vacuum test has no meaning on the fuel inlet.
If you are applying a p/v test to the purge port on the carb, it should not hold pressure, the air being forced in would just dump out through the main jet and the idle ports. If you apply a vacuum to the port, it probably won't hold, the main diaphragm will just be sucked down and open the inlet valve, however, if you block the fuel inlet port and apply a vacuum, it SHOULD hold a vacuum if the check valves in the carb are working properly (main jet check valve and may be another body check valve, depending on carb)
The purge bulb contains 2 check valves that keep the fuel moving in one direction only, from the carb to the tank, so the suction port of the purge should hold vacuum but not pressure. Generally, the purge is hooked up with clear lines and if you can see fuel being pumped through in the right direction, it's fine.
Click to expand...
Now to remember all of this and/or be able to reference it when needed Thanks
 
455 Ranchers. Worked on a thousand. Mostly solving the oil leak. So, is there any good reason to NOT simply buy another carb? Seems like you are "trusting" so much, when replacing parts to solve the problem, is the end game. Get a carb, return it if there is no change, after you replace the entire fuel system, including cap and vent. Thats about 40 bucks...and you are spending 10 x that in personal time.....for a simple fix, on a chainsaw...maybe you just like to chat?
 
Captain Bruce said:
Thats about 40 bucks...and you are spending 10 x that in personal time.....for a simple fix, on a chainsaw...maybe you just like to chat?
Click to expand...
There is merit in learning, especially hands on, making mistakes and learning from those, which makes it easier to remember (been there done that kinda situations).
There is also merit in learning, or just recapping ideas, by reading along in discussions with other people chatting about their learning process, like it is for me in this discussion. Imho that's pretty much the basic idea around a forum like this, isn't it? It's fantastic that we have these tools (forum) today to exchange ideas with (more experienced) colleagues and learn from one another.
If we'd all just throw new parts at will at any problem that we can't get our head around and don't chat/ask about it, chances are we die as dim-witted as born...
From me +1 for this discussion - thanks to OP and all involved!
 
Old2stroke said:
popoff pressure is a test done on the fuel input port and has NOTHING to dp with the impulse that operates the fuel pump
Click to expand...
When you say it like that There is no argument from me ... I will go back and read thru this thread . I am reading it to learn like "Pudsy" on post #31
Pudsy said:
From me +1 for this discussion - thanks to OP and all involved!
Click to expand...
 
Old2stroke said:
popoff pressure is a test done on the fuel input port and has NOTHING to dp with the impulse that operates the fuel pump
Click to expand...
With all the details in your replies I still let that get by me. Posts # 9 - 14-19 - 20 - 24 and 32 breaks things down for me. I read post #19 The OP ordered a new carb and now I will have to assume the nozzle/check valve part "Woodslasher" mentioned was(IS) the culprit. I have read other threads discussing the fuel circuits, nozzles, welch plugs and from lack of hands on use they grow fuzzy.
Old2stroke said:
These devices have a little rubbery disc that is often distorted and becomes intermittent so "bench testing" it doesn't mean it will continue to work in a running saw.
Click to expand...
The disc part being intermittent is new to me (Thanks) and is the main reason for OP buying a new carb.
I hope I retain all this STUFF.
AND THANKS again for all the DETAILED replies.
 
REJOICE, MY BROTHERS! WE ARE VICTORIOUS!

The issue was the new valve needle lever part from the rebuild kit. The pump/fuel intake would hold vacuum (so pump seal check valves ok), but NO pressure. If I took off the lever and spring and just held the needle down with my finger, it sealed perfectly. Put the old lever back on, 100% seal. Saw starts right up. For some reason, the new lever wasn’t allowing the needle to go down far enough to contact the seat.

Canceled the new carb, (saves me $90+) but I still got a new inlet pipe on order because one of the screw holes is stripped out. I suspect we still have an air leak there after the carb.

For anyone in the future reading this, the video below REALLY helped as well as all the help I received in this thread. THANKS GUYS!
 

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