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Carburetor Pop-Off Pressure TipsHi all, You need to understand how pop-off pressure works, and what its purpose is, which leads to how the carburetor works. The demand diaphragm is the VIP part - use nothing here but original Walbro because the copies leave a lot to be desired. This diaphragm is made out of some trick stuff; a rubber and fiberglass compound type of material. This is the black one with the alloy flat washer looking part in the center with one side having a small round pin sticking down. This pin on the diaphragm is in line with the fulcrum arm and comes down and pushes on one end of the fulcrum arm. This action releases the needle off the fuel inlet seat to let fuel from the pump enter the metering side of the carburetor. How quickly this action takes place is controlled by the strength of the spring that is under the fulcrum arm, to be pushed down by the pin, to release the needle off the seat, to let fuel flow (or to return back so the needle can seal the fuel flow off). The vacuum of the engine on the intake side controls the action of the demand diaphragm. Vacuum on the intake stroke will pull fuel out of the carburetor discharge ports which in turn makes vacuum that forms on the back side of the fuel as it leaves the carburetor and this vacuum pulls the demand diaphragm down to let the needle come off the seat to keep filling the metering side of the carburetor as needed. The pop off pressure measures the amount of pressure (or spring tension) that it takes to let the needle off the seat. It is where the lb. reading was at max on the gauge as you pump it up and then all of a sudden it drops a bit. Pop off pressure can be anywhere from 6 lbs. on up. Anything under 6 and the needle never really shuts off the fuel flow to the metering side of the carburetor. When you release the throttle the carb shutter goes closed and the high speed airflow (suction) is closed off from the carburetor. Now the vent on the cover plate (the little hole where we press to prime) allows the diaphragm return back up and the inlet needle drops back onto the seat due to the spring pressure. The way you use your pop off gauge if you are checking with the carburetor all together either on or off the engine is to replace the short piece of fuel line on the gauge with one about 3 ft. long. Fill this line up with fuel and then plug it onto the fuel inlet nipple on the carb. Remove the high side needle from the carburetor. Pump up the gauge till it pops off just to wet everything up. Now take 2 more readings. When the needle gets to the point where it won’t go any more and you hear a pop it will drop just a tick. This number in front of the needle in the gauge is what the pop off pressure is in lbs. When it pops it must pop and hold with no less than a ½ lb. of drop then hold hard here. A very slow seep is ok and shouldn’t hurt you much but most tuners want the reading to lock solid and hold at that little bit of drop from the number they want. If it won’t hold pressure and drops like a rock you can sometimes clean the seat and needle tip to cure problem. If this doesn’t work time to go get some rebuild parts. Pop off pressure you want for most carburetors is 10 to 12 lbs. for carbs on 100 cc and up engines. Smaller engines (Top 80) will like 15 to 20 lbs. These are round numbers and from here it will take some time to find the perfect happy spot. Looking forward to what you all find out. Best regards, Mike Forbes mike@michaelforbes.com
Additional Tips:Here is the video (put out by Walbro) I was telling you about guys about. It gives a good overview and makes it very easy to understand for pilots who have no idea how carbs work. Only the first half (up to the 13 min. mark) applies to our style butterfly valve carbs. The second half details a different kind of carb. It would be nice to have a similar video just for the WG8 Miniplane carb, and maybe a custom tool to set the correct metering lever value could be sent with the carb rebuild kits. Here is what Had found out. (Note: I edited this for clarity)
"Over the years, I have noticed that Miniplane ships out new Top 80 engines with metering levers on the carburetors set to a value different than the one given in their Italian service manual. The measured value on new Miniplane Walbro WG-8's has consistently been 1.7mm. This is also the same measured value of metering lever's in the rebuild kits from Walbro. The value given in Miniplane's service manual is 0.5-0.7mm. Extensive tests have demonstrated that the service manual value is correct rather than the value measured in carburetors on new engines from the factory. For a long time, I set metering lever's to the same value as on new engines (1.7mm). This was a mistake, as I now realize. The problems we experienced were poor throttle response and occasional stalling when suddenly going to full power. This 1.7mm value prevents fuel valve from opening fully (only about 0.5mm). At the 0.5-0.7mm value the valve will open 1mm. This is a significant difference -- 100% greater. I tested a newly rebuilt engine with both values. With the 1.7mm value, sudden operation of the throttle almost always stalled the engine (it was briefly starved of fuel). With the 0.5-0.7mm value, I could hit the throttle to fully open from idle and the engine would immediately accelerate to full speed without sputtering or stalling. It also seemed that the mid range was smoother. I wondered whether running at full load for extended periods might burn up the engine if the 1.7mm value was used? This is not something I wanted to find out. Obviously, engines with the 1.7mm value will run, but may fade when going to full power or have poorer throttle response. There is also the risk of leaning out the engine (burning it up)."
Based on this I reset my metering lever to 0.7 (30 thousands on my dial indicator). My pop off pressure is 18 psi. and reseats at 10 psi. and holds firm. I agree with Had. If you check the Miniplane manual it does indeed call for a 0.5-0.7mm metering lever value, and 17-18 psi. pop off pressure. It seem to be running good now. This makes sense, if the WG8 was originally specked for a 50cc motor, but Diego had them sized with a larger fixed high jet. BTW I believe years ago the Top 80 used a WG32 (which had adjustable high and low jets). The Supa-Tuna should tell us for sure whats going on. Here's a video of one on a paramotor. Terrible video but if you pause at 36 seconds you can see it. Best, Mike
Resources:Spring and shim kit: assorted springs and shims used to adjust to any pop off pressure (without cutting the spring or stretching the spring). Cutting and stretching the spring as taught by Alex Varve are not the correct way to achieve proper tune and will create erratic conditions and confuse you. Pop off pressure gauge: Anyone using a diaphragm carburetor needs to own a pop-off gauge to service their carburetor and trouble shoot any carburetor related problems. Walbro Metering Level Gauge (beware this will likely set the lever to the factory 1.7mm value, but could likely be modified) Walbro WG-8 Carb Disassembly A very good page with lots of pictures and exploded diagram.
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