View Full Version : carbs
05-05-2002, 08:02 PM
does anyone know what the difference is between the -12 and -32 carbs?
does one provide more fuel flow than the other?
05-06-2002, 08:54 PM
I know the jets and float bowls are different, venturies are the same. I would assume that the -32 would have more flow for the higher compression engine, but I could be wrong.
05-07-2002, 03:02 PM
Sort ah on the same subject.
I have a PA12 with stock tanks, with the forward fitting welded in, cross vented and (No header tanks) I have put this plane in what I think is every attitude/fuel configuration possible and never experienced a problem loosing fuel flow to the carb? My question is (2 fold)
1) what is the head pressure that is required of the carburator on an 0320/160hp? Also as to venting?
2) Has anyone else had this problem: I have the snorkel vent caps (built by Interior Pacific Aviation) they were what they install as part of the Charley Center STC? for the 180hp conversion) I installed them cause it seemed like a good Idea? Mine had the stock cub vented vented caps that would leak all over heck when the tanks were full and if at all parked on banked surface (the fuel would run across the vent and overfill the lower tank and out the freaking vented cap and fill the darn wing!!
Also I have another condition that is agrrivating (there is a high pitched whistle that appears to be coming from the vent snorkel (that will drive you nuts in the cockpit!) could this be eliminated by drilling a small hole in the vertical standpipe part of the snorkel? (if so would this static port effect the fuel pressure?
NEXT question! I am considering installing a JPI 450 fuel flow computer on my plane? does anyone know what of any restriction the transducer causes in the fuel line/flow?
I would be really interested in a diagram of some of the plumbing methods used in the PA 18 and the PA 12 without header tanks, also to what STC or approval process they are associated with?
05-07-2002, 09:28 PM
Head pressure should be .5 psi minimum, 8 psi max. You should have this with any high wing plane.
Whistling caps? Try bending the snorkle to a different angle.
I don't think the JPI transducer would cause any restriction, but why would you want such a silly, expensive contraption in a Cub?
Most Fat Cubs have really bad, cobbled together fuel systems. The parts manual shows how it should be originally. I'm putting stock Super Cub systems in them now for simplicity, and reliability.
05-07-2002, 11:46 PM
MD, I agree the PA18 system is what I would have preferred! I never had a problem with the header tanks etc. My system seems is the same as stock except for the cross vent in the front of the tank (I assume this was required when the header tank was removed?) It would be major to somewhat major to replumb now, the interior was fabric/vinyl covered instead of aluminum paneled as I would prefer for maintenance. (when I have a need to get into the fabric, I will most likely modify/simplify the system (as well as to put an extra fitting in the left tank for a (future belly tank!)
The reason for the JPI is due to the fact that this plane was built to be a float plane (not so for me) I have put it up on the long legs and big tires! The nose being as high as it is makes the (PA18 sight gages) that were welded into the tank ends, unreadable at 3 point till there is at least 6.5 gal of fuel on each side! I don't like taking off when I can't see (any fuel) sloshing around! I fly by the tacktime and can usually count on 7.5 gph and better at cruise. As you know most of our real cub flying is at minimum empty weight!
The JPI unit appears to be a pretty good value at $600 and I never could get used to running out of gas!
I will be doing a series of fuel flow tests at differing attitudes next week!
Thanks for the imput!
05-08-2002, 01:20 AM
PA-12 Man, I have a set of CC's drawings for the STC'ed "headerless" fuel system for a PA-18. I copied the system on the PA-14 I just finished and am trying to get a field approval for this. Thats one of the things I don't like about the PA-12/14 is the stock fuel system is junk from the tanks on down. One of the reasons your tank over flows is your fuel selector is most likely modified for the headerless system. The STC requires drilling out the selector so it will feed from both tanks at the same time in one position. The problem is, even in the off position the tanks cross feed so the up hill tank drains into the low wing tank. Its a thing call gravity that does it. Once the low tank fills up the only place fuel can go is out the vent. To be a true headerless system you should have a line going from the RH tank front fitting across the firewall to a T, joining up with the line from back fitting and from there to the selector. You need drains at the lowest point of the system, in the three point attitude with all you extended gear and big tires (guys forget that all this slope screws the fuel system sump up). They have got to be at the lowest point in the system and everything should slope to the drains. Any low spots in the line will collect water and you can't get it out of the system by sumping the drains. Gas will pass through the water until one cold night in September and the water freezes in the line. Then you know what happens next...Crash
-12 used on earlier 0320s with tapered riser in sump. -32 used on later 0320s with straight riser. Thats what I was told, dont know ifs its true or not. -32 are rumered to run smoother.
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