The tan is also called a check valve.
McCulloch Chain Saws
- Thread starter Bruce Hopf
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Ahh OK. This was on a Zama carb for an 850, but I took it to be a general comment.
There was no tan diaphragm in the carb to begin with.I added it to try to get the carb to hold pressure,then I deleted it later.
The tan (Teflon, snake skin, whatever you want to call it) affects the fuel pump portion of the carburetor but does not have an impact on whether the carburetor will hold pressure when testing it through the fuel fitting. Holding test pressure is purely a function of of the metering needle and seat (needle, seat, spring, lever, and metering diaphragm).
The backup diaphragms on the fuel pump side are only there to insure the flaps seat tightly enough for the fuel pump to develop the pressure needed with the saw in operation.
If the carburetor did hold pressure when you installed the Teflon back up diaphragm I would guess you accidentally achieved a good seat of the metering needle whether you intended to or not.
Mark
The backup diaphragms on the fuel pump side are only there to insure the flaps seat tightly enough for the fuel pump to develop the pressure needed with the saw in operation.
If the carburetor did hold pressure when you installed the Teflon back up diaphragm I would guess you accidentally achieved a good seat of the metering needle whether you intended to or not.
Mark
A good pump side seal between the cover and carb body is normally required to provide enough pressure to the needle and seat for a successful test. A bit of fuel in the carb can also help in the case of a marginal test result.
I am 99.9% certain that when pressure testing a carburetor on the bench with the positive pressure being generated outside the carburetor, both check valves will effectively be "open" and the needle is the only thing that actually holds the pressure. When the carburetor is in operation, both check valves in the fuel pump must be working alternately as the diaphragm moves up and down creating higher and lower pressure on the wet side of the diaphragm.
Mark
Often wrong but seldom in doubt...
Mark
Often wrong but seldom in doubt...
Please note my "normally required" disclaimer above.
All this carb talk makes my head spin. One good thing about my off-brand - no carb. Then again, electronics and computer stuff make my head spin. I think I will go lie down and contemplate the simplicity of a one man cross-cut saw.
Ron
Ron
That could explain the way everything went down.
Or the pump cover wasn't tight enough.
Not trying to be argumentative.., and certainly not with Mark, but let's think this through in very simple terms.
If the pump diaphragm cover isn't tight enough to provide adequate back pressure from a test source to at least equal the pressure of the metering lever spring, (and atmospheric pressure), the carb will leak at the cover/gasket before a true test of the needle and seat can be achieved..., provided the fuel inlet of the carb is being used as the pressure insertion point. Depending on the type of carb and cover, a pump diaphragm isn't necessarily even required for such a test when using an external pressure source -- which essentially agrees with Mark's general position on the diaphragm aspect.
As stated, (and demonstrated), a reasonably tight pump cover is "normally required" to perform an effective pressure test on a typical chainsaw carburetor. A loose cover may not provide the back pressure required to achieve meaningful results otherwise. And when attempting to locate an evasive leak when testing a carb, test it while it's submerged in a jar of fuel and look for the bubbles. It can save a lot of time.
Not trying to be argumentative.., and certainly not with Mark, but let's think this through in very simple terms.
If the pump diaphragm cover isn't tight enough to provide adequate back pressure from a test source to at least equal the pressure of the metering lever spring, (and atmospheric pressure), the carb will leak at the cover/gasket before a true test of the needle and seat can be achieved..., provided the fuel inlet of the carb is being used as the pressure insertion point. Depending on the type of carb and cover, a pump diaphragm isn't necessarily even required for such a test when using an external pressure source -- which essentially agrees with Mark's general position on the diaphragm aspect.
As stated, (and demonstrated), a reasonably tight pump cover is "normally required" to perform an effective pressure test on a typical chainsaw carburetor. A loose cover may not provide the back pressure required to achieve meaningful results otherwise. And when attempting to locate an evasive leak when testing a carb, test it while it's submerged in a jar of fuel and look for the bubbles. It can save a lot of time.
I had an HS (Tilly) carb that gave me fits about 2 mo.ago.I finally ended up sending it to a fellow out in Oregon who was stumped almost as much as me.He did the test as I did with the carb underwater.The bubbles came out of the air vent in the fuel pump cover.This should not have happened with the cover screwed on tight & a new gasket in place,yet it did.The only way he finally got the carb to hold pressure was to use a sealer on the gasket.There were actually 2 HS 132A Carbs that did this & one of them was brand new.Sometimes ya get a carb that likes to bite you in the behind.
GoBigRed
ArboristSite Operative
Let me guess who it was....Leon?
Yes sir!
GoBigRed
ArboristSite Operative
Great guy. I’ve sent him some stuff that I was totally stumped on. And somehow he figures them out. I do all my own wrench work but it’s nice to get a better mechanic on the jobs that give me troubles. Shipping is a ***** but you can’t beat his labor costs.
He did a pretty thorough video on those carbs..., at least the main troublemaker. It still leaked when he was finished, but was within generally acceptable limits. A small leak on the pump side is not necessarily uncommon and is usually acceptable -- within certain limits, of course. (Walbro even explains that in one of their repair guides.) No leak is obviously a better feeling, but not always achievable.
What must be remembered is that some carbs are intentionally vented on the pump side. The HD in my video is, and will not hold pressure without the diaphragm. If the cover was solid, a diaphragm wouldn't even be needed for testing the needle and seat. Depending on how and why, the correct diaphragm usually provides a seal when testing with an external pressure source, but not always. Some will indeed leak when air is applied through the fuel inlet -- which is not what a carb pump side wants to see in the first place. It wants to see fuel. It wants to see air from the crankcase to pump the fuel. Apples and oranges when it comes to how a pump diaphragm reacts to each condition. The presence of fuel in the pump chambers alone can eliminate a leak that would otherwise occur when testing with air.
What must be remembered is that some carbs are intentionally vented on the pump side. The HD in my video is, and will not hold pressure without the diaphragm. If the cover was solid, a diaphragm wouldn't even be needed for testing the needle and seat. Depending on how and why, the correct diaphragm usually provides a seal when testing with an external pressure source, but not always. Some will indeed leak when air is applied through the fuel inlet -- which is not what a carb pump side wants to see in the first place. It wants to see fuel. It wants to see air from the crankcase to pump the fuel. Apples and oranges when it comes to how a pump diaphragm reacts to each condition. The presence of fuel in the pump chambers alone can eliminate a leak that would otherwise occur when testing with air.
My point was that the fuel pump check valves do not have any affect on whether the carburetor will hold pressure or not. On a cube type carburetor like the SDC or HS the fuel pump cover, diaphragm, and gasket would need to be in place and tightened down to pressure test, otherwise the pressure would just vent back through the impulse port.
Mark
Mark
My point was to illuminate and clarify what could have been easily misinterpreted otherwise. Hopefully I did so in a helpful and non-contentious manner.
Not sure how successful I was, but I'm 0.1% sure that was my intention.
Not sure how successful I was, but I'm 0.1% sure that was my intention.
Some McCulloch bling I recently acquired. You may have seen this on eBay, I contacted the seller and was able to get it for a reduced rate. I am not sure what Richard V. Dempster's role was at McCulloch but his name appears on several documents I have in the R.V. Jackson file. Robert Vernon Jackson was an Engineer at McCulloch and R.V. Dempster was cc'd on some of R.V. Jackson's patent award letters.
It looks a little more like a beaver in person...
Mark
It looks a little more like a beaver in person...
Mark
As promised, my update on the PM700 shim for the 5-10. The thin one has a little intake "flare" and the thick one does not, but aside from that the smaller one is identical and will work perfectly on a 5-10. Also, the 5-10 fired right up, it still needs to be tuned but I also still need to find a log to tune it in.
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