Carburator theory/circuits

[Allenk]

Ok it's those Zeniths again. I finally got some rpms out of them but what a light show in the process. Ive had them completely apart and cleaned/flush/replaced to the point that I believe they are functional but can't get an idle. I get a lot of fuel(have put in a in line regulator set at 2.5).

My question is does anyone know the theory behind these things? I assume the idle jets allow fuel to flow and the mixture screws control the fuel/air. When do the main jets begin to flow? Do the accelerator pumps/jets provide a short flow of fuel between the idle and main jets until there is enough demand? This would help me understand how to set these up.

And YES, if I know this cars (it's a project) motor is going to be sound, I'm getting WEBERS.

[A Quiet Boom]

Your idle circuit works until there is enough draw on the venturis to pull fuel from the main jets. The accelerator pump jets prevent a temporary lean condition during this transition when the throttle is opened quickly. I understand a lot of guys prefer Webers or PMOs but I'm perfectly happy with my Zeniths now that I've tuned them correctly. Proper balancing is very important.

[BK911]

Haynes has a Weber Manual that is decent. Since Zeniths are basically a clone of Webers, the book should prove to be useful.

Good luck!
BK

[Allenk]

Ok good that's what I figured interms of the accelerator jets. I've actually got the engine to idle but the mixture screws are all the way in and it's still getting a bunch of fuel spitting. One problem was that someone put a variety of sized jets in for idle jets and air. now I'm wondering where all the fuel is coming from. Should I open the air bypass more? Turn in the flow adjustment?

[A Quiet Boom]

Have you checked your timing? The proper way to adjust idle mixture on a carburator (any carb) is to adjust the mixture screws towards lean until the rpms pick up a few hundred rpm, then close the butterflies until the idle rpm returns to normal repeat the process until the idle mixture is as lean as possble and the butterflies are as closed as possble. with six barrels to adjust this can be a bit difficult, I like to set a starting point by truning all the mixture screws all the way in then backing them out one full turn each I then start the car and follow above procedure adjusting each screw the same or I adjust using a carb synchronizer to get the highest reading and then match all the other cylinders.

[911pcars]

"I've actually got the engine to idle but the mixture screws are all the way in and it's still getting a bunch of fuel spitting."

Can you elaborate, "getting a bunch of fuel spitting."? Where is the fuel spitting?

Sherwood Lee
http://members.rennlist.org/911pcars

[clevy70911T]

Good news.

I went to my local library and found some large books that had fuel, emission and ignitions systems from every auto manufacturer in a range of years in the late 60's to mid 70's. This book had each circuits theory pictured and described in detail for the Zeniths and the webers.

Bad news.

The volumes were so large that there wasn't anyway to copy due to the deep binder and I can't remember the name. Could have been "Rogers", but I hadn't heard of it before. You might try your library and I will post the name of the books after I contact the library.

[Allenk]

Interms of the fuel/spitting, The carbs are still supplying a ton of fuel to the engine-very rich smell/smoke-not your bluish oil burn. They due pop but more often spit raw fuel back up. I get my more typically described popping when I give throttle and build some rpms. I'm starting to think ignition here. The mixture screws are in good shape. The other possiblility is the enrichment circuit which is disconnected right now but not plugged off so maybe fuel is passing by there.

[911pcars]

The books say to adjust the carburetors ..... last.

Do you know for certain the mechanical compression is okay, the valves are adjusted and the ignition timing is set correctly? Carbs can't be set if the other engine systems are out of whack.

At idle, does fuel spill out of the discharge tube? Use a light and a mirror and take a look in the carb. throats. If you see raw fuel spilling out, the float level is too high.

Spitting back at idle, off-idle might indicate a lean condition cause by air bypass via the throttle shafts or a vacuum leak somewhere. A leaking intake valve due to insufficient valve clearance may cause this as well.

Hope this helps,
Sherwood Lee
http://members.rennlist.org/911pcars

[Allenk]

Well, I only know this much: I've lowered the float levels by reducing washer size at the needle and seat. I just did a physical check of the ignition and turned the Z1 to TDC at the #1 cylinder compression stroke and the distributor was in line w/ #1. TDC seemed to correspond with the Z1 mark. I have noticed however that the points are extremely closely gapped to the point where I could barely see them open so I'll re-gap and fire it up again. I checked the valves when I sealed up the covers but the car had sat for years and I'm not sure if they are 100% accurate. The carbs where pretty messy when I took them apart and if there is no difference after resetting the points I'm sure a rebuild will be next. Any opinion?

Oh ya, Where would I get a vacuum leak? the only vacuum lines I have that I'm aware of are the lines to the Aux.enrichment.