question for the carb experts.

[bugfart]

How do you know if the diaphram needs replaced?
The new kits are nice and plyable... mine seems close when in gas, but if you set it on the table it gets some-what rigid. No holes or tears, but... it may have stretched a little bit.

 

[NevadaWalrus]

Most modern 2 cycles rev at least 10,000 rpm. The diphragm flexes twice during each rev (once in, once out)so that's 20,000 flexes per minute. Do the math per second and you'll realize how critical it is that it can work as intended. If in doubt, replace, if not sure and its more then a year old, replace. Echo says to maintain performance and emission regs to rebuild carb every year for homeowner use and twice a year for commercial users.

 

[Lakeside53]

Just replace it if in doubt. Generally thought, if its flexible in gas, it will work fine. If distorted or wrinked, dump it for sure.

Hey Nevada, I think you're on the wrong diaphragm - the pump side works as you suggest, but the metering side (I think that's what we are talking about here) flexes much less - only when the needle calls for more gas as it drawn down by use. It's still "a lot", but not per cycle.

 

[MacDaddy]

How do you know if the diaphram needs replaced?.

When your girlfriend gets knocked up

 

[bugfart]

How do you know when your girlfriend needs replaced?

 

[SWE#Kipp]

when you know you should replace her it's already to late, no one will trade her in for the latest model

 

[p38lightning]

I made a carb pressure tester for nearly nothing and it works good. I make my own beer and had a bubble pump that I use to keg with.(Like the pump on a blood pressure cuff) I went to a cookware store and bought a pressure guage for a pressure cooker. I then went to the hardware and bought a T fitting and put barbs on the sides and twisted the guage down into the top. The threads in the T were pipe threads and the guage wasn't so I twisted it in as best I could then sealed it with T-71 kinda like JB weld which Im sure would work. Pump it to 7psi and if it holds the pressure, there are no leaks. You can also use it to test crankcase pressure if you cut you a plate and gasket to cover the exhaust port. You would be suprised how often you find leaking crank seals though I do work on mostly older saws. Those things cost quiet a bit for no more than they are, this way you can have one for a few dollars. I'm a bit cheap!

 

[Al Smith]

Good idea on the pressure tester.I made one from a blood pressure cuff,e-bay,cheap.

 

[bugfart]

I really don't know how to change a seal yet, so I better not check...

Actually, I was going over the IPL and was surprised to see there was only one. What's up with that?

 

[NevadaWalrus]

Just replace it if in doubt. Generally thought, if its flexible in gas, it will work fine. If distorted or wrinked, dump it for sure.

Hey Nevada, I think you're on the wrong diaphragm - the pump side works as you suggest, but the metering side (I think that's what we are talking about here) flexes much less - only when the needle calls for more gas as it drawn down by use. It's still "a lot", but not per cycle.

The metering diaphragm is activated by engine pulses, not the inlet needle. Every piston down stroke pushes it out, up strokes suck it back in. Two flexes per rev.

 

[Al Smith]

I really don't know how to change a seal yet, so I better not check...

Actually, I was going over the IPL and was surprised to see there was only one. What's up with that?

I have a post on another site,I dare not mention,of how to change a seal.It's up to you to find that site.If all else fails after 3 days,send me a pm an I will send you a picture.In the mean time,best of luck.

 

[Lakeside53]

The metering diaphragm is activated by engine pulses, not the inlet needle. Every piston down stroke pushes it out, up strokes suck it back in. Two flexes per rev.

The metering diaphragm is exposed to the atmospheric pressure on it non-fuel side (or the inside of the air filter on the "intelligent design"). The fuel side is via the inlet needle which lets gas into the metering diaphragm chamber when the reservoir is depleted enough to allow the needle to become unseated. The metering diaphragm is pushed by atmospheric pressure in response to the lowering pressure in the venturi.

For a complete description of this (and there are many other sources) look at pages 9 and 10 of the Stihl Carburetor manual.

 

[NevadaWalrus]

The lowered pressure in the venturi IS an engine pulse from the same source as the fuel pump side. It arrives in a different manner but comes from the same source, changes in pressure within the crankcase. I under stand what you're saying, I think. At high rpm the diaphragm may not fully close the inlet needle because the pulses are so close together but it still has some flex twice per rev. Any deviation in the diaphragms ability to move as intended will affect fuel flow within the carb.

This is why on modern emission engines diaphragm condition is more critical then ever because there is no longer fully adjustable needle valves to compensate for worn or damaged parts.

 

[Lakeside53]

maybe

 

[spacemule]

How do you know when your girlfriend needs replaced?

Oh, that's easy. Shortly after your diaphram wears out.