Cleaning carb idle drillings without removing welch plug

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Okie said:
I did not read all of this but in answer to your question about removing welch plugs or clearing idle passages.
When I'm first flogging a carb of any kind, small to large I do not remove welch plugs.
I clean them good, and now days use a Usonic. I use low pressure air (from a air compressor) and/or spray can air to check the [passages.
If the carb indicates it's not right after doing a performance test on the eq I then go back and very carefully drill a itty bitty hole in the center of the welch plug and if it's aluminum I can usually persuade the plug out with a ice pick or a o ring pick, if steel I may have to casrefully use a sharp pointed small screw as a puller.
After the plug is out I can then reform the plug using a soft leather backing and a round punch or a ball bearing from my stock pile. I also save welch plugs and keep both the ball bearing and welch plugs in same place.
When I re-install the welch plug I have some really good strong slow set non runny epoxy about consistency of margarine (24 hours to fully cure) and coat the complete out surface of the plug to the edges. Mine is actually Brownells acraglass gel and can be mixed with aluminum or metal particles.
About $22 at this ebay linkhttps://www.ebay.com/itm/264504234121?epid=1700207420&hash=item3d95ae5089:g:w7YAAOSwKFlfU-BD
This stuff stores really good, but if you open the white cap jug and it's kinda grainy months or years later, just get some hot water and set both of them into the water for few minutes then slowly stir as the plastic container warms and they will become the proper consistency again. Do not get water into the epoxy. Also using warm water will result in a faster cure time, but the warmer you get it the thinner it becomes, but it will still not flow freely.
Very good stuff. I've use it for lots of piddling projects and it will glue rubber to metal.
Mainly use for glass bedding gunstocks. You can review release agents if you have a place you do not want it to bond to paint the surface with the release agent. Kiwi Neutral shoe wax is a good release agent. (their are others)

Fast set epoxy is not as good as my slow set type and I do not trust any JB weld stuff for long term bonding use of carb welch plugs.
Click to expand...
Thanks. I'll file this one.
Have decided not to remove as the passages are clear.
 
frank_ said:
the scored piston not sealing the inlet port properly tom
you can usually see a haze of fuel in the mouth of the carb, and get a laggy throttle response
Click to expand...
Thanks Frank.
The thing is after I replaced the pistons and ring i'm still getting the same symptom.
Compression is noticeably stronger but I don't have a gauge.
Tom.
 
SteveSr said:
Then leave the welch plug alone


Possibly... this test is hard to do as you have to seal off the other feeds into the "H" circuit. With this carb it looks like closing the "H" screw should be enough. Use generic fuel line squared off with a sharp razor blade to apply pressure and vacuum for the "straw" test. You should hear the valve click and flow much more in than out.

This looks like a "bypass" carb where most of the fuel goes through the jet in the main nozzle and only about 25% goes through the "H" screw. This provides a limited range of adjustability but much less sensitivity.


Probably.


No. They were designed like this. As you can see the screws are different with different screw threads to keep from getting them mixed up.

Keep in mind that the saw may have other air leaks that need to be fixed which is why it may have burned up in the first place. A main nozzle check valve usually won't cause this. If it sticks closed the saw won't rev up (no fuel). If it sticks or leaks open it will create an internal air leak and will stall at idle or the "L" screw will have to be open much further than spec to get it to idle. Symptoms can also be erratic which is why the rest of the saw has to be pristine to rule-in the carb.

I also found the Zama metering lever adjustment. It is interesting that they were too lazy to put the correct "bump" on their gauge for some of their carb models. File this under "Watch Out With Zama!" i.e. don't trust the gauge!
Click to expand...
I did the test with the h screw all the way on and could hear the valve popping. I can definitely suck through but not much.
With the screw open I can get air moving fairly freely in both directions.
So following Zama's own troubleshooting sequence the nozzle seems to be faulty?

'Test the main nozzle by blowing air by mouth through the H needle feed hole with a small hose. With the
needle open 2 turns open air should flow through, but you should not be able to suck air back.'
 
I'm trying to learn about fixing those check valves myself. As best i understand they are supposed to allow fuel to flow into the throttle body from the H circuit but to close and not allow air the other direction from the throttle body into the H speed circuit. The reason being if air is allowed into the H circuit it causes erratic behavior at low throttle or idle.
It would make sense that you should be able to blow air through the H needle feed hole and into the carb throttle body, but not suck air back in.
Here is my currently disassembled Walbro MDC with the Welch plug removed.
20210624_163927_HDR.jpg
 
MacAttack said:
I'm trying to learn about fixing those check valves myself. As best i understand they are supposed to allow fuel to flow into the throttle body from the H circuit but to close and not allow air the other direction from the throttle body into the H speed circuit. The reason being if air is allowed into the H circuit it causes erratic behavior at low throttle or idle.
It would make sense that you should be able to blow air through the H needle feed hole and into the carb throttle body, but not suck air back in.
Here is my currently disassembled Walbro MDC with the Welch plug removed.
View attachment 915010
Click to expand...
This carb has a capillary check valve. That is what that screen in the nozzle is. Capillary action of the fuel mix in the screen keeps the carb from sucking air back in through the "H" circuit. Usually these don't need replaced, just cleaned.
 
tomalophicon said:
I did the test with the h screw all the way on and could hear the valve popping. I can definitely suck through but not much.
With the screw open I can get air moving fairly freely in both directions.
So following Zama's own troubleshooting sequence the nozzle seems to be faulty?

'Test the main nozzle by blowing air by mouth through the H needle feed hole with a small hose. With the
needle open 2 turns open air should flow through, but you should not be able to suck air back.'
Click to expand...
You need to look at the flow schematic for your particular carb (or figure it out by inspection) before jumping to any conclusions. The Zama manual may or may not apply to your specific carburetor. If you are testing the main nozzle you have to plug or bypass any other possible leak sources to get a valid test.
 
SteveSr said:
This carb has a capillary check valve. That is what that screen in the nozzle is. Capillary action of the fuel mix in the screen keeps the carb from sucking air back in through the "H" circuit. Usually these don't need replaced, just cleaned.
Click to expand...
It you're talking about my Walbro carb, it does actually have a nitrile - fabric disc behind that screen that deteriorates. Here's what I found inside:

20210624_165253_HDR.jpg

And a new one:

20210626_083237_HDR.jpg

With the screen removed:

20210624_165336.jpg
 
MacAttack said:
One is OEM Walbro, one I just made from a nitrile pumper diaphragm using a leather punch....can you tell which is which?

I may have found a way to make some sweet check valve discs. :)

View attachment 915099
Click to expand...
Great job. I can't tell which is which.
I don't think I can remove mine without cutting into the housing unfortunately.
 
MacAttack said:
It you're talking about my Walbro carb, it does actually have a nitrile - fabric disc behind that screen that deteriorates. Here's what I found inside:

View attachment 915084
Click to expand...
That's not nitrile... It is fiberglass. True nitrile is not very fuel resistant. your new one may or may not stay in the main nozzle under running conditions. At least it shouldn't hurt the motor if it gets ingested. You may have better luck with the tan (teflon?) fuel pump diaphragms. There are also plenty of threads on here about messing with / repairing these old check valves.
 
SteveSr said:
You need to look at the flow schematic for your particular carb (or figure it out by inspection) before jumping to any conclusions. The Zama manual may or may not apply to your specific carburetor. If you are testing the main nozzle you have to plug or bypass any other possible leak sources to get a valid test.
Click to expand...
Ok.
I pressed out the nozzle and found another drilling in the side which I believe leads to the hole close to the valve in the metering chamber.
The nozzle seemed to work some of the time when tested in isolation. I could hear it flapping but could still get air through by sucking.

I then reassembled the saw thinking that if the valve seems to function when the H is closed all the way then it should idle. No dice. No matter where the high is it still won't idle.

One thing I haven't done is a vacuum test which i'll try to do soon. Gotta rig up a device for that.

Tom.
 
SteveSr said:
That's not nitrile... It is fiberglass. True nitrile is not very fuel resistant. your new one may or may not stay in the main nozzle under running conditions. At least it shouldn't hurt the motor if it gets ingested. You may have better luck with the tan (teflon?) fuel pump diaphragms. There are also plenty of threads on here about messing with / repairing these old check valves.
Click to expand...
In the picture of my finger, the rubber disc on the left is the one I made from a walbro pumper diaphragm, the "rubber" disc on the right is a brand new genuine Walbro replacement check valve disc for an MDC carburetor. Maybe I used the wrong term "nitrile", perhaps it's viton? Buna? But in any case it is absolutely a "rubbery" disc, .2mm thickness.
It's not possible to be ingested unless it completely disintegrated, it is held in place on the inside by the fine mesh screen, and on the other side is the pinhole orifice into the carb throttle body.
I do have some unused teflon diaphragms like you described, and I'm tempted to try making a check valve from it to try.
 
tomalophicon said:
Ok.
I pressed out the nozzle and found another drilling in the side which I believe leads to the hole close to the valve in the metering chamber.
The nozzle seemed to work some of the time when tested in isolation. I could hear it flapping but could still get air through by sucking.
Click to expand...
This is not a great sign although it sometimes takes the capillary action of the fuel to completely seal the valve.

tomalophicon said:
I then reassembled the saw thinking that if the valve seems to function when the H is closed all the way then it should idle. No dice. No matter where the high is it still won't idle.
Click to expand...
No, I think that you are confusing fuel flow with air flow. The "H" needle will cut off about 25% of the "H" circuit fuel but none of the air. This was done only to test the check valve. The issue with the check valve is that if it doesn't seal it will let air into the metering chamber disrupting the idle fuel flow.

tomalophicon said:
One thing I haven't done is a vacuum test which i'll try to do soon. Gotta rig up a device for that.
Click to expand...
There are lots of other places for leaks. If you are lucky and don't find any then you will likely need a new carb or a main nozzle for your existing one if it is available.
 
MacAttack said:
In the picture of my finger, the rubber disc on the left is the one I made from a walbro pumper diaphragm, the "rubber" disc on the right is a brand new genuine Walbro replacement check valve disc for an MDC carburetor. Maybe I used the wrong term "nitrile", perhaps it's viton? Buna? But in any case it is absolutely a "rubbery" disc, .2mm thickness.
Click to expand...
That is interesting as both Zama and Walbro have done away with that design at least several decades ago.

MacAttack said:
It's not possible to be ingested unless it completely disintegrated, it is held in place on the inside by the fine mesh screen, and on the other side is the pinhole orifice into the carb throttle body.
Click to expand...
That is an older design that I am not familiar with. I guess I don't understand how fuel gets past the check valve disc and into the venturi. The new designs place the disc inside the main nozzle near the tip and the fuel flows around it. All of these discs are made out of a stiff fiberglass like material.
 
tomalophicon said:
Ok.
I pressed out the nozzle and found another drilling in the side which I believe leads to the hole close to the valve in the metering chamber.
The nozzle seemed to work some of the time when tested in isolation. I could hear it flapping but could still get air through by sucking.

I then reassembled the saw thinking that if the valve seems to function when the H is closed all the way then it should idle. No dice. No matter where the high is it still won't idle.

One thing I haven't done is a vacuum test which i'll try to do soon. Gotta rig up a device for that.

Tom.
Click to expand...
Does it run at WOT?
 
The shape of the venturi determines how the valve opens at a determined vacuum draw = high vacuum at idle holds the valve closed , wot low vacuum opens the valve.
My guess is that the valve is not closing and is dumping fuel otherwise it would die when you stab the throttle. Like I said guess? Could be your welch plug leaking around the edges, usually you can see sealant around them from the factory.
 
SteveSr said:
This is not a great sign although it sometimes takes the capillary action of the fuel to completely seal the valve.


No, I think that you are confusing fuel flow with air flow. The "H" needle will cut off about 25% of the "H" circuit fuel but none of the air. This was done only to test the check valve. The issue with the check valve is that if it doesn't seal it will let air into the metering chamber disrupting the idle fuel flow.


There are lots of other places for leaks. If you are lucky and don't find any then you will likely need a new carb or a main nozzle for your existing one if it is available.
Click to expand...
Thanks.
Do you know if I plug the check valve (temporarily) that will give me a definitive answer?
Tom.
 

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