Amazon: Large fuel leak from Zenith 36VN carb on my 1966 131

[fishyboy]

Hi all,

Last weekend I started up my '66 131 after standing idle for a couple of months (with only 17,000 miles on the clock it doesn't get used too often). As ever it started after only a few attempts and I left the car to idle on the choke for a few minutes and warm up ready for a drive. Returning to the car after 5mins I noticed a smell of petrol and a puddle of fuel under the car. Quickly turning the ignition off and opening the bonnet I found fuel dripping from the Zenith 36VN onto the hot manifold and feeding a largish pool of fuel on the garage floor!

I last rebuilt the Zenith carbs only 3 years ago and it has been well behaved since (https://www.volvoforums.org.uk/showthread.php?t=312839). Given the volume of fuel expelled I was expecting to find that the float had maybe sunk and fuel was leaking via the gasket at the top of the fuel reservoir. Removing the carbs and taking apart it looked like the float was fine and working correctly and was the valve.

I have a new gasket set and plan to replace all the old gaskets with new.

Would it be wise (or foolish) to use two gaskets on the fuel reservoir lid or would this be asking for trouble?

Also it seems unlikely to me that neat fuel leaking the carb to manifold gasket as it should be vaporized at this point. Any ideas?

Phil

[Rustinmotion]

Originally Posted by fishyboy

Hi all,

Last weekend I started up my '66 131 after standing idle for a couple of months (with only 17,000 miles on the clock it doesn't get used too often). As ever it started after only a few attempts and I left the car to idle on the choke for a few minutes and warm up ready for a drive. Returning to the car after 5mins I noticed a smell of petrol and a puddle of fuel under the car. Quickly turning the ignition off and opening the bonnet I found fuel dripping from the Zenith 36VN onto the hot manifold and feeding a largish pool of fuel on the garage floor!

I last rebuilt the Zenith carbs only 3 years ago and it has been well behaved since (https://www.volvoforums.org.uk/showthread.php?t=312839). Given the volume of fuel expelled I was expecting to find that the float had maybe sunk and fuel was leaking via the gasket at the top of the fuel reservoir. Removing the carbs and taking apart it looked like the float was fine and working correctly and was the valve.

I have a new gasket set and plan to replace all the old gaskets with new.

Would it be wise (or foolish) to use two gaskets on the fuel reservoir lid or would this be asking for trouble?

Also it seems unlikely to me that neat fuel leaking the carb to manifold gasket as it should be vaporized at this point. Any ideas?

Phil

Could have been a stuck needle valve and the act of removing the lid has in stuck it, if it was leaking out of the lid it means the fuel bowl was over full as the level should not be that high if float and valve are working, I’d make sure it’s all clean and put it back together

[142 Guy]

Zenith carburettors do seem to be a bit of a popular topic.

Before ripping things apart, check your fuel pressure. The Zenith carbs do seem to be susceptible to overflow caused by high fuel pressure. Do a search on this forum and you can find threads discussing this topic. My recollection is that the recommended maximum fuel pressure is remarkably low.

Do you have evidence of where the gas was originating? If it was coming from the float bowl vent then re gasketing is not the solution. If the float continues to 'float' then I would suspect dirt in the needle valve preventing a compete seal. If gas is originating from that mating flange between the float assembly and main body then gasket replacement might be appropriate.

Double gasketing is never a good solution. If the mating flanges have distorted the best method is to try to resurface those surfaces to bring them back closer to flat (valve grinding compound and a flat of heavy glass can be effective / slow). For flange surfaces that appear acceptable, I would go with a single gasket and a fuel resistant thin-bodied non-hardening dressing. I like Hylomar Blue / Permatex Permashield; but, there are other options if you search around.

[samaron]

Had a thread a while ago about fuel dripping from the discharge beak inside the carburetor, causing the car to become flooded. In this post in that thread, I quote the following from the Volvo shop manual: "The pump is adjusted for a maximum pressure of 3.5 psi (0.25 kg/cm2)."

This information is for a B16 with a glass bowl fuel pump and a Zenith 34 VN carburetor. I assume the number is still relevant for a Zenith 36 VN, as they're very similar. There are also references in various bulletins and shop manuals that state the Zenith carburetors are very sensitive to pressure.

The gasket between the float bowl and body is, from what I can tell, supposed to seal for air. The emulsion block gasket inside the float bowl would seal for fuel. It can leak from a lot of strange places if one of the seals are not 100% and it is overfilled. In my experience, the float valve likes to stick. Often times shut, stalling the car. Sometimes open, causing fuel to hemorrhage and irratic running.

Since you have a 1966 model, I assume it does have a B18/20 engine as well. I'd seriously consider to retrofit anything else, as the options are available.

[fishyboy]

Thanks for the replies.
I have replaced the needle valve with a new one, although I think the original was fine.

I plan to replace all the paper gaskets and try running the engine to see where, if any, the leaks are coming from.

Looking at the 1968 service manual (attached) they show a fuel pump which looks like the one on my car. The text describes the process whereby the pumping action ceases when the pressure build ups following the closing of the float valve Is it possible that this has failed and the build up of pressure in the delivery pipe overcomes the needle valve or float causing the flooding. If so it might be worth me getting a new pump?



Phil

[142 Guy]

Vacuum / fuel pressure gauges are relatively inexpensive.
https://www.amazon.ca/DASBET-Vacuum-...8950907&sr=8-7
Just make sure that you do not get a tester meant for fuel injection systems which will not have the required accuracy at the low pressures that carb systems operate at.

I would test to confirm that the pressure is out of spec before I went and fiddled with the fuel pump. Since your leak problem occurred at idle it should be relatively easy to diagnose the problem by testing the pressure at idle. If the pressure is excessive then time to explore what is going on with the pump.

The original needle valve may have been just fine if it was a spec of foreign matter that was preventing it from closing. Do you have a proper micronic fuel filter or do you have one of those plastic mesh lump filters on the fuel line?

Personally I would check to see where the leak is before disassembling the carb. Disassembly and then reassembly does not always improve things.

[fishyboy]

Hi all,
I finally found some time today to rebuild the Zenith 36VN carb. I replaced the needle valve and added a second washer, which might reduce the level a little. All the gaskets were replaced. I also spent some time re-facing the lower surface of the carb, where it mounts onto the manifold. It was clear that the surface was slightly bowed with a raised area around the opening (Photo - part way through the process). Plenty of working from 600 to 1200 grade seemed to flatten things out, with the resultant surface of fresh metal and the gentle warping removed. With the carb refitted the car started fine and ran for 5 mins with no obvious leaks.

I have the test kit as suggested by '142 Guy' but haven't yet tested the pump pressure. I'm slightly confused as to how a gauge connected directly to the fuel pump would read the correct pressure (given that the pump with continue pushing against a closed system) with the pressure presumably building until the diaphragm stops moving. I was wondering about connecting a t-piece in the fuel line with the gauge connected to the t-piece. Would this work and give a more meaningful fuel pump pressure?

To be honest I think the Zenith might be a 'ticking time-bomb" and I am thinking of replacing it with a Weber 34, which is apparently a straight swap for the Zenith 36VN (https://www.skandix.de/en/spare-part...4-kit/1022670/).

Thanks for all the replies.

Phil

[142 Guy]

An in-line Tee fitting would be the correct way to measure the running pressure. Kits often come with tee fittings in different diameters to facilitate the measurement with different size fuel line. Easy to do if the fitting on the carb is a barbed fitting. Just add a short stub hose to the carb's barbed fitting. Shove the Tee into the stub and then the original supply hose on to the other side of the Tee with the gauge off of the tapped arm of the Tee. Clamps are always a good safety feature.

If more convenient you can do the measurement at the pump outlet. The distance between the pump and carb will be immaterial.