Zenith carb leaking air

[bernd_st]

Great ! Are you talking about the pump discharge nozzle as shown in the picture attached ?

 

[62SNBMR]

No. In the parts list it's identified as the pump check valve (#48 in the diagram). Others have called it the accel pump bleed orifice, and another diagram called it the non-return check valve. I noticed that the good valve provided resistance in one direction, then free flowing in the other. The bad valve was free flowing in both directions. Luckily I had an old carb set up that had good check valves, though slightly different. In any event, it's running great, no lag! Strong stream out of both discharge nozzles. Thanks for you help Berndt! And to the others who commented. It's great having these resources for help.

 

[bernd_st]

Alright then. Number 48 "Pump check valve" is what I referred to under thread posts #25 & 33. Good that you had a used pair. Wasn't quite aware myself that there are two different types. Just checked my stocks and only found 1 pc. loose (see the pictures). Looks like the 0.5mm hole allows a tiny bit of flow in the opposite direction. Still need to study that more.
Anyway and thanks to your persistence you ultimately found the culprit - congratulations...

 

[albeam]

well done guys,

Are really good diagnostic thread to solve the problem. i have certainly learnt a few things. there truly is a wealth of knowledge on this web site.

albeam

 

[bernd_st]

Positive feedback appreciated, thanks.
Just to close it off. The Pump check valve with the small hole is the later type listed under #5037774 in the Rootes Parts List. Furthermore the correct one for your S2 carbs #1744 F/R.

P.S. Even though I spent a great deal of time refurbishing Zeniths, amazingly I'm also still learning new things...

 

[62SNBMR]

Berndt,
if you find out why they put the small hole in, please let me know. Thanks

 

[bernd_st]

Can only guess why the Zenith engineers went for that hole. Looks to me like a fuel pressure release into the float chamber after pumping action. Perhaps the original design without the hole was leading to excessive duration of additional fuel squirt or even fuel dripping...

 

[RootesRacer]

Can only guess why the Zenith engineers went for that hole. Looks to me like a fuel pressure release into the float chamber after pumping action. Perhaps the original design without the hole was leading to excessive duration of additional fuel squirt or even fuel dripping...

Nearly all carburetors have a "hole" like this. Its the accel pump bleed orifice, as I said above its so that small or slow throttle openings do not get unnecessary enrichment. Most carbs have a separate bleed valve jet from the check valve (Zenith combined the two into one part).
Without the bleed, the entire pump piston stroke of fuel will be forced through the accel pump jet no matter how slow you open the throttle.

 

[bernd_st]

Confirms my theory. Only remaining question would be then how did it work in the early design (w/o hole). Or they just lived with the over enrichment and higher fuel consumption...

 

[RootesRacer]

Confirms my theory. Only remaining question would be then how did it work in the early design (w/o hole). Or they just lived with the over enrichment and higher fuel consumption...

Ive seen the same function implemented with a groove staked in the sealing surface of the one way valve so it doesnt seal perfectly in the reverse direction. Perhaps earlier valves had this feature and Zenith redesigned with a more reliable valve with an orifice.

 

[bernd_st]

Sounds as if we have to cut an early one up to find out. Time for the Dremel & magnifying glasses. If I find one

 

[62SNBMR]

Ive seen the same function implemented with a groove staked in the sealing surface of the one way valve so it doesnt seal perfectly in the reverse direction. Perhaps earlier valves had this feature and Zenith redesigned with a more reliable valve with an orifice.

Wonder what failed on mine? Repeated cleanings, soaking in carb cleaner, air, etc. hasn't worked. It certainly doesn't seal in either direction. Doesn't look damaged.

 

[bernd_st]

Haven't cut one apart yet, but think there should be a little spring /diaphragm valve inside. Must be faulty in your case. Have a Dremel ? Then time to enlighten us. Pls. share some pictures here

 

[RootesRacer]

I would wager on corrosion of the sealing face of the one way valve. You could soak the valve in some carb cleaner and blow it out and maybe bring it back from the dead (this would only work if its varnish or crud contamination).