Zenith 40 TIN - Why do the atomizers melt?

[MisterJalopy]

I'm new to 911s, new to the Registry, just submitted application/money to join.

Background:
I purchased 1971 911T that is an amateur restoration. There are eccentricities to be sure. The car was largely finished but then sat for about 6 years. I drained the gas, ran the pump to empty the lines, replaced the fuel filter, opened carbs and cleaned bowls which had a little lacquer sludge, changed plugs, fogged cylinders, Sea Foam, turned over engine by hand with plugs out several times and got car started. Have only driven so far for short test drives.

Issues exhibited:
- idles well at about 1000 RPM, misses occasionally
- rare backfire through exhaust, haven't heard through carbs
- when cool, it idles. When warm, if I come to stop at light, idle races to 2000-2200. Hand throttle not used.
- two of the atomizers are melted a little bit (cylinders 4 and 5)
- enrichment circuit deleted previously with JB weld.

Current:
- I have disassembled one of the carbs. Planning to complete carb one and use carb two as reference
- On the bench, if I operate the throttle, the accelerator pump did not pump on what would be cylinder 4 and 5. Upon pulling the accelerator valve, I discovered a tiny piece of red silicone sealant under the valve of 4. Mystery solved! 5 didn't have a tell tale obstruction, but generally gummed.

Plan
- soak carb, clean out and compress air for all passages. Same for jets, emulsion tubes, etc.
- rebuild carbs
- set float with the little acrylic pipe tool

Questions:
1. Why do the atomizers melt? A lean condition? backfire?
2. The Royze carb kit seems to be the standard/only option and gets lukewarm reviews here and at outlets for sale. Is there another option?
3. Beyond http://www.suggate.co.uk/new_site/articles/page2.htm, is there another source for carb rebuilding/tuning info? I have read the Haynes manual, 101 Projects and about 200 forum posts. I don't have the two volume Workshop Manual, but need one.


Thanks a ton in advance!

Peter of Newhall, California.

[edmayo]

You answered your own question,,,they melt due to a backfire, usually during starting. Early on there was a spray tube system that Porsche later advised removing due to this problem.

[dho]

Peter, Alfa 1750's eBay store is an excellent source for rebuild kits and parts. Good selection and great service. Paul Abbott at Performance Oriented is the carb expert. He deals primarily with Webers, but some of the info is applicable to Zeniths. His website has a lot of good info.

dho

[smedley]

Carburetor fires were common problem with the Zeniths back in the day....many actually came off the boats/docks with melted atomizers and plastic velocity stacks. I had three or four startup fires with my '70T. Actually learned from the sound and throttle response to jump out. pop lid, and throw pad I kept in the car to smother the damned thing. I had the Stoddard Zenith Sport Kit which at the time was blamed for the issue. There was not a lot of knowledge about this at the time...dealer or otherwise..."they do this." Duh. The fix at the time was Webers.

It actually did the car in when the owner twice removed from me didn't recognize the problem and the rear of the car went up! The car had won three Parade trophies with me back in the 70s.....oh well.

[Mark Morrissey]

Before I even broke into reading this thread, I knew Ed had the answer

[1QuickS]

Webers can suffer the same issue.

Hot re-starts generally have a fuel rich mixture in the air cleaner and a spit-back through the intakes can ignite it. Spitbacks are typically a lean mixture from carb setting or from air leaks, Zeniths have many opportunities for these air leaks but another source is worn throttle shaft journals.

Camshafts for our carburetted cars have overlap which means both intake & exhaust valves can be open at the same time so combustion gasses can access up the intake tract & ignite fuel rich fumes in the air cleaner.

When setting float levels I recommend to let the engine idle & then shut it down to read the level using the vial. Static setting is inaccurate, you want dynamic fuel delivery when setting floats. Also, floats can stick from the tip of the needle valve resting in a divot on the tab where they contact the float, Webers especially. The exchanging of parts during a rebuild will move floats to new locations so the divot will not align with the tip of the needle valve. The resulting misalignment can cause the needle valve to not close completely which leads to flooding.

[edmayo]

In addition Paul, I believe it is made worse by people who don't understand how to start a carbureted car. Can't tell you how many times I see someone pumping the throttle while CRANKING the engine! You pump first,,,,,then crank while holding throttle part way down. And actually on a hot engine just hold throttle mostly open, then crank. That dumps any fuel that may have accumulated on the plates into the manifold, and dumps in a lot of air to overcome the richness. It's even more fun watching someone who has only ever driven modern injected cars try and cold start a carbureted car that hasn't run for a few days! I tell them that with these 'old' cars the driver has to be 'involved' !

[Richy]

Zenth carbs have the idle over-run mixture adjustment . From my little experience with these carbs , the fast idle when warm can be addressed by adjusting this .

[MisterJalopy]

Thanks everybody!

I did a lot of googling in the forums and found very little about the melted atomizer cause. This is very helpful. I am going to be extremely careful on hot starts.

Car background - 1971 911 T was an amateur restoration about 7 years ago and then sat in the garage. The carbs were supposedly 'gone through' by a knowledgeable person and jetted for the cams being ground to "S-specs." Ignition is MSD and Pertronix. Enrichment circuit deleted. Stock air cleaner. I replaced the melted atomizers. Initially, to get it started, I removed carbs and sprayed as I could without disassembly. Now I have rebuilt.

I have rebuilt the carbs, have them back on the car and have done my best to tune according to Paul's excellent Weber resources on Performance Oriented. Although it is for Webers, so much of it is applicable and I have found it to be the most comprehensive source.

I cleaned the carb bodies in a ultrasonic cleaner. Previous owner, had deleted the enrichment circuit and plugged the inlets with JB Weld or similar. Either from my ultrasonic cleaning or general failure, it turns of some of them were passing air causing a vacuum leak. I put on vacuum caps from Autozone and a dab of epoxy on each to keep them steadfast.

I have verified fuel pump pressure at 4 psi, changed fuel filter, set both float levels with car at idle and post-idle. Timing is set, rotor and cap are good, new plugs and wires. Idle is currently about 1000 RPM. I tuned the car according to Performance Oriented Lean Best approach with the slide rule style synchronizers. The idle mixture on all carbs leans out as expected, but when I back out to toward rich, I find that they don't really respond. I ended up leaning each carb, backing it off until RPMs were well stablized and then an extra 1/2 turn. My rationale, which may be faulty, was to run them richer per the Performance Oriented section on Tuning Worn Carbs.

How it runs now -
- Garage idle - pretty stable RPM, no spitting from carbs, no back fires
- Driving - some initial hesitation, then strong throughout, occasional spitting
- Slowing down - if pretty heavy on the throttle and I let off and take out of gear, the idle sometimes drops to nothing and dies
- Slowing down - more gradual stop, idle sometimes 'hangs' at about 2000-2100 RPM. If I am at light, I can put in second to put on a little load and it immediately drops and stabilizes at about 1000 RPM

This is all a great improvement from where I was, but not ideal. I am modest shadetree mechanic that is new to Porsche. I can follow directions and have some common sense, but don't have the experience of tuning a car with GOOD carbs.

Where I am at - I feel like the issue is worn throttle shafts. If I wiggle the throttle shafts, it feels like considerable play and more than enough to allow air in. I have squirted the shafts while running with WD-40, but to no effect on idle. Based on all that I have read, my best guess is that the throttle shafts are creating uneven amounts of auxiliary air coming into the system.

I am happy to hear I am wrong in 20 ways with other things to try!

Thanks so much! This forum has been very helpful.