Carb & Fuel Flow Question

[Bartman]

Would anyone happen to have a fresh pump diaphragm to compare to the pic below? It looks like the membrane of the pump should be tighter, no? Thanks!

 

[acerc]

Just went and looked at one of my Walbro's and it is no where near that much flex or excess. But if I remember right you said this carb was a Bing on a ZDZ, it might be right for it, I don't know.​

 

[acerc]

Just looked up a Bing diaphragm and is does not have that flex either, pretty much like my Walbro, very little movement.

 

[49dimes]

That's the metering diaphragm. Don't know if the one in the K-20 WAT walbro kit is the same. It very well may be. If you are inclined to stay with the Bing carb try cleaning/spraying carb cleaner in all the tiny passages. Varnish build up from evaporated fuel can cause problems / restrictions and also cause the check valve before the HS jet to stick close or open. If the engine started but wont stay running leads me think your problem lies there. A stiff or distorted metering diaphragm usually allows an engine to run but not tune well if at all or not start. A check valve can be cleaned of deposits and "unstuck" but they are easily damaged. Do not insert small wire or the tip of a spray tube all the way in to the HS jet. It will damage it. That's why I say replace the carb. It's the EASY BUTTON and I'm all for easy these days



Pictures show a WT1115 Walbro 40-50cc carb. The scribe is pointing to the check valve permanently sealed cover. The last picture shows the very tiny idle ports (jets). To clean those remove both HS LS needles and spray carb cleaner from there. Cover one opening with your finger while spraying the other and vice versa. It also cleans the HS jet and check valve.

 

[acerc]

I have always found it odd that the part that moves to pull fuel is a metering and the part called a pump is hard flaps. For me the diaphragm is the pump, if it don't move from the push and pull of the pulse no fuel moves. Of course if the flaps don't open and close properly no fuel moves either. I have had engines, or airframes, that would sit for long spells and did not want to start pulling fuel like a new engine. So I would run a tube from the cover to an exterior opening and use a syringe to operate the diaphragm, works like a charm to get the fuel started flowing again. I also use a syringe on new engines on the stand, just works quicker and easier than all the flipping.

 

[49dimes]

The "pump" is actually the underside of the piston and engine crankcase. The "leaflets" open and close like valves with the reciprocating motion. The top of the piston and cylinder is the "pump" in 4cycles like your Valach's. But simply these carbs have two sides. The pump side and metering side. The metering side allows the diaphragm to move up and down only because the fuel under the diaphragm is also under the same pulse pressure the pump side sees. This is the force that allows the metering plunger to open and close. The metering plunger is mechanically attached to the diaphragm via a cantilever and is "Biased" with that cantilever/spring and atmospheric pressure.
This "Biase" determines how far the metering plunger moves off its seat for a "fixed amount" of fuel to flow.

 

[acerc]

Thanks for clearing that up, I don't know what I was thinking.

 

[Bartman]

I can see how you'd think that the diaphragm is the pump, I thought it was the pressure pulses from the piston/case that worked on the diaphragm to make it pulsate and move fuel from the tank.

Thanks for all the info @49dimes, I'm going to order a new carb tomorrow. Parts aren't available for the Bing and I'm officially fed up at this point.

 

[acerc]

I can see how you'd think that the diaphragm is the pump, I thought it was the pressure pulses from the piston/case that worked on the diaphragm to make it pulsate and move fuel from the tank.

Thanks for all the info @49dimes, I'm going to order a new carb tomorrow. Parts aren't available for the Bing and I'm officially fed up at this point.

Sad thing is I know it is not the pump, I've been to school on these things although it was a long time ago. What I actually do when I hook a syringe to the diaphragm metering side, and I believe I leave this part out most the time, is apply pressure to the tank via the inlet hose in my mouth and pull on the syringe. What this is doing is opening the metering needle with the syringe and the pressure will blow fuel through the pump diaphragm, through the metering side, through the needle ports into the carb orifice. Hence, priming a dry carb.

 

[Bartman]

i stopped by the local small engine repair shop and the guy hooked me up with a pump diaphragm from a Tillotson carb. He seemed to think that it would work just like the original Bing carb worked. The holes didn't line up exactly but it was close enough so I'll give it a try, maybe tomorrow.
If not, then it's off to DA to order a Walbro WJ-71 for a reasonable $85. The Husqvarna dealer wanted $140!!!