Burnt Zenith Carburetor Stacks

[Mr.P 911T 1970]

Forum – Please Help.

I have a 1970 911T 2.2 with original Zenith carburetors. I removed the air intake to clean the carburetors and notice that the right set of stacks was dirty but looked normal, however, on the left set of stacks looked melted with a tar substance (See pictures).

Engine runs normal, no issues, cranks over perfect, no issues. Can anyone identify what the heck is going on?

[buster73]

Wow - looks like you had a nice carb fire melting down the inlet trumpets. You can consider yourself lucky the fire did not spread any further. Could be several culprits like sticky needle valves, heavy backfire and/or any other type of fuel leak inside the carb. Ignition should also be checked. Make sure to disassemble the carb and check what else might be damaged inside. Molten metal might have even reached the inlet valve or cylinders. The black tar-like substance might just be the remains of the smoldering fire, so just developed during the fire. One other possibility would be collected oil residue from the vent hoses at the back of the air filter housing. However I would expect this to look similar on both banks. How does you air filter insert look like ?

[Trackrash]

^^^ Good advice. Yes I would replace the needles and seats in the carbs, if they haven't already been done. They can stick and shoot gas up into that area.

I like the shiny metal trumpets like you have on the right.

While in there, if your fuel hoses are more than a few years old you should probably replace ALL of them. And carry a fire extinguisher in your car.

BTW, I like those Zeniths on a stock or near stock 2,2s.

[1QuickS]

Zeniths like to accumulate fuel in the recesses created by the reinforcing ribs in the tops of the throttle housings, Webers have these reinforcing webs internal to the tops which helps shed fuel since the tops are smooth. A intake backfire can ignite the fuel vapor in the air cleaner as a result.

Check:

• fuel supply pressure while engine is running, something like 3 psi is fine with plus/minus .5 psi as a tolerance
• check fuel level in each float bowl with external gauge. This is to be performed with engine running. Adjust float needle valves to achieve fuel level using shims.
• Todays's fuels boil at lower temperatures than the fuels of 50 years ago. When you shut down your engine heat can easily be conducted from the heads to the carbs and boil the fuel in the bowls. This fuel will escape the bowls via the vent pipes located between the air horns (brass tubes) and through the auxiliary venturi which spills onto the throttle valve. This conduction may be minimized by installing insulator plates between heads and manifolds. You can install more than one set but you will need to remove OEM studs & replace with longer ones.
• The boiling fuel after shutdown fills the recesses in the top cover of the carbs which makes these little fuel reservoirs that evaporate gas into the air cleaner which is then susceptible to catch on fire with an intake backfire.
• A fuel pump cut-off switch can be used to good effect to kill fuel delivery in advance of shutting down the engine. This has been used by many in the past to control boiling fuel in the float bowls. It is also a good theft deterrent device.
• Another item to consider is to drill discrete drain holes in the reservoirs in the top covers to allow fuel to drain but this would drain the fuel over the carb bodies which would stain the bodies and possibly drain onto something hot...
• As in Webers, there is the possibility to drill internal passageways to re-direct boiling fuel down the throttle bores as opposed to it going up the vent pipes. Hot re-starts are to be endured.

[911 SLANT]

Heres a picture of mine after 5 years.

[Hi_Fi_Guy]

Everything 1quickS says is true. I’ve rebuilt my Zeniths once and after a carb fire and tow had no option but to pay to have one rebuilt again. If I had to do it over again I would have rather had a nice set of PMO or Weber carbs.

I live in Phoenix and the issue with fuel percolating out of the tops of the carbs at anything over 80-90 degrees Fahrenheit is a real problem. Running 95 octane with low ethanol in the Summer helps a bit.

Zeniths are not bad carbs but They always seemed like a compromise designed to meet emissions first with drivability and performance secondary considerations. In my opinion the other options may be better but Zeniths can be made to work well. Just watch out for fires in the warmer months. Usually happens after driving the car, parking, and starting up again after fuel has percolated out of the tops of the carbs.

[911 SLANT]

I did go with PMOs now. I have heard only good things about the PMOs.

[buster73]

Originally Posted by 1QuickS

[*]As in Webers, there is the possibility to drill internal passageways to re-direct boiling fuel down the throttle bores as opposed to it going up the vent pipes. Hot re-starts are to be endured.[/LIST]

Hi Paul, I am also just getting more involved in Zeniths and when searching the forum found you have been looking into anti-percolation measures for Zenith already long ago. Is the drilling proposal from the following older thread still the last evolution ?

http://forums.pelicanparts.com/porsche-911-technical-forum/569397-zenith-fuel-percolation-fix-need-feedback-please.html?highlight=zenith+percolation

Thinking about to go this way for my next Zenith project.

Appreciate your thoughts.

[911 SLANT]

Just go with PMO. Avoid all the problems with Zeniths. Been there done that!... waist of money

[1QuickS]

I'm glad to see I have already provided info regarding drilling the top covers for Zeniths, thank you for providing the link.

Zeniths work well if in good condition and the emission stuff is bypassed. I know of a respected shop in Anaheim where owner & his long time coworker both have Zeniths installed on their 2.5 liter 911s. They have been happy with them for at least 10 years and use them as daily drivers. This is a Porsche all-air-cooled shop that has been wrenching Porsches for 50 years for which I have serviced 19 projects including MFI throttle bodies, Solex 40PI, Zenith 40TIN and Weber IDA3C carbs. They are not the only examples I can think of that are happy with Zeniths on their 911s, especially if the Zeniths are installed on a stock 2.2T.

New PMOs are an option compared to the expense of a full-on mechanical refurbishment but Zeniths tend to have a longer-lived throttle shaft/journal life expectancy for various reasons. They do have some design details that have driven me to stop servicing them and focus only on the Webers.

[VFR750]

One issue with the float valve is the nut. When they machined the hex they slightly deformed the sealing surface on the opposite side. That periodic (6) wave make it very hard to seal completely.

Fuel would slowly leak out and accumulate. Fire could be a risk.

I sanded the surface smooth Using a block with a hole in it. 600-800 grit sand paper with the same hole. Slowly spin the sealing surface against the sandpaper. Initially you will see the (6) ridges. Keep going until the whole surface is smooth. Reinstall with a new seal and it won’t leak.

[VFR750]

Originally Posted by buster73

Hi Paul, I am also just getting more involved in Zeniths and when searching the forum found you have been looking into anti-percolation measures for Zenith already long ago. Is the drilling proposal from the following older thread still the last evolution ?

http://forums.pelicanparts.com/porsche-911-technical-forum/569397-zenith-fuel-percolation-fix-need-feedback-please.html?highlight=zenith+percolation

Thinking about to go this way for my next Zenith project.

Appreciate your thoughts.

Paul,

I’m pretty sure you did the fuel vent modification to my zeniths back in 2014.

Plus the Weber transition port modification.

[Mr.P 911T 1970]

Appreciate the insight folks. All the best.